Quenched and tempered steel plates and their production methods

Abstract

This application provides a quenched and tempered steel plate and its production method. The quenched and tempered steel plate comprises the following components by mass percentage: C: 0.15–0.20%; Si: 0.20–0.40%; Mn: 1.20–1.50%; Cr: 0.20–0.50%; Ni: 0.10–0.20%; Nb: 0.020–0.040%; Ti: 0.010–0.040%; Mo: 0.30–0.50%; B: 0.0010–0.0025%; P≤0.015%, S≤0.004%; N≤0.004%, Als: 0.010–0.060%, with the remainder being Fe and unavoidable impurities. This application manufactures a thick, high-strength quenched and tempered steel plate with high surface quality by adjusting the elemental composition and content of the quenched and tempered steel plate, specifically by adding certain amounts of Ni, Cr, and B.

Description

Technical Field

[0001] This application relates to the field of high-strength steel manufacturing technology, specifically to quenched and tempered steel plates and their production methods. Background Technology

[0002] Hot-rolled strips of 20-25mm thickness are produced using continuous hot rolling lines. Compared to medium and heavy plates, this method offers higher production efficiency, higher yield, and lower production costs. However, due to the high strength and deformation resistance of this thickness, the rolling speed is slow (F1-F7 ≤ 2.0m / s), and the dwell time in the high-temperature zone is long. Furthermore, to ensure smooth production, thick-gauge high-strength steel is coiled at higher temperatures to reduce its strength and facilitate coiling. Consequently, the surface warms up quickly after cooling, resulting in the formation of a large amount of iron oxide scale. Summary of the Invention

[0003] This application provides a quenched and tempered steel plate and its production method, aiming to provide a high-strength quenched and tempered steel plate with high surface quality in thick specifications.

[0004] In a first aspect, embodiments of this application provide a quenched and tempered steel plate comprising the following components by mass percentage: C: 0.15–0.20%; Si: 0.20–0.40%; Mn: 1.20–1.50%; Cr: 0.20–0.50%; Ni: 0.10–0.20%; Nb: 0.020–0.040%; Ti: 0.010–0.040%; Mo: 0.30–0.50%; B: 0.0010–0.0025%; P ≤ 0.015%; S ≤ 0.004%; N ≤ 0.004%; Als: 0.010–0.060%; the remainder being Fe and unavoidable impurities.

[0005] According to an embodiment of the first aspect of this application, the components simultaneously satisfy the following relationship:

[0006] 32.4[%Ni]≥9.87[%Si]+1.8[%Mn]-[%Cr];

[0007] 20≤38.6[%C]+5.7[%Mn]+9.6[%Mo]+3.3[%Cr]≤22.

[0008] According to the embodiment of the first aspect of this application, the quenched and tempered steel plate satisfies the following conditions: the depth of the surface pits is ≤0.1mm, and the number is ≤1 pit / m. 2 .

[0009] According to the embodiment of the first aspect of this application, the quenched and tempered steel plate satisfies: yield strength ≥ 960 MPa, tensile strength R m ≥980MPa, elongation A≥12%; -60℃, impact energy KV2≥60J.

[0010] According to an embodiment of the first aspect of this application, the metallographic structure of the quenched and tempered steel plate is: 100% tempered sorbite.

[0011] Secondly, embodiments of this application provide a method for producing quenched and tempered steel plates, comprising the following steps:

[0012] Continuous casting is used to obtain a billet containing the following components by mass percentage: C: 0.15–0.20%; Si: 0.20–0.40%; Mn: 1.20–1.50%; Cr: 0.20–0.50%; Ni: 0.10–0.20%; Nb: 0.020–0.040%; Ti: 0.010–0.040%; Mo: 0.30–0.50%; B: 0.0010–0.0025%; P≤0.015%, S≤0.004%; N≤0.004%, Als: 0.010–0.060%, with the remainder being Fe and unavoidable impurities.

[0013] The billet is heated in a furnace, descaled, rough rolled, finish rolled, laminar flow cooled, coiled, leveled, quenched and tempered to obtain quenched and tempered steel plate.

[0014] The roughing temperature is 1060–1120℃, the finishing temperature is 860–900℃, and the finishing speed is 2.5–3.0 m / s.

[0015] According to an embodiment of the second aspect of this application, the descaling pressure is 180-200 bar.

[0016] According to the embodiments of the second aspect of this application, the dephosphorizing water pressure between the roughing mill stands is 110-130 bar; the dephosphorizing water pressure between the finishing mill stands is 100-120 bar.

[0017] According to an embodiment of the second aspect of this application, the heating temperature in the quenching process is 900±20℃, the holding time is 1.5*plate thickness + 15±3min, and the cooling rate is ≥35℃ / s.

[0018] According to an embodiment of the second aspect of this application, the heating temperature in the tempering process is 600±20℃, the holding time is 1.5*plate thickness + 35±3min, and air cooling is used.

[0019] Compared with the prior art, the present invention has at least the following beneficial effects:

[0020] This application manufactures high-strength quenched and tempered steel plates with thicker dimensions and higher surface quality by adjusting the elemental composition and content in the steel plate. Specifically, by adding a certain amount of Ni element, the oxidation rate of the grains is reduced due to Ni's tendency to accumulate at the grain boundaries on the steel strip surface, resulting in a thinner iron oxide scale. Furthermore, the Ni element accumulated at the grain boundaries acts as a anchor for the iron oxide scale, improving its adhesion and reducing its peeling during leveling, fundamentally improving the problem of pitting on the surface of thick-gauge quenched and tempered steel. Simultaneously, Ni element can improve the low-temperature impact toughness of the steel plate, giving it good low-temperature impact performance. In addition, a certain amount of Cr element is added to the quenched and tempered steel plate in this application embodiment. The iron oxide scale consists of an outermost layer of Fe2O3, a middle layer of Fe3O4, an inner layer of FeO, and a Cr-rich oxide layer on the surface of the matrix. A Cr-rich spinel layer, FeCr2O4, exists between the matrix and the iron oxide scale. The spinel phase is a solid solution of FeCr2O4 formed by FeO and Cr2O3. During the deformation of the steel plate, the FeO layer also underwent the same deformation without cracking, indicating that the spinel structure formed by Cr in the FeO layer has certain plasticity and good adhesion. At the same time, certain amounts of Cr and B elements increase hardenability, ensuring that all thick-gauge high-strength steels form tempered sorbite structures during quenching, thereby improving the uniformity of its properties in the thickness direction. Attached Figure Description

[0021] The features, advantages, and technical effects of exemplary embodiments of this application will now be described with reference to the accompanying drawings.

[0022] Figure 1 Macroscopic photograph of the surface quality of the high-strength quenched and tempered steel sheet prepared in Example 1 of this application;

[0023] Figure 2 A macroscopic photograph of the surface quality of the steel plate in Comparative Example 3 of this application;

[0024] Figure 3 The morphology of the iron oxide scale of the high-strength quenched and tempered steel plate prepared in Example 1 of this application is shown, and the thickness of the iron oxide scale is <20μm;

[0025] Figure 4 The iron oxide scale morphology of the steel plate in Comparative Example 3 of this invention is shown, and the iron oxide scale thickness is >60μm. Detailed Implementation

[0026] The embodiments of this application will be described in further detail below with reference to the accompanying drawings and examples. The detailed description of the following embodiments and the accompanying drawings are used to illustrate the principles of this application by way of example, but should not be used to limit the scope of this application, that is, this application is not limited to the described embodiments.

[0027] In the description of this application, it should be noted that, unless otherwise stated, "a plurality of" means two or more; the terms "upper," "lower," "left," "right," "inner," and "outer," etc., indicating orientation or positional relationships, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this application. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance. "Vertical" does not require strict verticality, but may include permissible errors. "Parallel" does not require strict parallelism, but may include permissible errors.

[0028] The inventors of this application noted that during the leveling process of hot-rolled steel sheets, the iron oxide scale on the surface gradually pulverizes under the action of the straightening machine. This pulverized iron oxide scale adheres to the straightening rollers of the straightening machine under the action of grease and water. Because the yield strength of hot-rolled tempered steel is as high as 600-700 MPa and the dimensions are thick, a large straightening force is used during the straightening process. The iron oxide scale on the straightening rollers easily forms a large number of pit defects on the surface of the strip steel, with a depth exceeding 0.4 mm and more than 10 per square meter. Simultaneously, the pulverized iron oxide scale on the steel sheet surface deteriorates the working environment under the blowing action of the blower. Furthermore, the large amount of iron oxide powder on the steel sheet surface easily accumulates in the processing molds, reducing the mold clearance and causing damage to the stamping process. After coating, these pits are still clearly visible, requiring an additional "sanding + puttying" process before coating to compensate for them. With improvements in coating processes, many users are now using spraying for surface coating, making the "sanding + puttying" process impossible. Furthermore, users' requirements for appearance parts are becoming increasingly stringent. Therefore, high-strength steel with higher surface quality has become an important aspect of product competitiveness.

[0029] In view of this, the inventors of this application, through extensive experimental research, have provided a quenched and tempered steel plate and its production method, aiming to provide a high-strength quenched and tempered steel plate with high surface quality at thicker thicknesses.

[0030] Quenched and tempered steel plate

[0031] In a first aspect, embodiments of this application provide a quenched and tempered steel plate comprising the following components by mass percentage: C: 0.15–0.20%; Si: 0.20–0.40%; Mn: 1.20–1.50%; Cr: 0.20–0.50%; Ni: 0.10–0.20%; Nb: 0.020–0.040%; Ti: 0.010–0.040%; Mo: 0.30–0.50%; B: 0.0010–0.0025%; P ≤ 0.015%; S ≤ 0.004%; N ≤ 0.004%; Als: 0.010–0.060%; the remainder being Fe and unavoidable impurities.

[0032] This application manufactures high-strength quenched and tempered steel plates with thicker dimensions and higher surface quality by adjusting the elemental composition and content in the steel plate. Specifically, by adding a certain amount of Ni element, the oxidation rate of grains is reduced due to Ni's tendency to accumulate at grain boundaries on the steel strip surface, resulting in a thinner iron oxide scale. The Ni element accumulated at the grain boundaries acts as a anchor for the iron oxide scale, improving its adhesion and reducing its peeling during leveling, fundamentally improving the surface pitting problem of thick-gauge quenched and tempered steel. Simultaneously, Ni element improves the low-temperature impact toughness of the steel plate, giving it good low-temperature impact performance. Furthermore, a certain amount of Cr element is added to the quenched and tempered steel plate in this application embodiment. The iron oxide scale consists of an outermost layer of Fe2O3, a middle layer of Fe3O4, an inner layer of FeO, and a Cr-rich oxide layer on the surface of the matrix. A Cr-rich spinel layer, FeCr2O4, exists between the matrix and the iron oxide scale. The spinel phase is a solid solution of FeCr2O4 formed by FeO and Cr2O3. During the deformation process, the FeO layer also underwent the same deformation trend but did not crack, indicating that the spinel structure formed by Cr in the FeO layer has certain plasticity and good adhesion. At the same time, certain Cr and B elements increase hardenability, ensuring that all thick-gauge high-strength steels form tempered sorbite structure during quenching, thereby improving the uniformity of its properties in the thickness direction.

[0033] The role of each chemical component in quenched and tempered steel plate will be explained in detail below:

[0034] Carbon (C): As an interstitial atom in steel, carbon plays a crucial role in improving steel strength, having the greatest impact on yield strength and tensile strength. It is a key element determining the material's strength and hardness. C stabilizes austenite and can control the transformation of ferrite during laminar and air cooling. To obtain high-strength steel with a tensile strength of 980 MPa, a certain carbon content must be ensured. However, excessive carbon content can affect weldability. Therefore, in this application, the C content is set at 0.15–0.20%. For example, the mass percentage of C in the quenched and tempered steel plate can be 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.20%, or any combination of two of the above values.

[0035] Si: Si is a solid solution strengthening element. During air cooling in the two-phase region, it promotes the accelerated diffusion of carbon into austenite, which has a purifying effect on ferrite and improves the purity of ferrite in dual-phase steel, thereby obtaining a lower yield strength ratio. However, too high a Si content can easily form high-melting-point oxides on the surface of the steel plate, affecting the surface quality of the steel plate. Therefore, the Si content in this application is set to 0.20% to 0.40%. For example, the mass percentage of Si element in the quenched and tempered steel plate can be 0.20%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, 0.30%, 0.31%, 0.32%, 0.33%, 0.35%, 0.36%, 0.37%, 0.38%, 0.39%, 0.40%, or any range of two of the above values.

[0036] Mn: Mn is a substitutional element that plays a role in solid solution strengthening. It can expand the austenite phase region, reduce the critical quenching rate of steel, stabilize austenite, refine grains, and delay the transformation of austenite to pearlite. To ensure a tensile strength ≥980MPa, the Mn content should be controlled above 1.00%. If the Mn content is too low, the supercooled austenite is not stable enough and is prone to transforming into a pearlite-type structure during air cooling; if the Mn content is too high, it will affect the alloy cost. Therefore, the Mn content in this application is set at 1.20-1.50%. For example, the mass percentage content of Mn element in the quenched and tempered steel plate can be 1.20%, 1.21%, 1.22%, 1.23%, 1.24%, 1.25%, 1.26%, 1.27%, 1.28%, 1.29%, 1.30%, 1.31%, 1.32%, 1.33%, 1.35%, 1.36%, 1.37%, 1.38%, 1.39%, 1.40%, 1.41%, 1.42%, 1.43%, 1.44%, 1.45%, 1.46%, 1.47%, 1.48%, 1.49%, 1.50%, or a range of any two of the above values.

[0037] P and S: P and S are harmful inclusions in steel, which have a great detrimental effect on the formability, low-temperature toughness, weldability and fatigue performance of steel. The present invention aims to reduce production costs and improve product quality by controlling the P content to ≤0.015% and the S content to ≤0.004%, so that the influence of P and S on the formability is reduced to the lowest possible level.

[0038] B: Bode (B) is an element with a strong hardening effect in steel. Even trace amounts of B can greatly improve the hardenability of steel. Therefore, the B content in this invention is set to 0.0010–0.0025%. For example, the mass percentage of B in the quenched and tempered steel plate can be 0.0010%, 0.0011%, 0.0012%, 0.0013%, 0.0014%, 0.0015%, 0.0016%, 0.0017%, 0.0018%, 0.0019%, 0.0020%, 0.0021%, 0.0022%, 0.0023%, 0.0024%, 0.0025%, or any range of two of the above values.

[0039] Ti: Ti has certain grain refinement and precipitation strengthening effects. A small amount of Ti can also improve welding performance. In this invention, the main functions of Ti are grain refinement and welding performance, therefore the Ti content is set to 0.010-0.040%. For example, the mass percentage content of Ti element in the quenched and tempered steel plate can be 0.010%, 0.011%, 0.012%, 0.013%, 0.014%, 0.015%, 0.016%, 0.017%, 0.018%, 0.019%, 0.020%, 0.021%, 0.022%, 0.023%, 0.024%, 0.025%, 0.026%, 0.027%, 0.028%, 0.029%, 0.030%, 0.031%, 0.032%, 0.033%, 0.034%, 0.035%, 0.036%, 0.037%, 0.038%, 0.039%, 0.040%, or any range of two of the above values.

[0040] Mo: Mo can refine grains and improve strength and toughness. Mo exists in both solid solution and carbide phases in steel, thus molybdenum-containing steel exhibits both solid solution strengthening and carbide dispersion strengthening. Mo can shift the C-curve of steel to the right, significantly improving hardenability and tempering stability. Mo can also improve the high-temperature brittleness of alloy tempered steel and the toughness of tempered sorbite. During high-temperature tempering, Mo₂C precipitates in situ at dislocations and remains coherent with the matrix, preventing agglomeration and growth, resulting in a strong secondary hardening effect. Mo dissolves in ferrite, increasing the self-diffusion activation energy of iron and raising the recovery and recrystallization temperatures of steel. Mo is one of the commonly used alloying elements in steel. Therefore, the Mo content in this application is set at 0.30–0.50%. For example, the mass percentage content of Mo in the quenched and tempered steel plate can be 0.30%, 0.31%, 0.32%, 0.33%, 0.34%, 0.35%, 0.36%, 0.37%, 0.38%, 0.39%, 0.40%, 0.41%, 0.42%, 0.43%, 0.44%, 0.45%, 0.46%, 0.47%, 0.48%, 0.49%, 0.50%, or any range of two of the above values.

[0041] Niobium (Nb): Niobium can inhibit austenite recrystallization, precipitate NbC to refine ferrite grains, and improve strength and toughness. Niobium can improve the tempering stability of steel and reduce its temper brittleness. In the solid solution state, Nb can effectively inhibit the transformation of austenite to ferrite, pearlite, and bainite, and improve the hardenability of steel plates. Therefore, the Nb content in this application is set at 0.02–0.04%. For example, the mass percentage content of Nb element in the quenched and tempered steel plate can be 0.020%, 0.021%, 0.022%, 0.023%, 0.024%, 0.025%, 0.026%, 0.027%, 0.028%, 0.029%, 0.030%, 0.031%, 0.032%, 0.033%, 0.034%, 0.035%, 0.036%, 0.037%, 0.038%, 0.039%, 0.040%, or a range of any two of the above values.

[0042] Ni: Ni can increase the strength of steel without reducing its plasticity and improve its low-temperature toughness. It reduces the critical cooling rate of steel and improves its hardenability. It expands the austenite region and is an effective element for austenitization. It has a certain degree of corrosion resistance and good resistance to some reducing acids. However, Ni is a precious metal, and adding a large amount will increase its production cost. Therefore, the Ni content in this application is set at 0.10-0.20%. For example, the mass percentage of Ni in the quenched and tempered steel plate can be 0.10%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.20%, or any combination of two of the above values.

[0043] Cr: Cr element can improve the hardenability of steel and reduce the adhesion of iron oxide scale on the surface of strip steel, reduce the powdering of iron oxide scale on the surface of steel plate, and improve the surface quality of steel plate. Therefore, the Cr content in this invention is set to 0.20-0.50%. For example, the mass percentage content of Cr element in the quenched and tempered steel plate can be 0.20%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, 0.30%, 0.31%, 0.32%, 0.33%, 0.35%, 0.36%, 0.37%, 0.38%, 0.39%, 0.40%, 0.41%, 0.42%, 0.43%, 0.44%, 0.45%, 0.46%, 0.47%, 0.48%, 0.49%, 0.50%, or any range of two of the above values.

[0044] N: N is a harmful element in steel. Controlling the N content to ≤0.004% can reduce the risk of forming coarse TiN inclusions.

[0045] Als: Als plays a deoxidizing role in steelmaking, improving the purity of molten steel. Furthermore, Als can fix nitrogen (N) in steel and form stable compounds with N, effectively refining the grain size. Therefore, the Als content in this application is set at 0.010–0.060%. For example, the mass percentage content of Al element in the quenched and tempered steel plate can be 0.010%, 0.011%, 0.012%, 0.013%, 0.014%, 0.015%, 0.016%, 0.017%, 0.018%, 0.019%, 0.020%, 0.021%, 0.022%, 0.023%, 0.024%, 0.025%, 0.026%, 0.027%, 0.028%, 0.029%, 0.030%, 0.031%, 0.032%, 0.033%, 0.034%. %, 0.035%, 0.036%, 0.037%, 0.038%, 0.039%, 0.040%, 0.041%, 0.042%, 0.043%, 0.044%, 0.045%, 0.046%, 0.047%, 0.048%, 0.049%, 0.050%, 0.051%, 0.052%, 0.053%, 0.054%, 0.055%, 0.056%, 0.057%, 0.058%, 0.059%, 0.060%, or a range consisting of any two of the above values.

[0046] In summary, this application manufactures high-strength quenched and tempered steel plates with thicker dimensions and higher surface quality by adjusting the elemental composition and content in the quenched and tempered steel plates.

[0047] In some embodiments, the composition of the quenched and tempered steel plate simultaneously satisfies the following relationship:

[0048] 32.4[%Ni]≥9.87[%Si]+1.8[%Mn]-[%Cr];

[0049] 20≤38.6[%C]+5.7[%Mn]+9.6[%Mo]+3.3[%Cr]≤22

[0050] The composition of the quenched and tempered steel plate simultaneously satisfies the above relationships. The first relationship enables the quenched and tempered steel plate in this application to have good surface quality, while the second relationship enables the quenched and tempered steel plate in this application to have high strength and good toughness. This application achieves high surface quality and high strength in thicker specifications by controlling the elemental composition and content in the quenched and tempered steel plate.

[0051] In some embodiments, the quenched and tempered steel plate meets the following requirements: the depth of surface pits is ≤0.1mm, and the number is ≤1 pit / m. 2In this embodiment of the application, by adjusting the elemental composition and content in the quenched and tempered steel plate, the prepared quenched and tempered steel plate can exhibit better surface quality.

[0052] In some embodiments, the quenched and tempered steel plate meets the following requirements: yield strength ≥ 960 MPa, tensile strength R m ≥980MPa, elongation A≥12%; -60℃, impact energy KV2≥60J. In the embodiments of this application, by adjusting the elemental composition and content in the quenched and tempered steel plate, the prepared quenched and tempered steel plate can exhibit better strength.

[0053] In some embodiments, the metallographic structure of the quenched and tempered steel sheet is 100% tempered sorbite. The 100% tempered sorbite structure gives the quenched and tempered steel sheet of this application good toughness and plasticity, while also having high strength, thus exhibiting good comprehensive mechanical properties.

[0054] Quenched and tempered steel plate

[0055] Secondly, embodiments of this application provide a method for producing quenched and tempered steel plates, comprising the following steps:

[0056] Continuous casting is used to obtain a billet containing the following components by mass percentage: C: 0.15–0.20%; Si: 0.20–0.40%; Mn: 1.20–1.50%; Cr: 0.20–0.50%; Ni: 0.10–0.20%; Nb: 0.020–0.040%; Ti: 0.010–0.040%; Mo: 0.30–0.50%; B: 0.0010–0.0025%; P≤0.015%, S≤0.004%; N≤0.004%, Als: 0.010–0.060%, with the remainder being Fe and unavoidable impurities.

[0057] The billet is heated in a furnace, descaled, rough rolled, finish rolled, laminar flow cooled, coiled, leveled, quenched and tempered to obtain quenched and tempered steel plate.

[0058] The roughing temperature is 1060–1120℃, the finishing temperature is 860–900℃, and the finishing speed is 2.5–3.0 m / s.

[0059] The method for producing quenched and tempered steel plates provided in this application, by adjusting and controlling the elemental composition and content in the quenched and tempered steel plates and adopting a high-temperature fast rolling process, can effectively slow down the further oxidation of the cast billet. The increase in rolling temperature reduces the strength of the cast billet, which can effectively reduce the rolling force, achieve rapid rolling, and reduce the iron oxide scale generated during hot rolling.

[0060] Meanwhile, in this embodiment, a powerful descaling process is used in the hot rolling process to remove the iron oxide scale generated during the hot rolling heating process and the hot rolling process, preventing the iron oxide scale from being pressed in during the rolling process.

[0061] In some embodiments, the descaling pressure is 180–200 bar. The above-mentioned strong descaling pressure is used in the hot rolling process of this application, which can thoroughly remove the iron oxide scale generated during the hot rolling heating process and the rolling process of the quenched and tempered steel plate, and prevent the iron oxide scale from being pressed in during the rolling process.

[0062] In some embodiments, the dephosphorizing water pressure between roughing mill stands is 110–130 bar; and the dephosphorizing water pressure between finishing mill stands is 100–120 bar.

[0063] In some embodiments, laminar cooling employs front-stage cooling at a rate of 50–150 °C / s. In this application embodiment, front-stage cooling is used after finishing rolling to prevent prolonged residence at high temperatures, which could lead to the formation of a thicker quaternary oxide scale.

[0064] In some embodiments, the winding temperature is 600–650°C. The winding temperature in this embodiment is the aforementioned temperature, which can promote the eutectoid reaction of iron oxide scale on the surface of the hot-rolled plate, thereby generating Fe3O4 with stronger deformability and thus improving the deformability of the iron oxide scale.

[0065] In some embodiments, the leveling and straightening pressure is 10,000–13,000 KN, and the fine straightening pressure is 21,000–23,000 KN. In the embodiments of this application, the leveling and straightening pressure is within the above range, which ensures that thick-gauge high-strength steel yields steel plates with good shape.

[0066] In some embodiments, the heating temperature in the quenching process is 900±20℃, the holding time is 1.5*plate thickness + 15±3min, and the cooling rate is ≥35℃ / s.

[0067] In some embodiments, the heating temperature in the tempering process is 600±20℃, the holding time is 1.5*plate thickness + 35±3min, and air cooling is used.

[0068] This application employs the aforementioned temperature and time-matched quenching process to ensure austenitization of the microstructure at the center of the plate thickness, allowing for the complete formation of martensite during quenching. The aforementioned temperature and time-matched tempering process ensures that the microstructure at the center of the plate thickness undergoes sufficient high-temperature tempering to obtain tempered martensite, thereby achieving a good balance between high strength and high toughness.

[0069] In summary, this application manufactures a quenched and tempered steel plate with a reasonable balance of high strength and high toughness by adjusting the elemental composition and content of the quenched and tempered steel plate and combining it with the high-temperature fast rolling process flow and process parameters.

[0070] Example

[0071] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all components, percentages, and ratios reported in the following embodiments are based on weight, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0072] Examples 1-8

[0073] Thick, high-strength quenched and tempered steel plates with high surface quality and their preparation method. The chemical composition and mass percentage of the high-strength quenched and tempered steel plates in each embodiment are shown in Table 1. The mass percentage of the chemical components shown in Table 1 simultaneously satisfies the following relationship:

[0074] 32.4[%Ni]≥9.87[%Si]+1.8[%Mn]-[%Cr]

[0075] 20≤38.6[%C]+5.7[%Mn]+9.6[%Mo]+3.3[%Cr]≤22

[0076] The process for preparing thick, high-strength quenched and tempered steel plates with high surface quality is as follows:

[0077] Converter smelting—ladle refining—continuous casting (electromagnetic stirring)—heating—dephosphorization—rough rolling—finish rolling—laminar cooling—coiling—leveling—quenching—tempering—packaging.

[0078] The main process parameters of the preparation methods of each embodiment of this specific implementation are shown in Table 2.

[0079] The performance and surface test results of the products obtained in each embodiment of this specific implementation are shown in Table 3.

[0080] Table 1 shows the mass percentage of chemical composition of the high-strength steel plate used for the fuel tank bracket in each embodiment.

[0081]

[0082]

[0083]

[0084] Table 3 Performance test results of the products prepared in each embodiment

[0085]

[0086] Figure 1 Macroscopic photograph of the surface quality of the high-strength quenched and tempered steel sheet prepared in Example 1 of this application; Figure 2 A macroscopic photograph of the surface quality of the steel plate in Comparative Example 3 of this application; Figure 3 The morphology of the iron oxide scale of the high-strength quenched and tempered steel plate prepared in Example 1 of this application is shown, and the thickness of the iron oxide scale is <20μm; Figure 4 The iron oxide scale morphology of the steel plate in Comparative Example 3 of this invention is shown, and the iron oxide scale thickness is >60μm.

[0087] From Tables 1-3 and Figures 1-4 It is evident that the elemental composition and content of the tempered steel plates in Examples 1-9 all meet the requirements of this application. Furthermore, the high-temperature fast rolling process and parameters result in tempered steel plates with a reasonable balance of high strength and high toughness, and good surface quality. However, the Ni content in Comparative Example 1 does not conform to the formula of this application. Even if other processes meet the requirements and the finished product performance meets the requirements, a large number of oxidation pits still exist on the surface of the finished product. The elemental contents of Comparative Example 2 do not conform to the formula of this application. Although the surface quality is good, the tensile strength is only 906 MPa, which is lower than the normal mechanical property range of this invention. The components of Comparative Example 3 are all within the range of this application and meet the formula of this invention, but the process did not follow the preparation method required by this invention, resulting in many oxidation pits still existing on its surface.

[0088] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A quenched and tempered steel sheet, characterized by, The components include the following components by mass percentage: C: 0.15~0.20%; Si: 0.20~0.40%; Mn: 1.20~1.50%; Cr: 0.20~0.50%; Ni: 0.10~0.20%; Nb: 0.020~0.040%; Ti: 0.010~0.040%; Mo: 0.30~0.50%; B: 0.0010~0.0025%; P≤0.015%, S≤0.004%; N≤0.004%, Als: 0.010~0.060%, the remainder being Fe and unavoidable impurities; The tempered steel plate satisfies: surface pit depth ≤ 0.1 mm, and number ≤ 1 / m 2 ; Yield strength ≥ 960 MPa, tensile strength R m ≥ 980 MPa, elongation A ≥ 12%; -60°C, impact energy KV2≥ 60 J; The metallographic structure of the quenched and tempered steel plate is: 100% tempered sorbite; The quenched and tempered steel plate is prepared by the following steps: Continuous casting is used to obtain a billet, which comprises the following components in mass percentage: C: 0.15~0.20%; Si: 0.20~0.40%; Mn: 1.20~1.50%; Cr: 0.20~0.50%; Ni: 0.10~0.20%; Nb: 0.020~0.040%; Ti: 0.010~0.040%; Mo: 0.30~0.50%; B: 0.0010~0.0025%; P≤0.015%, S≤0.004%; N≤0.004%, Als: 0.010~0.060%, the remainder being Fe and unavoidable impurities; The billet is heated in a furnace, descaled, rough rolled, finish rolled, laminar flow cooled, coiled, leveled, quenched and tempered to obtain a quenched and tempered steel plate. The descaling pressure is 180~200 bar, the roughing temperature is 1060~1120℃, the finishing temperature is 860~900℃, and the finishing speed is 2.5~3.0 m / s. The quenching process involves heating at 900±20℃, holding for (1.5*plate thickness + 15)±3min, and cooling at a rate ≥35℃ / s. The tempering process involves heating at 600±20℃, holding for (1.5*plate thickness + 35)±3min, and air cooling.

2. The tempered steel sheet according to claim 1, characterized by The components simultaneously satisfy the following relationship: 32.4[%Ni]≥9.87[%Si]+1.8[%Mn]-[%Cr]; 20≤38.6[%C]+5.7[%Mn]+9.6[%Mo]+3.3[%Cr]≤22.

3. The tempered steel sheet according to claim 1, characterized by, The dephosphorizing water pressure between roughing mill stands is 110~130 bar; the dephosphorizing water pressure between finishing mill stands is 100~120 bar.

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