A ni-cr-mo steel and a method for producing the same

Abstract

This invention discloses a NiCrMo steel and its preparation method, belonging to the technical field of marine engineering steel. It solves the problems of poor uniformity and poor strength-toughness matching in existing thick-gauge marine engineering steels. The preparation method includes: rough rolling, finish rolling, warm rolling, and direct online quenching after warm rolling. The warm rolling is performed within the Ar1-Ms range. The steel plate prepared by the method of this invention exhibits good uniformity and high strength and toughness.

Description

Technical Field

[0001] This invention relates to the field of steel technology for marine engineering, and in particular to a NiCrMo steel and its preparation method. Background Technology

[0002] In recent years, with my country's increasing emphasis on marine resources, shipbuilding, oil and gas extraction platforms, and offshore wind power platforms all require high-performance marine welding engineering steel. Due to the harsh working environment at sea, marine engineering steel is increasingly developing towards higher strength, higher toughness, higher corrosion resistance, easier welding, and extra-thickness specifications. As marine engineering equipment continues to grow in size, the thickness of steel plates is constantly increasing. This increased thickness leads to uneven cooling during production, resulting in uneven microstructure and significant performance fluctuations. Moreover, the larger the steel plate thickness, the more pronounced this problem becomes.

[0003] Therefore, improving the uniformity and strength of thick-gauge marine engineering steel has become an urgent problem to be solved. Summary of the Invention

[0004] In view of the above, the present invention aims to provide a NiCrMo steel and its preparation method to solve one of the following technical problems: the existing thick-gauge marine engineering steel has poor uniformity and poor strength and toughness matching.

[0005] The objective of this invention is mainly achieved through the following technical solutions:

[0006] This invention provides a method for preparing NiCrMo steel, which includes rough rolling, finish rolling, warm rolling, and direct online quenching after warm rolling, wherein the warm rolling is performed in the Ar1-Ms range.

[0007] Furthermore, the composition of NiCrMo steel, by mass percentage, includes: Ni: 5.0%–10.0%, Cr: 0.50%–0.70%, and Mo: 0.40%–0.70%.

[0008] Furthermore, the initial rolling temperature for warm rolling is Ar1-(10~20)℃.

[0009] Furthermore, the final rolling temperature of the warm rolling is Ar1-(30~50)℃.

[0010] Furthermore, the deformation during warm rolling is 40% to 80%.

[0011] Furthermore, the warm rolling process includes: after finishing rolling, the intermediate billet is subjected to high-pressure water cooling to rapidly cool to near the Ar1 temperature, followed by air cooling for 10 to 30 seconds before warm rolling begins.

[0012] Furthermore, the step of online direct quenching after warm rolling includes: immediately water cooling to room temperature after warm rolling.

[0013] Furthermore, the preparation method also includes critical quenching. The critical quenching steps are as follows: after online direct quenching, the steel plate is reheated to around Ac3+(5~10)℃, and quenched after holding for a time t1 of (0.8~1.2)d1, where d1 is the thickness of the steel plate, the unit of d1 is mm, and the unit of t1 is min.

[0014] Furthermore, the preparation method also includes high-temperature tempering. The steps of high-temperature tempering are as follows: the critically quenched steel plate is heated to 600-650℃, the holding time t2 is (1.0-1.5)d2, and then air-cooled. Here, d2 is the thickness of the steel plate, the unit of d2 is mm, and the unit of t2 is min.

[0015] The present invention also provides a NiCrMo steel, which is prepared by the above-described preparation method.

[0016] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0017] In the preparation method of NiCrMo steel of the present invention, the mechanical properties of the steel plate are improved by means of rough rolling, fine rolling, warm rolling and online direct quenching after warm rolling, and the warm rolling is controlled in the Ar1-Ms range.

[0018] In the NiCrMo steel preparation method of this invention, by controlling the cooling method of the intermediate billet before warm rolling, the temperature during warm rolling, the deformation amount, the cooling rate after rolling, and the critical quenching temperature, it is possible to avoid temperature inhomogeneity between the surface and the core, which can easily lead to the formation of a gradient material with a larger core grain size and a smaller surface grain size during rolling. Precise control of the deformation amount can introduce a higher density of dislocations into the metastable austenite structure, providing sufficient nucleation sites for martensite crystallization during subsequent quenching, thereby refining the martensite grains. Simultaneously, dislocations in the metastable austenite can be inherited by the transformed martensite, increasing the dislocation density in the transformed martensite structure. In other words, a suitable deformation amount can sufficiently refine the original austenite grain size, making the transformed martensite structure finer. This improves the mechanical properties of the steel through grain refinement strengthening and dislocation strengthening. The preparation method of this invention can be used to prepare thick-gauge NiCrMo steel and has wide applicability.

[0019] The NiCrMo steel of the present invention exhibits good uniformity and high strength and toughness. For example, the tensile strength R of the 9NiCrMo steel prepared by the method of the present invention is high. m The yield strength R is above 1350 MPa. elThe tensile strength R of the 10CrNi7Mo steel prepared by the method of this invention is above 1270 MPa, the elongation A is above 16%, the impact energy at -84℃ is above 200 J, and the impact performance fluctuation is below 5 J. This represents a strength increase of over 200 MPa compared to steel plates prepared by traditional methods (which include rough rolling, finish rolling followed by air cooling to room temperature, quenching, and high-temperature tempering). m For a strength of 1190 MPa or higher (e.g., 1196–1260 MPa), the yield strength R el The tensile strength R of the 10CrNi5MoV steel prepared by the method of this invention is 1150 MPa or higher (e.g., 1150–1200 MPa), the elongation A is 16% or higher (e.g., 16.5%–17.0%), the impact energy at -84℃ is 230 J or higher (e.g., 236–250 J), and the impact performance fluctuation is less than 5 J (e.g., 2–4 J), resulting in a strength increase of more than 200 MPa compared to steel plates prepared by conventional methods. m For a strength of 1100 MPa or higher (e.g., 1108–1200 MPa), the yield strength R el The strength of the steel plate is 1050 MPa or higher (e.g., 1050-1150 MPa), the elongation A is 17% or higher (e.g., 17.0%-17.5%), the impact energy at -84℃ is 200 J or higher (e.g., 204-300 J), the impact performance fluctuation is less than 5 J (e.g., 1-5 J), and the strength is increased by more than 200 MPa compared with the steel plate prepared by the traditional method.

[0020] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of what is particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0021] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0022] Figure 1 Here is a SEM image of the microstructure of Example 1;

[0023] Figure 2 This is a TEM image of the microstructure of Example 1;

[0024] Figure 3 Here is a SEM image of the microstructure of Example 2;

[0025] Figure 4 TEM image of the microstructure of Example 2;

[0026] Figure 5 TEM image of the microstructure of Example 3;

[0027] Figure 6 Here is a SEM image of the microstructure of Example 4;

[0028] Figure 7 Here is a SEM image of the microstructure of Example 5;

[0029] Figure 8 This is a microstructure diagram of Comparative Example 1;

[0030] Figure 9 This is a microstructure diagram of Comparative Example 2;

[0031] Figure 10 This is a microstructure diagram of Comparative Example 3. Detailed Implementation

[0032] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of the present invention and, together with the embodiments of the present invention, serve to illustrate the principles of the present invention.

[0033] This invention provides a method for preparing NiCrMo steel, which includes: rough rolling, finish rolling, warm rolling, and direct online quenching after warm rolling, wherein the warm rolling is performed in the Ar1-Ms range.

[0034] Specifically, the Ar1 temperature mentioned above is the starting temperature of the transformation from austenite to pearlite during cooling. The Ms temperature is the starting temperature of the transformation from austenite to martensite.

[0035] Specifically, the composition of the above-mentioned NiCrMo steel, by mass percentage, includes: Ni: 5.0% to 10.0%, Cr: 0.50% to 0.70%, and Mo: 0.40% to 0.70%.

[0036] Specifically, the NiCrMo steel has a high Ni content, resulting in a larger metastable austenite phase region during cooling. This ensures that ferrite and bainite phase transformations will not occur due to deformation during rolling.

[0037] Specifically, if the starting rolling temperature of the above-mentioned warm rolling is too high, the recovery of deformed grains and the annihilation of dislocations inside the grains during the deformation process will not be conducive to the formation of subsequent ultrafine martensite structure. Therefore, the starting rolling temperature of warm rolling should be controlled at Ar1-(10~20)℃.

[0038] Specifically, while a low final rolling temperature is beneficial for the transmission of rolling to the center and for sufficient grain deformation, it can also exceed the processing capacity of the equipment and damage it. Therefore, the final rolling temperature of warm rolling should be controlled at Ar1-(30~50)℃.

[0039] Specifically, if the deformation during warm rolling is too small, the metastable austenite cannot be sufficiently deformed, failing to achieve the goal of grain refinement; if the deformation is too large, the temperature drop during rolling will be too large, increasing the hardness of the steel plate and making rolling more difficult. Therefore, the deformation during warm rolling should be controlled at 40%–80%. A deformation of 40%–80% can introduce a higher density of dislocations into the metastable austenite structure, providing sufficient nucleation sites for martensite crystallization during subsequent quenching, thereby refining the martensite grains. At the same time, dislocations in the metastable austenite can be inherited by the transformed martensite, increasing the dislocation density in the transformed martensite structure. In other words, a suitable deformation can sufficiently refine the original austenite grain size, making the transformed martensite structure finer, thus improving the mechanical properties of the steel through grain refinement strengthening and dislocation strengthening.

[0040] Specifically, the above-mentioned warm rolling steps include: after finishing rolling, the intermediate billet is subjected to 1 to 3 passes of high-pressure water cooling in the finishing mill to cool to near the Ar1 temperature, and then air-cooled for 10 to 30 seconds before warm rolling.

[0041] Specifically, the above preparation method controls the cooling method of the intermediate billet before warm rolling, the temperature during warm rolling, the deformation, and the cooling rate after rolling. This is because after high-pressure water cooling in the finishing mill before warm rolling, the surface temperature of the intermediate billet decreases rapidly, but the core temperature decreases more slowly. Cooling to around Ar1 temperature and then air-cooling for a period of time is to transfer heat from the core to the surface through thermal conduction, making the core and surface temperatures of the intermediate billet consistent. This avoids uneven temperature between the surface and the core, which can easily lead to a gradient material with larger core grains and smaller surface grains during rolling. The warm rolling temperature is set between Ar1 and Ms temperatures because if the rolling temperature is too high, the recovery of deformed grains and the annihilation of dislocations within the grains during deformation are not conducive to the formation of subsequent ultrafine martensite structures. If the final rolling temperature is too low, although it is beneficial for the conduction of rolling to the core and the full deformation of the grains, it will exceed the processing capacity of the equipment and damage it.

[0042] Specifically, the steps for online direct quenching after warm rolling include: immediately water cooling to room temperature after warm rolling to ensure that the steel plate microstructure is martensitic. Online direct quenching can ensure that austenite is less likely to form ferrite, pearlite, and bainite microstructures during cooling, can retain most of the dislocations generated during the warm rolling deformation process, and can increase the dislocation density by generating new dislocations through martensitic phase transformation.

[0043] Specifically, the above preparation method also includes critical quenching. The specific steps of critical quenching are as follows: After online direct quenching, the steel plate is reheated to around Ac3 + (5~10)℃, held for a time t1 of (0.8~1.2)d1, and then quenched. Here, Ac3 is the temperature at which ferrite begins to transform into austenite during heating, d1 is the thickness of the steel plate (in mm), and t1 is in minutes.

[0044] It should be noted that steel plates subjected to direct online quenching after warm rolling exhibit strong deformation texture and significant anisotropy in their properties. Therefore, it is necessary to eliminate anisotropy through re-austenitization to ensure the transverse mechanical properties of the steel plate. The critical quenching heating temperature is determined to be Ac3 + (5~10)℃, which is lower than the general quenching temperature, and the holding time is also shorter. This is because the martensitic phase transformation that occurs during direct online quenching is a shear-type phase transformation, and the elements in the steel do not diffuse and segregate. Therefore, high temperature is not required to ensure the homogenization of the elements in the steel, and at the same time, it can avoid the growth of austenite grains and refine the grains.

[0045] Specifically, the above preparation method also includes high-temperature tempering. The specific steps of high-temperature tempering are as follows: the critically quenched steel plate is heated to 600-650℃, and the holding time t2 is (1.0-1.5)d2. After holding, it is air-cooled. Wherein, d2 is the thickness of the steel plate, the unit of d2 is mm, and the unit of t2 is min.

[0046] Specifically, in the above preparation method, the billet heating is included before rough rolling. The specific steps of billet heating include: heating an electroslag billet with a thickness of 300-400 mm, with the first heating temperature in the heating furnace being 1070±50℃, the second heating temperature being 1150±50℃, the third heating temperature being 1220±20℃, the soaking temperature being 1200±20℃, and the total heating time t being (8-10) d, where t is in min and d is the thickness of the electroslag billet in cm.

[0047] Specifically, in the above preparation method, if the initial rolling temperature of the roughing mill is too high, the grains refined by dynamic recrystallization will grow significantly; if it is too low, the rolling force of the roughing mill will increase. Therefore, the initial rolling temperature of the roughing mill should be controlled to be ≥1070℃, and the final rolling temperature should be ≥1000℃. For example, the initial rolling temperature of the roughing mill is 1070~1130℃, and the final rolling temperature is 1000~1050℃.

[0048] Specifically, in the above preparation method, if the initial rolling temperature of the finishing mill is too high, the grain size of austenite recrystallization will be too large; if it is too low, it will easily enter the two-phase region of austenite and ferrite, forming mixed crystals. Therefore, the initial rolling temperature of the finishing mill should be controlled to ≤980℃, and the final rolling temperature should be 830~880℃. For example, the initial rolling temperature of the finishing mill is 930~980℃.

[0049] Specifically, the above preparation method can be used to prepare thick NiCrMo steel plates. For example, the thickness of the electroslag billet can be 300-500 mm, the thickness of the intermediate billet can be 150-250 mm, and the thickness of the final steel plate can be 40 mm or more, for example, 40-100 mm.

[0050] Specifically, the aforementioned NiCrMo steel has a large supercooled austenite phase region during cooling. By adopting three-stage rolling and critical quenching, the mechanical properties of the steel plate are improved through fine grain strengthening and dislocation strengthening.

[0051] Specifically, this invention also provides a NiCrMo steel prepared using the above-described method. The microstructure of the NiCrMo steel of this invention consists of ultrafine tempered martensite and second-phase precipitates, with the martensite laths of the ultrafine tempered martensite having a width on the order of nm, for example, below 600 nm. The steel of this invention exhibits a uniform microstructure along its thickness direction.

[0052] Specifically, the NiCrMo steel of the present invention has good uniformity and high strength and toughness. For example, the tensile strength R of the 9NiCrMo steel prepared by the method of the present invention is high. m The yield strength R is above 1350 MPa. el The tensile strength R of the 10CrNi7Mo steel prepared by the method of this invention is above 1270 MPa, the elongation A is above 16%, the impact energy at -84℃ is above 200 J, and the impact performance fluctuation is below 5 J, which is more than 200 MPa higher than that of steel plates prepared by traditional methods. m For a strength of 1190 MPa or higher (e.g., 1196–1260 MPa), the yield strength R el The tensile strength R of the 10CrNi5MoV steel prepared by the method of this invention is 1150 MPa or higher (e.g., 1150–1200 MPa), the elongation A is 16% or higher (e.g., 16.5%–17.0%), the impact energy at -84℃ is 230 J or higher (e.g., 236–250 J), and the impact performance fluctuation is less than 5 J (e.g., 2–4 J), resulting in a strength increase of more than 200 MPa compared to steel plates prepared by conventional methods. m For a strength of 1100 MPa or higher (e.g., 1108–1200 MPa), the yield strength R el The strength of the steel plate is 1050 MPa or higher (e.g., 1050-1150 MPa), the elongation A is 17% or higher (e.g., 17.0%-17.5%), the impact energy at -84℃ is 200 J or higher (e.g., 204-300 J), the impact performance fluctuation is less than 5 J (e.g., 1-5 J), and the strength is increased by more than 200 MPa compared with the steel plate prepared by the traditional method.

[0053] In the preparation method of NiCrMo steel of the present invention, the mechanical properties of the steel plate are improved by adopting three-stage rolling and critical quenching, and by means of fine grain strengthening and dislocation strengthening.

[0054] In the NiCrMo steel preparation method of this invention, by controlling the cooling method of the intermediate billet before warm rolling, the temperature during warm rolling, the deformation amount, and the cooling rate after rolling, it is possible to avoid temperature inhomogeneity between the surface and the core, which can easily lead to the formation of a gradient material with a larger core grain size and a smaller surface grain size during rolling. Precise control of the deformation amount can introduce a higher density of dislocations into the metastable austenite structure, providing sufficient nucleation sites for martensite crystallization during subsequent quenching, thereby refining the martensite grains. Simultaneously, dislocations in the metastable austenite can be inherited by the transformed martensite, increasing the dislocation density in the transformed martensite structure. In other words, a suitable deformation amount can sufficiently refine the original austenite grain size, making the transformed martensite structure finer, thus improving the mechanical properties of the steel through grain refinement strengthening and dislocation strengthening.

[0055] The NiCrMo steel of the present invention exhibits good uniformity and high strength and toughness. For example, the tensile strength R of the 9NiCrMo steel prepared by the method of the present invention is high. m The yield strength R is above 1350 MPa. el The tensile strength R of the 10CrNi7Mo steel prepared by the method of this invention is above 1270 MPa, the elongation A is above 16%, the impact energy at -84℃ is above 200 J, and the impact performance fluctuation is below 5 J. This represents a strength increase of over 200 MPa compared to steel plates prepared by traditional methods (which include rough rolling, finish rolling followed by air cooling to room temperature, quenching, and high-temperature tempering). m For a strength of 1190 MPa or higher (e.g., 1196–1260 MPa), the yield strength R el The tensile strength R of the 10CrNi5MoV steel prepared by the method of this invention is 1150 MPa or higher (e.g., 1150–1200 MPa), the elongation A is 16% or higher (e.g., 16.5%–17.0%), the impact energy at -84℃ is 230 J or higher (e.g., 236–250 J), and the impact performance fluctuation is less than 5 J (e.g., 2–4 J), resulting in a strength increase of more than 200 MPa compared to steel plates prepared by conventional methods. m For a strength of 1100 MPa or higher (e.g., 1108–1200 MPa), the yield strength R el The strength of the steel plate is 1050 MPa or higher (e.g., 1050-1150 MPa), the elongation A is 17% or higher (e.g., 17.0%-17.5%), the impact energy at -84℃ is 200 J or higher (e.g., 204-300 J), the impact performance fluctuation is less than 5 J (e.g., 1-5 J), and the strength is increased by more than 200 MPa compared with the steel plate prepared by the traditional method.

[0056] The advantages of the preparation method of the present invention will be demonstrated below with specific embodiments and comparative examples.

[0057] Example 1

[0058] This embodiment provides a NiCrMo steel and its preparation method.

[0059] In this embodiment, the steel used is 9NiCrMo steel, with the following chemical composition: 0.08% C, 0.48% Mn, 0.25% Si, 8.8% Ni, 0.60% Cr, 0.50% Mo, P≤0.010%, S<0.005%, and the balance being Fe. Its Ar1 and Ac3 temperatures are 610℃ and 780℃, respectively.

[0060] The preparation method of this embodiment includes: before the steel plate is rolled, the electroslag billet is 400mm thick. After heating, rough rolling, fine rolling, warm rolling, critical quenching and tempering, a steel plate with a thickness of 90mm is produced. The specific process is shown in Table 1-2, and the mechanical properties of the steel plate are shown in Table 3.

[0061] The microstructure of the steel prepared in this embodiment is as follows: Figure 1 and Figure 2 As shown, the microstructure consists of ultrafine tempered martensite and nanoscale second-phase precipitates. The martensite laths of the ultrafine tempered martensite have a width in the nm range, for example, below 600 nm.

[0062] Example 2

[0063] This embodiment provides a NiCrMo steel and its preparation method.

[0064] In this embodiment, the steel used is 10CrNi7Mo steel, with the following chemical composition: 0.08% C, 0.65% Mn, 0.18% Si, 6.75% Ni, 0.65% Cr, 0.52% Mo, P≤0.010%, S<0.005%, and the balance being Fe. Its Ar1 and Ac3 temperatures are 630℃ and 790℃, respectively.

[0065] The preparation method of this embodiment includes: before the steel plate is rolled, the electroslag billet is 350mm thick. After heating, rough rolling, fine rolling, warm rolling, critical quenching and tempering treatment, a steel plate with a thickness of 60mm is produced. The specific process is shown in Table 1-2, and the mechanical properties of the steel plate are shown in Table 3.

[0066] The microstructure of the steel prepared in this embodiment is as follows: Figure 3 and Figure 4 As shown, the microstructure consists of ultrafine tempered martensite and nanoscale second-phase precipitates. The martensite laths of the ultrafine tempered martensite have a width in the nm range, for example, below 600 nm.

[0067] Example 3

[0068] This embodiment provides a NiCrMo steel and its preparation method.

[0069] In this embodiment, 10CrNi5MoV steel was selected, with the following chemical composition: 0.09% C, 0.60% Mn, 0.30% Si, 5.05% Ni, 0.63% Cr, 0.45% Mo, 0.08% V, P≤0.010%, S<0.005%, and the balance being Fe. Its Ar1 and Ac3 temperatures were 650℃ and 800℃, respectively. (Performance after conventional treatment: R) el 810MPa, R m :860, A:20%,-84℃KV2 / J:198J)

[0070] The preparation method of this embodiment includes: before the steel plate is rolled, the electroslag billet is 300mm thick. After heating, rough rolling, fine rolling, warm rolling, critical quenching and tempering treatment, a steel plate with a thickness of 40mm is produced. The specific process is shown in Table 1-2, and the mechanical properties of the steel plate are shown in Table 3.

[0071] The microstructure of the steel prepared in this embodiment is as follows: Figure 5 As shown, the microstructure consists of ultrafine tempered martensite and nanoscale second-phase precipitates. The martensite laths of the ultrafine tempered martensite have a width in the nm range, for example, below 600 nm.

[0072] Example 4

[0073] This embodiment provides a NiCrMo steel and its preparation method.

[0074] The steel used in this embodiment has the same chemical composition as in Example 2. Before rolling, the electroslag billet is 400mm thick. After heating, rough rolling, finish rolling, warm rolling, critical quenching, and tempering, it is produced into a steel plate with a thickness of 80mm. The specific process is shown in Table 1-2, and the mechanical properties of the steel plate are shown in Table 3.

[0075] The microstructure of the steel prepared in this embodiment is as follows: Figure 6 As shown, the microstructure consists of ultrafine tempered martensite and nanoscale second-phase precipitates. The martensite laths of the ultrafine tempered martensite have a width in the nm range, for example, below 600 nm.

[0076] Example 5

[0077] This embodiment provides a NiCrMo steel and its preparation method.

[0078] The steel used in this embodiment has the same chemical composition as in Example 3. Before rolling, the electroslag billet is 400mm thick. After heating, rough rolling, finish rolling, warm rolling, critical quenching, and tempering, it is produced into a steel plate with a thickness of 100mm. The specific process is shown in Table 1-2, and the mechanical properties of the steel plate are shown in Table 3.

[0079] The microstructure of the steel prepared in this embodiment is as follows: Figure 7As shown, the microstructure consists of ultrafine tempered martensite and nanoscale second-phase precipitates. The martensite laths of the ultrafine tempered martensite have a width in the nm range, for example, below 600 nm.

[0080] The inventors conducted extensive experimental research during the research process, and some poorly performing solutions are now presented as comparative examples.

[0081] Comparative Example 1

[0082] The composition of the steel in this comparative example is the same as that in Example 1, and will not be repeated here. The preparation method adopts a conventional method, including the following steps:

[0083] The slab with a thickness of 400 mm was heated, rough rolled, and finished rolled using the same processes as in Example 1. After being rolled into a steel plate with a thickness of 90 mm, it was air-cooled to room temperature, then heated to 840°C and held for 120 min before quenching. It was then tempered at 600°C. The microstructure consisted of tempered martensite (lamellae size in the micrometer range) and second-phase precipitates, as detailed in [link to details]. Figure 8 The performance is shown in Table 3.

[0084] Comparative Example 2

[0085] The composition of the steel in this comparative example is the same as that in Example 2, and will not be repeated here. The preparation method adopts a conventional method, including the following steps:

[0086] The slab with a thickness of 350 mm was heated, rough rolled, and finished rolled using the same process as in Example 2. After being rolled into a steel plate with a thickness of 60 mm, it was air-cooled to room temperature, then heated to 860°C and held for 90 min before quenching. It was then tempered at 620°C. The microstructure consisted of tempered martensite (lamellae size in the micrometer range) and second-phase precipitates, as detailed in [link to details]. Figure 9 The performance is shown in Table 3.

[0087] Comparative Example 3

[0088] The composition of the steel in this comparative example is the same as that in Example 3, and will not be repeated here. The preparation method includes the following steps:

[0089] The slab with a thickness of 400 mm followed the same overall process steps as in Example 5, but the warm rolling start temperature was 660℃ (i.e., Ar1 + 10℃). Due to the relatively high warm rolling start temperature, the core deformation of the steel plate was less complete, resulting in greater performance, especially impact fluctuations. Specific properties are shown in Table 3. The microstructure is as follows: Figure 10 As shown, its microstructure consists of tempered martensite (lamellae size in the micrometer range) and second-phase precipitates.

[0090] Comparative Example 4

[0091] The composition of the steel in this comparative example is the same as that in Example 3, and will not be repeated here. The overall steps of its preparation method are the same as those in Example 4. The difference is that the warm rolling deformation is only 20%. Due to the insufficient deformation, the metastable austenite is not sufficiently refined, so the final microstructure is tempered martensite, and the properties are shown in Table 3.

[0092] Table 1. Examples of steel plate roughing and finishing processes

[0093]

[0094]

[0095] Table 2 Examples of Steel Plate Warm Rolling and Quenching / Tempering Processes

[0096]

[0097] Table 3 Mechanical properties of steel plates from examples and comparative examples

[0098]

[0099]

[0100] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for the production of a NiCrMo steel, characterized in that, The preparation method comprises rough rolling, finish rolling, warm rolling and online direct quenching after warm rolling, wherein the warm rolling is performed in the Ar1-Ms interval; The NiCrMo steel comprises, in percentage by mass, Ni: 5.0%-10.0%, Cr: 0.50%-0.70%, and Mo: 0.40%-0.70%. The warm rolling comprises high-pressure water cooling of the intermediate blank obtained after finish rolling, rapid cooling to the Ar1 temperature, and air cooling for 10-30 seconds before starting the warm rolling. The rough rolling temperature is controlled to be 1070-1130 DEG C, and the rough rolling final rolling temperature is 1000-1050 DEG C. The finish rolling temperature is controlled to be 930-980 DEG C, and the finish rolling final rolling temperature is 830-880 DEG C. The warm rolling temperature is controlled to be Ar1-(10-20) DEG C, and the warm rolling final rolling temperature is Ar1-(30-50) DEG C. The online direct quenching after warm rolling comprises water cooling to room temperature immediately after warm rolling. The preparation method further comprises critical quenching, and the critical quenching comprises the following steps: after online direct quenching, reheating the steel plate to Ac3+(5-10) DEG C, holding, and then quenching; the holding time t1 is (0.8-1.2)d1, wherein d1 is the thickness of the steel plate, and the unit of d1 is mm, and the unit of t1 is min. The thickness of the obtained steel plate is greater than or equal to 40 mm; the microstructure of the NiCrMo steel is ultra-fine tempered martensite and second-phase precipitates, the width of the martensite lath of the ultra-fine tempered martensite is less than or equal to 600 nm, and the microstructure is uniform in the thickness direction; the NiCrMo steel has good uniformity and high strength and toughness.

2. The production method according to claim 1, characterized by, The NiCrMo steel comprises, in percentage by mass, Ni: 5.0%-8.8%, Cr: 0.50%-0.70%, and Mo: 0.40%-0.70%.

3. The method of claim 1, wherein, The preparation method further comprises high-temperature tempering, and the high-temperature tempering comprises the following steps: heating the steel plate after critical quenching to 600-650 DEG C, holding, and then air cooling.

4. A NiCrMo steel, characterized in that, The NiCrMo steel is prepared by the preparation method in any one of claims 1-3.

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