NiCrMo steel and preparation method thereof

Abstract

The invention discloses NiCrMo steel and a preparation method thereof, belongs to the technical field of steel for ocean engineering, and solves the problems of poor uniformity and poor strength and toughness matching of thick steel for ocean engineering in the prior art. The preparation method comprises the steps of rough rolling, finish rolling, warm rolling and on-line direct quenching after warm rolling, and warm rolling is conducted in the Ar1-Ms interval. The steel plate prepared through the method is good in uniformity and high in obdurability.

Description

Technical Field

[0001] The invention relates to the technical field of marine engineering steel, and in particular to NiCrMo steel and a preparation method thereof. Background Art

[0002] In recent years, as my country attaches great importance to marine resources, shipbuilding, oil and gas production platforms, offshore wind power generation platforms, etc. all need excellent performance marine welding engineering steel. Because it serves in the marine environment and the working environment is relatively harsh, marine engineering steel is gradually developing in the direction of high strength, high toughness, high corrosion resistance, easy welding, and extra thick specifications. As marine engineering equipment continues to develop in the direction of large-scale, the thickness of steel plates continues to increase. Due to the increase in thickness, the steel plates will produce uneven organization due to uneven cooling during the production process, resulting in large performance fluctuations. Moreover, the larger the thickness of the steel plate, the more prominent the problem.

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

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

[0005] The purpose of the present invention is mainly achieved through the following technical solutions:

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

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

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

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

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

[0011] Furthermore, the warm rolling step includes: cooling the intermediate billet obtained after the finish rolling with high pressure water, rapidly cooling it to near the Ar1 temperature, air cooling it for 10 to 30 seconds and then starting the warm rolling.

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

[0013] Furthermore, the preparation method also includes critical quenching, and the steps of critical quenching are as follows: after online direct quenching, the steel plate is reheated to about Ac3+(5~10)℃, and quenched after the holding time t1 is (0.8~1.2)d1, wherein 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, and the steps of high-temperature tempering are as follows: heating the steel plate after critical quenching to 600-650°C, holding time t2 is (1.0-1.5) d2, and air cooling after holding, wherein d2 is the thickness of the steel plate, the unit of d2 is mm, and the unit of t2 is min.

[0015] The invention also provides a NiCrMo steel which is prepared by the above 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, rough rolling, finish rolling, warm rolling and online direct quenching after warm rolling are adopted, and the warm rolling is controlled to be rolled in the Ar1-Ms range, so as to improve the mechanical properties of the steel plate by means of grain refinement strengthening and dislocation strengthening.

[0018] In the preparation method of NiCrMo steel of the present invention, the temperature unevenness between the surface and the core can be avoided 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, and it is easy to form a gradient material with a larger grain size in the core and a smaller grain size on the surface during the rolling process. Accurately controlling the deformation amount can introduce a higher density of dislocations in the metastable austenite structure, provide sufficient nucleation sites for martensite crystallization in the subsequent quenching process, thereby refining the martensite grains, and at the same time, the dislocations in the metastable austenite can be inherited to the martensite after the phase transformation, and the dislocation density in the martensite structure after the phase transformation is increased, that is, the appropriate deformation amount can fully refine the original austenite grain size, make the martensite structure after the phase transformation finer, and improve the mechanical properties of the steel through grain refinement strengthening and dislocation strengthening. The preparation method of the present invention can be used to prepare thick-gauge NiCrMo steel, and has wide applicability.

[0019] The NiCrMo steel of the present invention has good uniformity and high toughness. For example, the tensile strength R of the 9NiCrMo steel prepared by the method of the present invention is m The yield strength R is above 1350MPa. elThe tensile strength R of the 10CrNi7Mo steel prepared by the method of the present invention is 1270MPa or more, the elongation A is 16% or more, the impact energy at -84°C is 200J or more, and the impact performance fluctuation is less than 5J. The strength of the steel plate prepared by the traditional method (the traditional method includes rough rolling, air cooling to room temperature after fine rolling, quenching, and high temperature tempering) is increased by more than 200MPa. m 1190MPa or more (e.g. 1196-1260MPa), yield strength R el The tensile strength R of the 10CrNi5MoV steel prepared by the method of the present invention is 200 MPa or more, and the tensile strength R of the 10CrNi5MoV steel prepared by the method of the present invention is 1150 MPa or more (for example, 1150-1200 MPa), the elongation A is 16% or more (for example, 16.5%-17.0%), the impact energy at -84°C is 230 J or more (for example, 236-250 J), and the impact performance fluctuation is 5 J or less (for example, 2-4 J). m 1100MPa or more (e.g. 1108-1200MPa), yield strength R el It is above 1050MPa (for example, 1050-1150MPa), the elongation A is above 17% (for example, 17.0%-17.5%), the impact energy at -84°C is above 200J (for example, 204-300J), and the impact performance fluctuation is below 5J (for example, 1-5J). The strength of the steel plate is increased by more than 200MPa compared with that prepared by traditional methods.

[0020] Other features and advantages of the present invention will be described in the following description, and part of them will become obvious from the description, or will be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the contents particularly pointed out in the written description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

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

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

[0024] Figure 3 This is the microstructure SEM image 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 This is the SEM image of the microstructure of Example 4;

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

[0029] Figure 8 It is the microstructure diagram of comparative example 1;

[0030] Fig. 9 It is the microstructure diagram of comparative example 2;

[0031] Fig.10 This is the microstructure diagram of Comparative Example 3. DETAILED DESCRIPTION

[0032] Preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used to explain the principles of the present invention together with the embodiments of the present invention.

[0033] The invention provides a method for preparing NiCrMo steel. The 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 range.

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

[0035] Specifically, the components of the NiCrMo steel, in terms of mass percentage, include: Ni: 5.0% to 10.0%, Cr: 0.50% to 0.70%, and Mo: 0.40% to 0.70%.

[0036] Specifically, the Ni content of the NiCrMo steel is relatively high, and the phase region of the metastable austenite is relatively large during cooling, which can ensure that during rolling, no ferrite and bainite phase transformation occurs due to deformation induction.

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

[0038] Specifically, if the final rolling temperature of the warm rolling is too low, it is beneficial to the conduction of rolling to the center and the full deformation of grains, but it will exceed the processing capacity of the equipment and damage the equipment. Therefore, the final rolling temperature of the warm rolling is controlled to be Ar1-(30~50)℃.

[0039] Specifically, if the deformation amount of the above-mentioned warm rolling is too small, the metastable austenite cannot be fully deformed, and the purpose of grain refinement cannot be achieved; if the deformation amount is too large, the temperature drop during rolling is too large, the hardness of the steel plate increases, and the difficulty of rolling increases; therefore, the deformation amount of warm rolling is controlled to be 40% to 80%. The deformation amount of 40% to 80% can introduce a higher density of dislocations in the metastable austenite structure, providing sufficient nucleation sites for martensite crystallization during the subsequent quenching process, thereby refining the martensite grains. At the same time, the dislocations in the metastable austenite can be inherited to the martensite after phase transformation, increasing the dislocation density in the martensite structure after phase transformation, that is, the appropriate deformation amount can fully refine the original austenite grain size, make the martensite structure after phase transformation finer, and improve the mechanical properties of steel through grain refinement strengthening and dislocation strengthening.

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

[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 the high-pressure water cooling of the finishing mill before warm rolling, the surface temperature of the intermediate billet decreases faster, but the core is slower. Cooling to the Ar1 temperature and air cooling for a period of time is to transfer the heat of the core to the surface through heat conduction, so that the temperature of the core and the surface of the intermediate billet is consistent, avoiding uneven temperature between the surface and the core, and easily forming a gradient material with larger grain size in the core and smaller grain size on the surface during rolling. The warm rolling temperature is set between the Ar1-Ms temperature. This is because if the rolling temperature is too high, the recovery of the deformed grains during the deformation process and the annihilation of the dislocations inside the grains are not conducive to the subsequent formation of ultrafine martensitic structure. The final rolling temperature is too low. Although it is conducive to the conduction of rolling to the core and the full deformation of the grains, it will exceed the processing capacity of the equipment itself and damage the equipment if it is too low.

[0042] Specifically, the specific steps of online direct quenching after warm rolling include: water cooling to room temperature immediately after warm rolling to ensure that the steel plate structure is martensite. Online direct quenching can ensure that austenite is not easy to form ferrite, pearlite and bainite structures during cooling, and most of the dislocations in the warm rolling deformation process can be retained, and new dislocations are generated through martensitic phase transformation to increase the dislocation density.

[0043] Specifically, the preparation method further includes critical quenching, and the specific steps of critical quenching are as follows: after online direct quenching, the steel plate is reheated to about Ac3+(5-10)°C, and quenched after the holding time t1 is (0.8-1.2)d1. Wherein, Ac3 is the temperature at which ferrite begins to transform into austenite during heating, d1 is the thickness of the steel plate, the unit of d1 is mm, and the unit of t1 is min.

[0044] It should be noted that the steel plate after warm rolling and online direct quenching will produce a strong deformation texture, and the anisotropy of the steel plate performance is more obvious. Therefore, the anisotropy must be eliminated by re-austenization to ensure the transverse mechanical properties of the steel plate. The heating temperature of critical quenching 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 online direct quenching is a shear phase transformation, and the elements in the steel do not diffuse and segregate, so there is no need for high temperature 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 preparation method further includes high temperature tempering, and the specific steps of high temperature tempering are as follows: heating the critical quenched steel plate to 600-650°C, holding time t2 for (1.0-1.5) d2, and air cooling after holding. 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 rough rolling also includes heating the ingot, and the specific steps of heating the ingot include: heating the electroslag ingot with a thickness of 300 to 400 mm, the first heating temperature in the heating furnace is 1070±50°C, the second heating temperature is 1150±50°C, the third heating temperature is 1220±20°C, the uniform heating temperature is 1200±20°C, and the total heating time t is (8 to 10) d, wherein t is in units of min, d is the thickness of the electroslag ingot, and d is in units of cm.

[0047] Specifically, in the above preparation method, if the starting rolling temperature of rough rolling is too high, the grains refined by dynamic recrystallization will grow significantly, and if it is too low, the rolling force of the rough rolling unit will be increased. Therefore, the starting rolling temperature of rough rolling is controlled to be ≥1070°C, and the final rolling temperature of rough rolling is ≥1000°C. For example, the starting rolling temperature of rough rolling is 1070-1130°C, and the final rolling temperature of rough rolling is 1000-1050°C.

[0048] Specifically, in the above preparation method, if the start rolling temperature of the finishing rolling is too high, the grain size of the austenite undergoing recrystallization is large, and if it is too low, it is easy to enter the two-phase region of austenite and ferrite to form mixed crystals. Therefore, the start rolling temperature of the finishing rolling is controlled to be ≤980°C, and the final rolling temperature is 830-880°C. For example, the start rolling temperature of the finishing rolling is 930-980°C.

[0049] Specifically, the above preparation method can be used to prepare thick-gauge 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 more than 40 mm, for example, 40-100 mm.

[0050] Specifically, the NiCrMo steel has a large undercooled austenite phase region when cooled. By adopting three-stage rolling and critical quenching, the mechanical properties of the steel plate are improved by means of grain refinement strengthening and dislocation strengthening.

[0051] Specifically, the present invention further provides a NiCrMo steel, which is prepared by the above-mentioned preparation method. The structure of the NiCrMo steel of the present invention is ultrafine tempered martensite and second phase precipitates, and the martensite lath width of the ultrafine tempered martensite is in the nm level, for example, less than 600 nm. The structure of the steel of the present invention is uniform in the thickness direction.

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

[0053] In the preparation method of the NiCrMo steel of the present invention, three-stage rolling and critical quenching are adopted to improve the mechanical properties of the steel plate through methods such as grain refinement and dislocation strengthening.

[0054] In the preparation method of NiCrMo steel of the present invention, the temperature unevenness between the surface and the core can be avoided 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, and a gradient material with a larger grain size in the core and a smaller grain size on the surface can be easily formed during the rolling process. Accurately controlling the deformation amount can introduce a higher density of dislocations in the metastable austenite structure, provide sufficient nucleation sites for martensite crystallization in the subsequent quenching process, thereby refining the martensite grains, and at the same time, the dislocations in the metastable austenite can be inherited to the martensite after phase transformation, thereby increasing the dislocation density in the martensite structure after phase transformation, that is, the appropriate deformation amount can fully refine the original austenite grain size, make the martensite structure after phase transformation finer, and improve the mechanical properties of the steel through grain refinement strengthening and dislocation strengthening.

[0055] The NiCrMo steel of the present invention has good uniformity and high toughness. For example, the tensile strength R of the 9NiCrMo steel prepared by the method of the present invention is m The yield strength R is above 1350MPa. el The tensile strength R of the 10CrNi7Mo steel prepared by the method of the present invention is 1270MPa or more, the elongation A is 16% or more, the impact energy at -84°C is 200J or more, and the impact performance fluctuation is less than 5J. The strength of the steel plate prepared by the traditional method (the traditional method includes rough rolling, air cooling to room temperature after fine rolling, quenching, and high temperature tempering) is increased by more than 200MPa. m 1190MPa or more (e.g. 1196-1260MPa), yield strength R el The tensile strength R of the 10CrNi5MoV steel prepared by the method of the present invention is 200 MPa or more, and the tensile strength R of the 10CrNi5MoV steel prepared by the method of the present invention is 1150 MPa or more (for example, 1150-1200 MPa), the elongation A is 16% or more (for example, 16.5%-17.0%), the impact energy at -84°C is 230 J or more (for example, 236-250 J), and the impact performance fluctuation is 5 J or less (for example, 2-4 J). m 1100MPa or more (e.g. 1108-1200MPa), yield strength R el It is above 1050MPa (for example, 1050-1150MPa), the elongation A is above 17% (for example, 17.0%-17.5%), the impact energy at -84°C is above 200J (for example, 204-300J), and the impact performance fluctuation is below 5J (for example, 1-5J). The strength of the steel plate is increased by more than 200MPa compared with that prepared by traditional methods.

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

[0057] Example 1

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

[0059] The steel used in this embodiment is 9NiCrMo steel, whose chemical composition is: 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 is Fe. Its Ar1 and Ac3 temperatures are 610℃ and 780℃.

[0060] The preparation method of this embodiment includes: before the steel plate is rolled, the thickness of the electroslag billet is 400 mm, and after heating, rough rolling, finish rolling, warm rolling, critical quenching and tempering treatment, a steel plate with a thickness of 90 mm 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 is ultrafine tempered martensite and nanometer-scale second phase precipitates, and the width of the martensite lath of the ultrafine tempered martensite is in the nm level, for example, less than 600 nm.

[0062] Example 2

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

[0064] The steel used in this embodiment is 10CrNi7Mo steel, whose chemical composition is: 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 is Fe. Its Ar1 and Ac3 temperatures are 630℃ and 790℃.

[0065] The preparation method of this embodiment includes: before the steel plate is rolled, the thickness of the electroslag billet is 350 mm, and after heating, rough rolling, finish rolling, warm rolling, critical quenching and tempering treatment, a steel plate with a thickness of 60 mm 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 is ultrafine tempered martensite and nanometer-scale second phase precipitates, and the width of the martensite lath of the ultrafine tempered martensite is in the nm level, for example, less than 600 nm.

[0067] Example 3

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

[0069] The steel used in this embodiment is 10CrNi5MoV steel, whose chemical composition is: 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 is Fe. Its Ar1 and Ac3 temperatures are 650℃ and 800℃. (Performance after treatment by traditional method: 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 thickness of the electroslag billet is 300 mm, and after heating, rough rolling, finish rolling, warm rolling, critical quenching and tempering treatment, a steel plate with a thickness of 40 mm 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 is ultrafine tempered martensite and nanometer-scale second phase precipitates, and the width of the martensite lath of the ultrafine tempered martensite is in the nm level, for example, less than 600 nm.

[0072] Example 4

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

[0074] The chemical composition of the steel used in this embodiment is the same as that of embodiment 2. Before the steel plate is rolled, the thickness of the electroslag billet is 400 mm. After heating, rough rolling, finish rolling, warm rolling, critical quenching and tempering, a steel plate with a thickness of 80 mm is produced. 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 is ultrafine tempered martensite and nanometer-scale second phase precipitates, and the width of the martensite lath of the ultrafine tempered martensite is in the nm level, for example, less than 600 nm.

[0076] Example 5

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

[0078] The chemical composition of the steel used in this embodiment is the same as that of embodiment 3. Before the steel plate is rolled, the thickness of the electroslag billet is 400 mm. After heating, rough rolling, finish rolling, warm rolling, critical quenching and tempering, a steel plate with a thickness of 100 mm is produced. 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 is ultrafine tempered martensite and nanometer-scale second phase precipitates, and the width of the martensite lath of the ultrafine tempered martensite is in the nm level, for example, less than 600 nm.

[0080] The inventor has conducted a large number of experimental studies during the research process, and now uses some solutions with poor performance as comparative examples.

[0081] Comparative Example 1

[0082] The components of the steel in this comparative example are the same as those in Example 1, which will not be described in detail herein. The preparation method adopts a conventional method, comprising the following steps:

[0083] The slab with a thickness of 400 mm was heated, roughly rolled and finished rolled in the same manner as in Example 1. After being rolled into a steel plate with a thickness of 90 mm, it was air-cooled to room temperature, heated to 840° C. for 120 min, quenched, and then tempered at 600° C. The structure was tempered martensite (the lath size was micrometers) and second phase precipitates. Figure 8 , the performance is shown in Table 3.

[0084] Comparative Example 2

[0085] The components of the steel in this comparative example are the same as those in Example 2, which will not be described in detail herein. The preparation method adopts a conventional method, comprising the following steps:

[0086] The slab with a thickness of 350 mm was heated, roughly rolled and finished rolled in the same manner as in Example 2. After being rolled into a steel plate with a thickness of 60 mm, it was air-cooled to room temperature, heated to 860° C. for 90 min, quenched, and then tempered at 620° C. The structure was tempered martensite (the lath size was micrometers) and second phase precipitates. Fig. 9 , the performance is shown in Table 3.

[0087] Comparative Example 3

[0088] The components of the steel in this comparative example are the same as those in Example 3, which will not be described in detail herein. The preparation method comprises the following steps:

[0089] The overall process steps of the slab with a thickness of 400 mm are the same as those of Example 5, but the warm rolling start temperature is 660°C (i.e., Ar1+10°C temperature). Since the warm rolling start temperature is relatively high, the core of the steel plate is not fully deformed, resulting in large performance, especially impact fluctuations. The specific performance is shown in Table 3. Fig.10 Its structure is tempered martensite (lath size is micron level) and second phase precipitates.

[0090] Comparative Example 4

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

[0092] Table 1 Rough rolling and finishing rolling process of steel plate in Example

[0093]

[0094]

[0095] Table 2 Example steel plate warm rolling and tempering process

[0096]

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

[0098]

[0099]

[0100] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing 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 range.

2. The preparation method according to claim 1, characterized in that: The components of the NiCrMo steel, measured in percentage by mass, include: Ni: 5.0% to 10.0%, Cr: 0.50% to 0.70%, and Mo: 0.40% to 0.70%.

3. The preparation method according to claim 1, characterized in that: The starting temperature of the warm rolling is Ar1-(10~20)℃.

4. The preparation method according to claim 3, characterized in that: The final rolling temperature of the warm rolling is Ar1-(30~50)℃.

5. The preparation method according to claim 1, characterized in that: The deformation amount of the warm rolling is 40% to 80%.

6. The preparation method according to claim 1, characterized in that: The warm rolling step includes: cooling the intermediate billet obtained after the finish rolling with high-pressure water, rapidly cooling it to near the Ar1 temperature, air cooling it for 10 to 30 seconds and then starting the warm rolling.

7. The preparation method according to claim 1, characterized in that: The step of online direct quenching after warm rolling includes: water cooling to room temperature immediately after warm rolling.

8. The preparation method according to any one of claims 1 to 7, characterized in that: The preparation method also includes critical quenching, and the steps of critical quenching are as follows: after online direct quenching, the steel plate is reheated to about Ac3+(5-10)°C, kept warm and then quenched.

9. The preparation method according to claim 8, characterized in that: The preparation method further comprises high temperature tempering, and the steps of high temperature tempering are as follows: heating the critically quenched steel plate to 600-650° C., keeping the temperature and then air cooling.

10. A NiCrMo steel, characterized in that: The NiCrMo steel is prepared by the preparation method according to any one of claims 1 to 9.

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