A thick rare earth NM550 steel plate and its manufacturing method

Abstract

The invention provides a thick rare earth NM550 steel plate and a manufacturing method thereof. The components include C0.26-0.35%, Si0.1-0.3%, Mn0.75-1.0%, P≤0.015%, S≤0.005%, Alt0.015-0.04%, Cr0.65-0.9%, Nb0.01-0.03%, V0.005-0.01%, Ti0.01-0.02%, Mo0.1-0.2%, Ni0.1-0.2%, B0.001-0.003%, Re0.0015-0.004%, N0.003-0.005%, and the rest are Fe and residual elements. The hardness of the steel plate at 1 / 2 of the thickness is ≥500HB, the longitudinal impact energy at -40°C is ≥65J, various high mechanical properties meet national standards, the thickness reaches 80mm, and no cutting delayed cracks will occur.

Description

Technical Field

[0001] The invention belongs to the field of alloys, and in particular relates to a thick rare earth NM550 steel plate and a manufacturing method thereof. Background Art

[0002] NM550 steel plates are mainly used in mining machinery, loading and transportation equipment, large excavation equipment and other fields. With the rapid development of my country's economy, the size of equipment and spare parts has continued to increase, and the application area has become wider. The demand for the thickness of wear-resistant plates has also gradually increased. Not only does it need to meet the performance of the surface and 1 / 4 of the thickness, but it also puts forward higher requirements for the core performance. The hardness range needs to reach 475~580HB, and the longitudinal impact energy at -40℃ is ≥60J. At the same time, with the increase of the thickness of NM550 steel plates, the risk of cracking after cutting increases sharply. At present, domestic steel mills cannot stably achieve a core hardness of ≥475HB, a longitudinal impact energy of ≥60J at -40℃, and no delayed cracks after cutting.

[0003] At present, the 60-80mm thick NM550 steel plate is produced by 300mm continuous casting billet, mainly by increasing the content of C, Mn, Cr, Mo, and Ti to improve the hardenability of the steel plate. Its production method includes heating, rolling, strong cooling after rolling and heat treatment. However, the 60-80mm thick NM550 steel plate produced by the existing technology has the problems of delayed cracking during cutting and substandard impact toughness of the core. Summary of the invention

[0004] The purpose of the present invention is to provide a thick rare earth NM550 steel plate and a manufacturing method thereof. Through component design and production method, the produced NM550 steel plate not only meets the national standard requirements, but also meets the hardness of ≥500HB at 1 / 2 of the thickness, the longitudinal impact energy of ≥65J at -40℃, and various high standards of mechanical properties meet the national standards. The maximum thickness of the steel plate can reach 80mm, and no cutting delayed cracks will be generated, thereby meeting the market demand for producing thick NM550 steel plates with core performance.

[0005] The specific technical solutions of the present invention are as follows:

[0006] A thick rare earth NM550 steel plate, comprising the following components in mass percentage:

[0007] C: 0.26~0.35%, Si: 0.1~0.3%, Mn: 0.75~1.0%, P≤0.015%, S≤0.005%, Alt: 0.015~0.04%, Cr: 0.65~0.9%, Nb: 0.01~0.03%, V: 0.005~0.01%, Ti: 0.01~0.02%, Mo: 0.1~0.2%, Ni: 0.1~0.2%, B: 0.001~0.003%, Re: 0.0015~0.004%, N: 0.003~0.005%, the rest are Fe and residual elements.

[0008] The thickness of the thick rare earth NM550 steel plate is 60 to 80 mm.

[0009] The structure of the thick rare earth NM550 steel plate is martensite, and both the surface and the core 1 / 2 are martensite.

[0010] The thick rare earth NM550 steel plate has a yield strength of ≥1450Mpa, a tensile strength of ≥1700Mpa, an elongation of ≥21%, a surface Brinell hardness of ≥530HB, and a longitudinal impact energy of ≥65J at -40°C; the yield strength at 1 / 2 of the thickness is ≥1400Mpa, the tensile strength is ≥1650Mpa, the elongation is ≥21%, the Brinell hardness is ≥500HB, and the longitudinal impact energy of ≥65J at -40°C.

[0011] The present invention provides a method for manufacturing a thick rare earth NM550 steel plate, comprising the following steps:

[0012] Heating, rolling, post-rolling cooling and heat treatment;

[0013] The heating process adopts a hot delivery and hot charging segmented isothermal heating process, the billet hot delivery temperature is 700-800°C, the hot charging temperature is 500-650°C, the billet preheating section heating temperature is 500-900°C, the heating time is 120-140 minutes, the first heating section temperature is 900-1150°C, the heating time is 120-140 minutes, the second heating section temperature is 1200-1260°C, the heating time is 60-80 minutes, the soaking section temperature is 1200-1240°C, the heating time is 30-50 minutes, and the total heating time is 330-410 minutes. Due to the addition of rare earth elements Re During the smelting process, fine and dispersed modified inclusions are formed, which refine the grains of the steel plate, thereby reducing the amount of Ni alloy added and eliminating the need to cool the billet before it enters the furnace and brush the Ni-containing steel anti-oxidation coating process, so that the billet can be hot-delivered and hot-loaded into the furnace, and the overall temperature uniformity and composition uniformity of the billet before entering the furnace are improved. On this basis, a segmented isothermal process is adopted to change the previous uniform speed advancement mode of the walking-beam heating furnace, delay the heating time of the billet in the preheating section and the first heating section, shorten the heating time of the second heating section and the soaking section, and avoid staying too long in the extremely rapid growth zone of austenite above 1150°C, which makes the billet structure coarse.

[0014] The rolling adopts a two-stage controlled rolling process, specifically: a two-stage controlled rolling process of differential temperature rolling in the recrystallization zone + rolling in the non-recrystallization zone;

[0015] The recrystallization zone differential temperature rolling is as follows: the starting rolling temperature is ≥1050°C, the final rolling temperature is >950°C, and the surface of the steel plate is cooled by the front cooling device in the first and third passes.

[0016] 1300-1500L / min, cooling rate 6-7℃ / s, so that the temperature difference between the surface of the steel plate after cooling and the core reaches 150-200℃, the reduction rate of each of the 1st to 4th rolling passes after cooling is 9%-10%, and the final rolling temperature is greater than 950℃; the present invention adopts differential temperature rolling in the 1st and 3rd passes of the steel plate rough rolling stage, utilizes the 150-200℃ temperature difference between the surface and the core of the steel plate, and the fluidity of the core is better than that of the surface, thereby increasing the deformation of the core, and combined with the rare earth modified inclusions dispersed in the steel, the purpose of refining the core structure grains and improving the uniformity of the full thickness structure performance of the steel plate is achieved.

[0017] The rolling in the non-recrystallization zone is as follows: the starting rolling temperature is ≤880°C, the final rolling temperature is 840-860°C, the cumulative reduction rate is 30%-40%, and the single-pass reduction is less than 20mm.

[0018] The post-rolling cooling: interval cooling is used. After the steel plate is rolled, the water temperature is ≥820°C. The ACC nozzles 1-3, 6-8, 11-13, 16-18, 20-21 are opened at intervals, a total of 14 groups. The water volume is 500-550m3 for the 1st to 3rd group. 3 / h, 440-470m / h for groups 6-8 3 / h, groups 11-13 and 16-18 were 240-260m / h 3 / h, 150-170m for groups 20-21 3 / h, cooling roller speed 1.0-1.2m / s, red-return temperature after cooling 300-350℃, after cooling, offline stack cooling at 200-300℃ for 24-48 hours. The present invention performs forced interval cooling, the cooling red-return temperature reaches 300-350℃, and after cooling, a uniform martensitic structure is obtained in the thickness direction of the steel plate. In the subsequent offline quenching heating process, the lath bundle boundary and boundary in the original martensitic structure of the steel plate provide effective heterogeneous nucleation points for the nucleation of austenite grains during the austenitization process, which hinders the growth of austenite grains during the heating process, so that the grains of the steel plate structure are further refined after offline quenching, the comprehensive mechanical properties are improved, and the risk of cracking is reduced.

[0019] The heat treatment process adopts quenching + low temperature tempering;

[0020] The quenching: holding temperature: 860-880°C, storage time: (3×H)min, H represents the thickness of the steel plate, in mm; when calculating, the data before the steel plate thickness unit is substituted into the formula, and the storage time unit is minute min;

[0021] The low-temperature tempering: tempering temperature: 180-220°C, storage time: (1.5×H+150) minutes; H represents the thickness of the steel plate, in mm; when calculating, the data before the steel plate thickness unit is substituted into the formula for calculation, and the storage time unit is minute min.

[0022] In the prior art, NM550 steel plates are generally improved in hardenability by increasing the content of C, Mn, Cr, Mo, Ti, and Ni to ensure that the core hardness is ≥500HB. However, the inventors found that the elements Mn and Cr are very likely to form component segregation in the center of the ingot, resulting in a segregation zone in the core of the steel plate, making the surface and core structure of the thick NM550 uneven, which not only deteriorates the impact toughness of the core, but also generates large thermal stress and structural stress at the end when used for hot cutting, causing the problem of delayed cracking during cutting. The element Ti combines with N in the steel to form TiN inclusions. The edges of this type of inclusions are sharp and easily form stress concentration in the structure, further increasing the risk of delayed cracking during cutting. The element Mo is expensive and has a high cost of use; the element Ni content is high, and the billet must be painted with anti-oxidation paint before entering the furnace, and hot delivery and hot loading cannot be achieved; and the alloy content is high, the temperature of the steel plate structure is completely austenitized, resulting in a high quenching and heat preservation temperature. After quenching and water cooling, the internal stress of the steel plate is high, which increases the risk of cracking after cutting. The present invention adds 0.0015-0.004% rare earth Re, uses rare earth elements to purify molten steel, and modifies the composite inclusions of (Mn, Ca) S or TiN into smaller and more Re2O2S and Re x S y Inclusions can not only reduce the number of TiN inclusions in steel, but also the modified inclusions can provide more nucleation points for the nucleation of the organization in the subsequent rolling process, which is beneficial to the refinement of grains in the rolling process. By refining the hot-rolled grains of NM550 steel plates, the hardenability of the steel plates is improved. In addition, rare earth rhenium Re and elements such as Al, O, and S form fine inclusions enriched in the grain boundaries of the organization, which reduces the interfacial energy of the grain boundaries and increases the energy "potential barrier" of other elements segregating at the grain boundaries. It can inhibit the segregation of B elements at the grain boundaries to a certain extent, and more effectively exert the ability of B elements to improve hardenability, thereby achieving the effect of reducing the content of alloying elements such as Mn, Cr, and Mo, reducing production costs, reducing segregation in the core of steel plates, and optimizing the uniformity of organizational properties. Moreover, the invention refines the grains of the steel plate structure after adding rare earth Re, reduces the segregation of the strong hardenability element B, improves the hardenability of the steel plate, appropriately reduces the content of alloy elements such as Mn, Cr, and Mo, and reduces the complete austenitization temperature of the steel plate, which can appropriately reduce the quenching and heat preservation temperature, reduce the internal stress of the steel plate after quenching, and reduce the risk of cracking of the steel plate after cutting.

[0023] In the existing production method, the steel billet cannot be hot-delivered and hot-loaded due to its high alloy composition, especially the high Ni content. It needs to be cooled to below 100°C and coated with anti-oxidation coating before entering the furnace. In order to achieve uniform heating of the overall temperature of the billet, the heating time must be extended, resulting in coarse grains in the steel billet structure. At the same time, due to the limitation of the rolling force of the rolling mill, the single-pass reduction amount of the steel plate in the rough rolling stage is small, and the single-pass reduction rate is only 10%. The core position of the steel plate cannot undergo sufficient dynamic recrystallization to refine the grains, resulting in differences in the structure of the steel plate surface and the core, which not only affects the impact toughness of the core, but also after quenching, the surface and core of the steel plate will produce greater organizational stress, causing cutting delay cracking problems. The present invention adopts a differential temperature rolling process in the rough rolling stage of the steel plate, and uses a front intermediate cooling device to perform surface cooling on the steel billet before the first and third rough rolling passes, with a cooling water volume of 1300-1500L / min and a cooling rate of 6-7°C / s, so that the temperature difference between the surface of the steel plate after cooling and the core reaches 150-200°C, and the reduction rate of each of the first to fourth rolling passes after cooling is 9%-10%, and the final rolling temperature is greater than 950°C. Through differential temperature rolling, the deformation in the rolling process can be better penetrated into the core, the deformation amount of the core is increased, the driving force of the core nucleation is increased, and the purpose of refining the core structure grains and improving the uniformity of the full thickness structure performance of the steel plate is achieved by combining with the rare earth modified inclusions dispersed in the steel.

[0024] In addition, the steel plates produced by the prior art have uneven surface and core structures after rolling. Strong cooling after rolling will further increase the structural stress in the thickness direction of the steel plates, leading to cracking of the hot-rolled steel plates. Air cooling after rolling will cause the grains of the steel plates to continue to grow during the slow cooling process, affecting the final hardenability and impact toughness of the steel plates, leading to uneven core structures and performance degradation. The present invention, on the premise of improving the uniformity of the core structure of the steel plates by differential temperature rolling, forces the steel plates to cool after rolling. To ensure the uniformity of the surface and core structures of the steel plates, the cooling process uses interval cooling. After the steel plates are rolled, the water inlet temperature is ≥820°C. The ACC nozzles 1-3, 6-8, 11-13, 16-18, 20-21 are opened at intervals, totaling 14 groups. The water volume is: 500-550m for groups 1-3. 3 / h, 6-8 groups: 440-470m 3 / h, groups 11-13, 16-18 are 240-260m 3 / h, 150-170m for groups 20-21 3 / h, cooling roller speed 1.0-1.2m / s, red-returning temperature after cooling 300-350℃, after cooling is completed, it is offline for 24-48 hours at 200-300℃. Through interval cooling and slow roller speed, the steel plate can be red-returned during the cooling process, and the core and surface can better exchange heat, thereby improving the uniformity of the steel plate thickness direction. After cooling, the steel plate forms a martensitic structure. In the subsequent offline quenching heating process, the lath bundle boundaries and boundaries in the original martensitic structure of the steel plate provide effective heterogeneous nucleation points for the nucleation of austenite grains during the austenitization process, which hinders the growth of austenite grains during the heating process, and further refines the grain size of the steel plate after offline quenching, thereby improving the comprehensive mechanical properties and reducing the risk of cracking.

[0025] Compared with the prior art, the steel plate produced by the present invention using the above-mentioned components and processes not only meets the national standards, but also meets the hardness of ≥500HB at 1 / 2 of the thickness, the longitudinal impact energy of ≥65J at -40°C, and various high mechanical properties meet the national standards. The maximum thickness of the steel plate can reach 80mm, and no cutting delayed cracks will occur, meeting the market demand for the production of NM550 steel plates with large thickness and good core performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The surface organization of Example 1;

[0027] Figure 2 The structure at 1 / 2 thickness of Example 1;

[0028] Figure 3 This is a cutting photo after tempering in Example 1. Flame cutting sampling was performed and no delayed cracking occurred;

[0029] Figure 4 The surface organization of Example 2;

[0030] Figure 5 The structure at 1 / 2 thickness of Example 2;

[0031] Figure 6 This is a cutting photo after tempering in Example 2. Flame cutting sampling was performed and no delayed cracking occurred;

[0032] Figure 7 The surface structure of Comparative Example 1;

[0033] Figure 8 The structure at 1 / 2 thickness of Comparative Example 1;

[0034] Fig. 9 This is a photo of the cutting after tempering in comparative example 1. Flame cutting sampling was performed and delayed cracks appeared.

[0035] Fig.10 The surface structure of Comparative Example 2;

[0036] Fig.11 The structure at 1 / 2 thickness of Comparative Example 2;

[0037] Fig.12 This is a photo of the cutting after tempering in comparative example 2. Flame cutting sampling was performed and delayed cracks appeared.

[0038] Fig.13 This is the surface structure of Comparative Example 3;

[0039] Fig.14 This is the structure at 1 / 2 thickness of Comparative Example 3;

[0040] Fig.15 This is a photo of the cutting after tempering in comparative example 3. Flame cutting sampling was performed and delayed cracks appeared. DETAILED DESCRIPTION

[0041] Example 1

[0042] A method for manufacturing a thick rare earth NM550 steel plate, wherein the thickness is 60 mm, and the thick rare earth NM550 steel plate comprises the following components in mass percentage:

[0043] C: 0.29%, Si: 0.28%, Mn: 0.9%, P: 0.008%, S: 0.003%, Alt: 0.027%, Cr: 0.7%, Nb: 0.02%, V: 0.007%, Ti: 0.016%, Mo: 0.15%, B: 0.002%, Ni: 0.12%, Re: 0.0028%, N: 0.0046%, and the rest are Fe and residual elements.

[0044] The manufacturing method includes heating, rolling, cooling after rolling and heat treatment processes. The specific process steps are as follows:

[0045] Heating process: the steel billet containing the above-mentioned smelting components is transported to the raw material area of ​​the rolling mill. The end face size of the steel billet is 300mm, the hot delivery temperature of the steel billet is 750℃, the hot charging temperature is 560℃, the heating temperature of the steel billet preheating section is 500-900℃, the heating time is 136 minutes, the temperature of the first heating section is 900-1150℃, the heating time is 134 minutes, the temperature of the second heating section is 1200℃-1260℃, the heating time is 75 minutes, the temperature of the soaking section is 1200-1240℃, the heating time is 48 minutes, and the total heating time is 393 minutes for 300mm casting billet.

[0046] Rolling process: The first stage adopts differential temperature rolling, and the first stage rolling start temperature is 1065℃. The cooling device is used to cool the steel billet before the first and third rolling passes. The cooling water volume is 1300L / min, the cooling rate is 7℃ / s, and the roller speed is set to 1m / s during cooling, so that the temperature difference between the surface of the steel plate and the core reaches 150-200℃ after cooling. After cooling, normal rolling is carried out. The first four rolling reductions are 29mm, 27mm, 24mm and 21mm respectively, and the reduction rates are 9.6%, 9.9%, 9.8% and 9.5% respectively, and the final rolling temperature is 968℃; the second stage adopts non-recrystallization rolling, the second stage rolling start temperature is 874℃, the final rolling temperature is 846℃, the cumulative reduction rate is 30%, and the single-pass reduction is less than 20mm.

[0047] Post-rolling cooling process: After the steel plate is rolled, the water temperature is 835℃. The ACC nozzles 1-3, 6-8, 11-13, 16-18, 20-21 are opened at intervals, a total of 14 groups. The water volume is 500m for the 1st to 3rd group. 3 / h, 450m for 6-8 groups 3 / h, 240m for groups 11-13, 16-18 3 / h, 160m for group 20-21 3 / h, cooling roller speed 1.1m / s, red-return temperature after cooling 312℃, after cooling is completed, it is cooled offline at 220℃ for 24 hours.

[0048] Heat treatment process: quenching + low-temperature tempering, steel plate thickness 60mm, quenching and holding temperature: 870℃±10℃, storage time 180 minutes, low-temperature tempering process: tempering temperature: 200℃±20℃, storage time: 240 minutes (H represents the thickness of the steel plate, mm).

[0049] The microstructure of the steel plate in this example is shown in Figure 1-Figure 2 The mechanical properties of the steel plate are shown in Table 1, and the hardness properties are shown in Table 2. The maximum thickness of NM550 in the national standard GB / T24186-2009 is 60 mm. The performance testing method for manufacturing 60-80 mm NM550 steel plates of the present invention refers to the NM550 steel plate standard in GB / T24186-2009, and the performance at 1 / 2 of the thickness is added with reference to the standard.

[0050] Table 1 Mechanical properties of 60 mm NM550 steel plate in Example 1

[0051]

[0052] Table 2 Hardness properties of 60mm NM550 steel plate in Example 1

[0053]

[0054] It can be seen from the performance test results in Table 1 and Table 2 that the mechanical properties of the 60mm NM550 steel plate at 1 / 4 and 1 / 2 of the thickness are good, fully meeting the requirements of the corresponding standards, and have good strength and toughness matching. At the same time, the Brinell hardness of the surface and core also exceeds the standard requirements, and has good wear resistance.

[0055] Example 2

[0056] A method for manufacturing a thick rare earth NM550 steel plate with a thickness of 80 mm. The chemical composition of the steel plate and its mass percentage are: C: 0.32%, Si: 0.25%, Mn: 0.83%, P: 0.006%, S: 0.003%, Alt: 0.025%, Cr: 0.75%, Nb: 0.025%, V: 0.008%, Ti: 0.018%, Mo: 0.18%, B: 0.0018%, Ni: 0.17%, Re: 0.003%, N: 0.0044%, and the rest are Fe and residual elements.

[0057] The method for manufacturing a thick rare earth NM550 steel plate in this embodiment includes heating, rolling, and heat treatment processes, and the specific process steps are as follows:

[0058] Heating process: the steel billet containing the above-mentioned smelting components is transported to the raw material area of ​​the rolling mill. The end face size of the steel billet is 300mm, the hot delivery temperature of the steel billet is 770℃, the hot charging temperature is 595℃, the heating temperature of the steel billet preheating section is 500-900℃, the heating time is 126 minutes, the temperature of the first heating section is 900-1150℃, the heating time is 130 minutes, the temperature of the second heating section is 1200℃-1260℃, the heating time is 73 minutes, the temperature of the soaking section is 1200-1240℃, the heating time is 44 minutes, and the total heating time is 373 minutes. Rolling process: The first stage rolling start temperature is 1063℃. The billet is cooled by cooling device before the first and third rolling passes. The cooling water volume is 1500L / min, the cooling rate is 7℃ / s, and the roller speed is set to 1m / s during cooling, so that the temperature difference between the surface and the core of the steel plate reaches 150-200℃ after cooling. After cooling, normal rolling is carried out. The first four rolling reductions are 29mm, 27mm, 23mm and 20mm respectively, and the reduction rates are 9.6%, 9.9%, 9.4% and 9.0% respectively. The final rolling temperature is 959℃. The second stage rolling start temperature is 877℃, the final rolling temperature is 842℃, the cumulative reduction rate is 30%, and the single-pass reduction is less than 20mm.

[0059] Cooling process after rolling: After the steel plate is rolled, the water temperature is 832℃. The ACC nozzles 1-3, 6-8, 11-13, 16-18, 20-21 are opened at intervals, a total of 14 groups. The water volume is 550m3 for the 1st to 3rd group. 3 / h, 470m for 6-8 groups 3 / h, 260m for groups 11-13, 16-18 3 / h, 170m for group 20-21 3 / h, cooling roller speed 1.0m / s, red-return temperature after cooling 302℃, after cooling is completed, it is cooled offline at 211℃ for 36 hours.

[0060] Heat treatment process: quenching + low-temperature tempering, steel plate thickness 80mm, quenching and holding temperature: 870℃±10℃, storage time 240 minutes, low-temperature tempering process: tempering temperature: 200℃±20℃, storage time: 270 minutes.

[0061] The microstructure of the steel plate in this example is shown in Figure 4-Figure 5 ,The mechanical properties of the steel plate are shown in Table 3, and the hardness properties are shown in Table 4.

[0062] Table 3 Mechanical properties of 80 mm NM550 steel plate in Example 2

[0063]

[0064] Table 4 Hardness properties of 80 mm NM550 steel plate in Example 2

[0065]

[0066] It can be seen from the performance test results in Table 3 and Table 4 that the mechanical properties of the 80mm NM550 steel plate at 1 / 4 and 1 / 2 of the thickness are good, fully meeting the requirements of the corresponding standards, and have good strength and toughness matching. At the same time, the Brinell hardness of the surface and core also exceeds the standard requirements, and has good wear resistance.

[0067] It can be seen from the steel plate microstructure diagrams of Example 1 and Example 2 and the photos of the cut ends after flame cutting that the surface and core structures of the steel plates are martensite, and the grains of the structures are fine and uniform, ensuring the uniformity of the structural properties in the thickness direction of the steel plates, and no delayed cracks occur after flame cutting.

[0068] Comparative Example 1

[0069] A method for manufacturing a thick rare earth NM550 steel plate with a thickness of 80 mm. The chemical composition of the steel plate and its mass percentage are: C: 0.30%, Si: 0.25%, Mn: 1.3%, P: 0.008%, S: 0.003%, Alt: 0.03%, Cr: 0.9%, Nb: 0.026%, V: 0.008%, Ti: 0.018%, Mo: 0.36%, Ni: 0.48%, B: 0.0026%, and the rest are Fe and residual elements.

[0070] The comparative example NM550 steel plate production method includes heating, rolling, and heat treatment processes, and the specific process steps are as follows:

[0071] Heating process: the steel billet containing the above-mentioned smelting components is transported to the raw material area of ​​the steel rolling mill. The end face size of the steel billet is 300mm. Before entering the furnace, Ni-containing steel anti-oxidation coating is evenly applied on the upper and lower surfaces of the billet. After coating, it is put into the heating furnace. The temperature of the steel billet entering the furnace is 50°C, the heating temperature of the steel billet preheating section is 500-900°C, the heating time is 206 minutes, the temperature of the first heating section is 900-1150°C, the heating time is 164 minutes, the temperature of the second heating section is 1200°C-1260°C, the heating time is 105 minutes, the temperature of the soaking section is 1200-1240°C, the heating time is 78 minutes, and the total heating time is 553 minutes.

[0072] Rolling process: the first stage rolling starting temperature is 1062℃, the final rolling temperature is 963℃, the cumulative reduction rate is 55%, the second stage rolling temperature is 873℃, the final rolling temperature is 821℃, the cumulative reduction rate is 35%, air cooling after rolling, and stack cooling for 36 hours off the line.

[0073] Heat treatment process: high temperature quenching + low temperature tempering, quenching and holding temperature: 900℃±10℃, storage time: 240 minutes (H represents the thickness of the steel plate, mm), low temperature tempering process: tempering temperature: 200℃±20℃, storage time: 270 minutes.

[0074] The microstructure of the steel plate in this comparative example is shown in Figure 7-Figure 8 ,The mechanical properties of the steel plate are shown in Table 5, and the hardness properties are shown in Table 6.

[0075] Table 5 Mechanical properties of 80mm NM550 steel plate in comparative example 1

[0076]

[0077] Table 6 Hardness properties of 80mm NM550 steel plate in comparative example 1

[0078]

[0079] It can be seen from the performance test results in Table 5 and Table 6 that the -40℃ longitudinal impact performance and Brinell hardness of the 80mm NM550 steel plate at 1 / 2 of the thickness are relatively low, and there are differences in the performance between the surface and the core.

[0080] Comparative Example 1 is a prior art production method, and its composition and process do not meet the requirements of the present invention. It can be seen from the microstructure diagram of the steel plate and the photo of the cut end after flame cutting that the surface structure of the steel plate is martensite, and there is a certain amount of bainite in addition to the martensite structure at 1 / 2 of the thickness. There is a certain difference between the core and surface structures. After flame cutting, delayed cracks appear in the 80mm steel plate.

[0081] Comparative Example 2

[0082] A method for manufacturing a thick rare earth NM550 steel plate with a thickness of 80 mm. The chemical composition of the steel plate and its mass percentage are: C: 0.31%, Si: 0.26%, Mn: 0.9%, P: 0.008%, S: 0.004%, Alt: 0.028%, Cr: 0.85%, Nb: 0.022%, V: 0.009%, Ti: 0.018%, Mo: 0.17%, Ni: 0.19%, B: 0.0026%, Re: 0.003%, N: 0.0036%, and the rest are Fe and residual elements.

[0083] The comparative example NM550 steel plate production method includes heating, rolling, post-rolling cooling and heat treatment processes, and the specific process steps are as follows:

[0084] Heating process: the steel billet containing the above-mentioned smelting components is transported to the raw material area of ​​the steel rolling mill. The end face size of the steel billet is 300mm, the hot delivery temperature of the steel billet is 780℃, the hot charging temperature is 605℃, the heating temperature of the steel billet preheating section is 500-900℃, the heating time is 122 minutes, the temperature of the first heating section is 900-1150℃, the heating time is 125 minutes, the temperature of the second heating section is 1200℃-1260℃, the heating time is 70 minutes, the temperature of the soaking section is 1200-1240℃, the heating time is 48 minutes, and the total heating time is 365 minutes.

[0085] Rolling process: The first stage rolling starting temperature is 1069℃, the final rolling temperature is 953℃, the cumulative reduction rate is 50%, the second stage rolling temperature is 866℃, the final rolling temperature is 840℃, and the cumulative reduction rate is 35%.

[0086] Cooling process after rolling: After the steel plate is rolled, the water temperature is 830℃. The ACC nozzles 1-3, 6-8, 11-13, 16-18, 20-21 are opened at intervals, a total of 14 groups. The water volume is 550m3 for the 1st to 3rd group. 3 / h, 470m for 6-8 groups 3 / h, 260m for groups 11-13, 16-18 3 / h, 170m for group 20-21 3 / h, cooling roller speed 1.0m / s, red-return temperature after cooling 306℃, after cooling is completed, it is cooled offline at 220℃ for 36 hours.

[0087] Heat treatment process: quenching + low-temperature tempering, steel plate thickness 80mm, quenching and holding temperature: 870℃±10℃, storage time 240 minutes, low-temperature tempering process: tempering temperature: 200℃±20℃, storage time: 270 minutes.

[0088] The microstructure of the steel plate in this comparative example is shown in Figure 10-11 ,The mechanical properties of the steel plate are shown in Table 7, and the hardness properties are shown in Table 8.

[0089] Table 7 Mechanical properties of 80mm NM550 steel plate in comparative example 2

[0090]

[0091] Table 8 Hardness properties of 80mm NM550 steel plate in comparative example 2

[0092]

[0093] It can be seen from the performance test results in Tables 7 and 8 that the -40°C longitudinal impact performance and Brinell hardness of the 80mm NM550 steel plate at 1 / 2 of the thickness are relatively low, and there are differences in the performance between the surface and the core.

[0094] Although the composition of Comparative Example 2 is controlled according to the present invention, it is still produced according to the existing rolling process. It can be seen from the microstructure diagram of the steel plate and the photo of the cut end after flame cutting that the surface structure of the steel plate is martensite, and there is a certain amount of bainite in addition to the martensite structure at 1 / 2 of the thickness. There is a certain difference between the core and surface structures. After flame cutting, delayed cracks appear in the 80mm steel plate.

[0095] Comparative Example 3

[0096] A method for manufacturing a thick rare earth NM550 steel plate with a thickness of 80 mm. The chemical composition of the steel plate and its mass percentage are: C: 0.31%, Si: 0.24%, Mn: 1.4%, P: 0.008%, S: 0.003%, Alt: 0.033%, Cr: 0.88%, Nb: 0.025%, V: 0.009%, Ti: 0.016%, Mo: 0.34%, Ni: 0.44%, B: 0.0025%, and the rest are Fe and residual elements.

[0097] The comparative example NM550 steel plate production method includes heating, rolling, post-rolling cooling and heat treatment processes, and the specific process steps are as follows:

[0098] Heating process: the steel billet containing the above-mentioned smelting components is transported to the raw material area of ​​the steel rolling mill. The end face size of the steel billet is 300mm. Before entering the furnace, Ni-containing steel anti-oxidation coating is evenly applied on the upper and lower surfaces of the billet. After the coating is completed, it is put into the heating furnace. The temperature of the steel billet entering the furnace is 55°C, the heating temperature of the steel billet preheating section is 500-900°C, the heating time is 210 minutes, the temperature of the first heating section is 900-1150°C, the heating time is 160 minutes, the temperature of the second heating section is 1200°C-1260°C, the heating time is 95 minutes, the temperature of the soaking section is 1200-1240°C, the heating time is 70 minutes, and the total heating time is 535 minutes.

[0099] Rolling process: The first stage rolling start temperature is 1062℃. The billet is cooled by cooling device before the first and third rolling passes. The cooling water volume is 1500L / min, the cooling rate is 7℃ / s, and the roller speed is set to 1m / s during cooling, so that the temperature difference between the surface and the core of the steel plate reaches 150-200℃ after cooling. After cooling, normal rolling is carried out. The first four rolling reductions are 29mm, 27mm, 23mm and 20mm respectively, and the reduction rates are 9.6%, 9.9%, 9.4% and 9.0% respectively. The final rolling temperature is 956℃. The second stage rolling start temperature is 878℃, the final rolling temperature is 845℃, the cumulative reduction rate is 30%, and the single-pass reduction is less than 20mm.

[0100] Cooling process after rolling: After the steel plate is rolled, the water temperature is 833℃. The ACC nozzles 1-3, 6-8, 11-13, 16-18, 20-21 are opened at intervals, a total of 14 groups. The water volume is 550m3 for the 1st to 3rd group. 3 / h, 470m for 6-8 groups 3 / h, 260m for groups 11-13, 16-18 3 / h, 170m for group 20-21 3 / h, cooling roller speed 1.0m / s, red-return temperature after cooling 310℃, after cooling is completed, it is cooled offline at 212℃ for 36 hours.

[0101] Heat treatment process: quenching + low-temperature tempering, steel plate thickness 80mm, quenching and holding temperature: 870℃±10℃, storage time 240 minutes, low-temperature tempering process: tempering temperature: 200℃±20℃, storage time: 270 minutes.

[0102] The microstructure of the steel plate in this comparative example is shown in Figure 13-14 ,The mechanical properties of the steel plate are shown in Table 9, and the hardness properties are shown in Table 10.

[0103] Table 9 Mechanical properties of 80mm NM550 steel plate in comparative example 3

[0104]

[0105] Table 10 Hardness properties of 80mm NM550 steel plate in comparative example 3

[0106]

[0107] It can be seen from the performance test results in Tables 9 and 10 that the -40°C longitudinal impact performance and Brinell hardness of the 80mm NM550 steel plate at 1 / 2 of the thickness are relatively low, and there are differences in the performance between the surface and the core.

[0108] The composition and heating process of Comparative Example 3 do not meet the requirements of the present invention. It can be seen from the microstructure diagram of the steel plate and the photo of the cut end after flame cutting that the surface structure of the steel plate is martensite. In addition to the martensite structure, there is a small amount of bainite and segregation band at 1 / 2 thickness. There is a certain difference between the core and surface structures. After flame cutting, delayed cracks appear in the 80mm steel plate.

Claims

1. A thick rare earth NM550 steel plate, characterized in that: The thick rare earth NM550 steel plate includes the following components in percentage by mass: C: 0.26~0.35%, Si: 0.1~0.3%, Mn: 0.75~1.0%, P≤0.015%, S≤0.005%, Alt: 0.015~0.04%, Cr: 0.65~0.9 %, Nb: 0.01~0.03%, V: 0.005~0.01%, Ti: 0.01~0.02%, Mo: 0.1~0.2%, Ni: 0.1~0.2%, B: 0.001~0.003%, RE: 0.0015~0.004%, N: 0.003~0.005%, the rest is Fe and residual elements; The thickness of the thick rare earth NM550 steel plate is 60 to 80 mm; The surface and the core 1 / 2 of the thick rare earth NM550 steel plate are all martensite; The thick rare earth NM550 steel plate has a yield strength of ≥1450Mpa, a tensile strength of ≥1700Mpa, an elongation of ≥21%, a surface Brinell hardness of ≥530HB, and a longitudinal impact energy of ≥65J at -40°C; a yield strength of ≥1400Mpa, a tensile strength of ≥1650Mpa, an elongation of ≥21%, a Brinell hardness of ≥500HB, and a longitudinal impact energy of ≥65J at -40°C at 1 / 2 of the thickness; The method for manufacturing the thick rare earth NM550 steel plate comprises the following steps: heating, rolling, cooling after rolling and heat treatment; The heat treatment process adopts quenching and low-temperature tempering; the quenching: holding temperature: 860-880°C, storage time: (3×H) min, the low-temperature tempering: tempering temperature: 180-220°C, storage time: (1.5×H+150) minutes.

2. A method for manufacturing the thick rare earth NM550 steel plate according to claim 1, characterized in that: The manufacturing method comprises the following steps: heating, rolling, cooling after rolling and heat treatment.

3. The manufacturing method according to claim 2, characterized in that: The heating process adopts a hot delivery and hot charging segmented isothermal heating process, the billet hot delivery temperature is 700-800°C, the hot charging temperature is 500-650°C, the billet preheating section heating temperature is 500-900°C, the heating time is 120-140 minutes, the first heating section temperature is 900-1150°C, the heating time is 120-140 minutes, the second heating section temperature is 1200-1260°C, the heating time is 60-80 minutes, the soaking section temperature is 1200-1240°C, and the heating time is 30-50 minutes.

4. The manufacturing method according to claim 2, characterized in that: The rolling includes differential temperature rolling in the recrystallization zone, specifically: the starting rolling temperature is ≥1050°C, the final rolling temperature is >950°C, the surface of the steel plate is cooled by a front-of-machine cooling device in the first and third passes, the cooling water volume is 1300-1500L / min, the cooling rate is 6-7°C / s, so that the temperature difference between the surface and the core of the steel plate after cooling reaches 150-200°C, the reduction rate of each of the first to fourth rolling passes after cooling is 9%-10%, and the final rolling temperature is >950°C.

5. The manufacturing method according to claim 2 or 4, characterized in that: The rolling includes rolling in the non-recrystallization zone, specifically: the starting rolling temperature is ≤880°C, the final rolling temperature is 840-860°C, the cumulative reduction rate is 30%-40%, and the single-pass reduction is less than 20mm.

6. The manufacturing method according to claim 2, characterized in that: The post-rolling cooling: interval cooling is used. After the steel plate is rolled, the water temperature is ≥820°C. The ACC nozzles 1-3, 6-8, 11-13, 16-18, 20-21 are opened at intervals, a total of 14 groups. The water volume is 500-550m3 for the 1st to 3rd group. 3 / h, 440-470m / h for groups 6-8 3 / h, groups 11-13 and 16-18 were 240-260m / h 3 / h, 150-170m for groups 20-21 3 / h, cooling roller speed 1.0-1.2m / s, red-return temperature after cooling 300-350℃, after cooling is completed, it is offline at 200-300℃ for 24-48 hours.

7. The manufacturing method according to claim 2, characterized in that: The heat treatment process adopts quenching and low-temperature tempering; the quenching: holding temperature: 860-880°C, storage time: (3×H) min, the low-temperature tempering: tempering temperature: 180-220°C, storage time: (1.5×H+150) minutes.

Images