Ultrahigh-strength steel plate having high r value and manufacturing method therefor

Abstract

Disclosed in the present invention is an ultrahigh-strength steel plate having a high r value, comprising, in the thickness direction, an upper surface layer, a middle layer, and a lower surface layer. The main bodies of the microstructures of the upper surface layer and the lower surface layer are ferrite, and the carbon content of the upper surface layer and the carbon content of the lower surface layer are both less than or equal to 0.025%; the mass percentages of chemical elements in the upper surface layer and in the lower surface layer respectively meet: Ti-3.42N-3.98C ≥ 0; and the main body of the microstructure of the middle layer is at least one of tempered martensite and bainite. Correspondingly, also disclosed is a manufacturing method for the ultrahigh-strength steel plate having a high r value. The steel plate of the present invention has ultrahigh strength and a high r value. Lighter thickness reduction of the ultrahigh-strength steel plate having a high r value in a deformation process is achieved, and thus the steel plate presents good formability in both global stretching deformation and local deformation.

Description

Technical Field

[0001] The present invention relates to a steel plate and a manufacturing method therefor, and particularly relates to a high-strength steel plate and a manufacturing method therefor.Background Art

[0002] The r value represents the level of the thinning resistance of a steel plate during the deformation process of drawing forming. Among high-formability deep drawing steel plates (such as interstitial-free steel) with a tensile strength of 350 MPa or lower, the r value is one of the important technical indicators. A high r value means that the steel plate has better drawing forming performance.

[0003] However, existing high-strength steel plates, since they are not used for deep drawing, have not been focused on the r value of the steel plates and the performance it characterizes.

[0004] For example, the Chinese patent document with the publication number CN101768695A, the publication date of July 7, 2010, and the title "1000MPa Grade Ti Microalloyed Ultrafine-Grained Cold-Rolled Dual-Phase Steel and Its Preparation Process" discloses that its chemical composition is: 0.03-0.2% C, 0.2-0.8% Si, 1.2-2.0% Mn, Ti: 0.03-0.15%, ≤0.02% P, S≤0.015, 0.02-0.15% Al, and the rest are Fe and unavoidable impurities. After hot rolling and cold rolling, an annealing is performed in the critical region, and under the condition that the cooling rate is less than 50°C / s, a cold-rolled dual-phase steel with a strength of 980MPa or higher is obtained. It can be seen that this high-strength steel does not focus on the r value of the steel plate.

[0005] For example, the Chinese patent document with the publication number CN101363099A, the publication date of February 11, 2009, and the title "A Cold-Rolled Dual-Phase Steel Plate with Tensile Strength of 1000MPa Grade and Its Preparation Method" discloses: C: 0.14-0.21%, Si: 0.4-0.9%, Mn: 1.5-2.1%, P: ≤0.02%, S: ≤0.01%, Nb: 0.001-0.05%, and V: 0.001-0.02%. After hot rolling and cold rolling, the steel plate is held at a temperature between 760-820°C, cooled with a cooling rate of 40-50°C / s, and subjected to over-aging at 240-320°C for 180-300s. This high-strength steel does not focus on the r value of the steel plate.

[0006] Xiong Ziliu et al. introduced a dual-phase steel with a tensile strength of 980MPa in Formability of High-Strength Dual-Phase Steels, Heat Treatment of Metals (Vol. 46, No. 5, 2021), but its r value is less than 0.9.

[0007] Ge Delong et al. introduced martensitic steels with a strength range of 1000-1400MPa in Maximum Bending Angle of 1000MPa Grade Ultra-High Strength Martensitic Steel Based on Three-Point Bending, Journal of Plasticity Engineering (Vol. 21, No. 4, 2014), and their r value are less than 0.7.Summary

[0008] One object of the present invention is to provide an ultrahigh-strength steel plate having a high r value. This steel plate has an ultrahigh strength and a high r value. Slighter thickness reduction of the ultrahigh-strength steel plate having a high r value during the deformation process is achieved, and thus the steel plate presents good formability in both global tensile deformation and local deformation.

[0009] In the ultrahigh-strength steel plate having a high r value of the present invention, the r value represents the level of the thinning resistance of the steel plate during the deformation process of drawing forming. A steel plate with a high r value is less likely to have its thickness thinned during tensile deformation, and thus is less likely to fracture. For ultrahigh-strength steel plates, when severe local deformation occurs, if the r value is high, the local area is less likely to have thickness thinning, which can slow down the necking process and thus is beneficial to the formability of the steel plates.

[0010] To achieve the above objective, the present invention provides an ultrahigh-strength steel plate having a high r value, comprising, in the thickness direction, an upper surface layer, a middle layer, and a lower surface layer; the main bodies of the microstructures of the upper surface layer and the lower surface layer are ferrite, and the carbon content of the upper surface layer and the carbon content of the lower surface layer are both less than or equal to 0.025%; the mass percentages of chemical elements in the upper surface layer and in the lower surface layer respectively meet: Ti-3.42N-3.98C ≥ 0; and the microstructure of the middle layer comprises ferrite and tempered martensite.

[0011] One of the cores of the present invention lies in achieving a high r value by enabling the mass percentages of chemical elements in the upper surface layer and the lower surface layer to respectively meet Ti-3.42N-3.98C ≥ 0, and ensuring the carbon content of both the upper surface layer and the lower surface layer being ≤ 0.025%. Meanwhile, the overall high strength of the steel plate is mainly ensured by the structure of the middle layer. In some embodiments, the carbon content of both the upper surface layer and the lower surface layer is 0.02-0.025%. The mass percentages of chemical elements in the upper surface layer and the lower surface layer respectively meet: Ti-3.42N-3.98C ≥ 0.060, preferably ≥ 0.068. In some embodiments, the mass percentages of chemical elements in the upper surface layer and the lower surface layer respectively meet: 0.16 ≥ Ti-3.42N-3.98C ≥ 0.06.

[0012] Further, in the ultrahigh-strength steel plate having a high r value of the present invention, the average grain diameter of ferrite in the upper surface layer and the lower surface layer is ≤ 15µm. In some embodiments, in the ultrahigh-strength steel plate having a high r value of the present invention, the average grain diameter of ferrite in the upper surface layer and the lower surface layer can be ≤ 10µm. In some embodiments, in the ultrahigh-strength steel plate having a high r value of the present invention, the average grain diameter of ferrite in the upper surface layer and the lower surface layer is 6-10µm.

[0013] Further, in the ultrahigh-strength steel plate having a high r value of the present invention, the volume fraction of ferrite in the upper surface layer and the lower surface layer is ≥ 97% respectively. In some embodiments, the volume fraction of ferrite in the upper surface layer and the lower surface layer is 97-99%.

[0014] Further, in the ultrahigh-strength steel plate having a high r value of the present invention, the microstructure of the middle layer may further comprise bainite.

[0015] Furthermore, in the ultrahigh-strength steel plate having a high r value of the present invention, regardless of whether the middle layer comprises bainite or not, the volume fraction of tempered martensite + bainite is ≥ 40%. In some embodiments, the middle layer comprises ferrite and at least one of tempered martensite and bainite. In some embodiments, the volume fraction of ferrite in the middle layer is 55-60%, preferably 57-60%. In some embodiments, the volume fraction of tempered martensite in the middle layer is 40-45%, preferably 40-43%. In some embodiments, the volume fraction of bainite in the middle layer is 0-5%, preferably 0-3%.

[0016] Further, in the ultrahigh-strength steel plate having a high r value of the present invention, the ferrite in the upper surface layer and the lower surface layer comprises carbides, and the types of the carbides are Ti (C, N) and Nb (C, N).

[0017] Further, in the ultrahigh-strength steel plate having a high r value of the present invention, the thickness of the upper surface layer and the lower surface layer is respectively 100-200µm.

[0018] Further, in the ultrahigh-strength steel plate having a high r value of the present invention, the sum of the thicknesses of the upper surface layer and the lower surface layer does not exceed 40% of the total thickness of the steel plate.

[0019] Further, the thicknesses of the ultrahigh-strength steel plate having a high r value of the present invention is 0.8-2.5mm, preferably 1.2-2.0 mm.

[0020] Further, the ultrahigh-strength steel plate having a high r value of the present invention comprises Fe and inevitable impurities, wherein it further comprises the following chemical elements with mass percentages as follows: C: 0.08-0.20%; Si: 0.01-1.7%; Mn: 0.7-2.7%; Ti: 0.16-0.26%; Nb: 0.015-0.05%; Al: 0.02-0.06%; N≤0.005%.

[0021] Further, in the ultrahigh-strength steel plate having a high r value of the present invention, the mass percentages of each chemical elements are: C: 0.08-0.20%; Si: 0.01-1.7%; Mn: 0.7-2.7%; Ti: 0.16-0.26%; Nb: 0.015-0.05%; Al: 0.02-0.06%; N≤0.005% and a balance of Fe and inevitable impurities.

[0022] In some embodiments, the design principles of each chemical elements of the ultrahigh-strength steel plate having a high r value of the present invention are specifically as follows: C: In the ultrahigh-strength steel plate having a high r value of the present invention, C can enhance the strength of the steel by affecting the hardness of martensite. If the carbon content in the steel is too low, the strength of the steel after quenching is relatively low; whereas the higher the carbon content in the steel, the higher the strength of the steel after quenching. However, excessively high carbon is not conducive to achieving a high r value. Therefore, the content of C element in the ultrahigh-strength steel plate having a high r value of the present invention is controlled within the range of 0.08-0.20%.

[0023] Si: In the ultrahigh-strength steel plate having a high r value of the present invention, Si can play roles such as solid solution strengthening, inhibiting the precipitation of Fe3C, and promoting the formation of retained austenite. Based on this, the content of Si in the present invention is controlled to be 0.01-1.7%. In some embodiments, the content of Si is 0.4-1.7%.

[0024] Mn: Mn is the main element for improving the hardenability of steel, and its content matches the cooling capacity of the adopted cooling method. When the content of Mn is too low, the steel plate cannot be hardened, thus failing to achieve relatively high strength; whereas if the content of Mn is too high, it is unfavorable to the carbon equivalent. Therefore, the content of Mn element in the ultrahigh-strength steel plate having a high r value of the present invention is controlled within the range of 0.7-2.7%. In some embodiments, the content of Mn is 1.0-2.7%.

[0025] Ti: In the ultrahigh-strength steel plate having a high r value of the present invention, the content of Ti is closely related to the r value of the present invention. If the content of Ti is too low, it is insufficient to fix all C and N in the upper surface layer and lower surface layer, resulting in a relatively low r value; if the content of Ti is too high, Ti is excessive. Therefore, the content of Ti element in the ultrahigh-strength steel plate having a high r value of the present invention is controlled within the range of 0.16-0.26%.

[0026] Nb: In the ultrahigh-strength steel plate having a high r value of the present invention, Nb plays a role in assisting in fixing C and N, forming Nb (C, N), and ensuring that none of the C and N in the upper surface layer and lower surface layer is in a solid solution state. Therefore, the content of Nb element in the ultrahigh-strength steel plate having a high r value of the present invention is controlled within the range of 0.015-0.05%.

[0027] Al: The main role in the present invention is deoxidation during the smelting process, and its content range can be: 0.02-0.06%.

[0028] N: In the present invention, N is not conducive to forming the composition ratio of Ti-3.42N-3.98C ≥ 0 in the upper surface layer and lower surface layer, nor is it conducive to forming a state of interstitial-free atoms locally. Therefore, it is necessary to control N ≤ 0.005%.

[0029] In the present invention, the unavoidable impurities are mainly S and P. Under the condition that process conditions permit, it is expected that their content is as low as possible. In the ultrahigh-strength steel plate having a high r value of the present invention, the content of P element is ≤ 0.015%. In the ultrahigh-strength steel plate having a high r value of the present invention, the content of S element is ≤ 0.003%.

[0030] It can be understood that in the ultrahigh-strength steel plate having a high r value of the present invention, the basic components are: C: 0.08-0.20%, Si: 0.01-1.7%, and Mn: 0.7-2.7%. These basic components form the basis of the strength of the steel plate, but a high r value cannot be achieved based on these basic components. To achieve a high r value, the steel plate of the present invention further contains: Ti: 0.16-0.26%, Nb: 0.015-0.05%, and N ≤ 0.005%.

[0031] Further, in the ultrahigh-strength steel plate having a high r value of the present invention, its chemical elements further contain at least one of Cr, Mo, and B; wherein B is ≤ 0.005% and Cr + Mo is ≤ 0.5%. In some embodiments, when contained, the content of B does not exceed 0.005%, the content of Cr does not exceed 0.3%, and the content of Mo does not exceed 0.2%.

[0032] In the ultrahigh-strength steel plate having a high r value of the present invention, the alloying elements Cr, Mo, and B can effectively cooperate with Mn to achieve the effect of improving hardenability, so that the hardenability of the steel matches the cooling capacity of rapid cooling, thereby further avoiding failure to harden or excessively high carbon equivalent.

[0033] Further, a tensile strength of the ultrahigh-strength steel plate having a high r value of the present invention is ≥ 980MPa, an r value r 90 of the ultrahigh-strength steel plate having a high r value of the present invention is ≥ 1.2, a microhardness HV of the upper surface layer and the lower surface layer of the ultrahigh-strength steel plate having a high r value of the present invention is ≤ 130, and a microhardness HV of the middle layer of the ultrahigh-strength steel plate having a high r value of the present invention is ≥ 330. Preferably, a tensile strength of the ultrahigh-strength steel plate having a high r value of the present invention is ≥ 1020MPa. In some embodiments, a tensile strength of the ultrahigh-strength steel plate having a high r value of the present invention can be 980-1100MPa, preferably 980-1060MPa.

[0034] In some embodiments, an r value r 90 of the ultrahigh-strength steel plate having a high r value of the present invention is 1.2-1.4.

[0035] In some embodiments, the microhardness HV of the upper surface layer and the lower surface layer of the ultrahigh-strength steel plate having a high r value of the present invention are each independently 100-130.

[0036] In some embodiments, a microhardness HV of the middle layer of the ultrahigh-strength steel plate having a high r value of the present invention is 330-380.

[0037] In some embodiments, an elongation of the ultrahigh-strength steel plate having a high r value of the present invention is ≥ 12%, such as 12-16% or 13-15%.

[0038] In some embodiments, a yield strength of the ultrahigh-strength steel plate having a high r value of the present invention is ≥ 570MPa, such as 570-660MPa.

[0039] Correspondingly, another object of the present invention is to provide a manufacturing method for an ultrahigh-strength steel plate having a high r value, which can produce an ultrahigh-strength steel plate having a high r value by optimizing the process.

[0040] To achieve the above objective, the present invention further provides a manufacturing method for the ultrahigh-strength steel plate having a high r value as described above, which comprises the steps of: smelting and casting; hot rolling; cold rolling after pickling; and annealing; wherein the annealing step comprises: a high-temperature humidification decarburization process: spraying water vapor in the annealing furnace for humidification, simultaneously controlling the heating temperature of the steel plate to 880-950°C and maintaining it for 100-300s, and controlling the dew point in the annealing furnace to be 0°C or higher; an ordinary slow cooling process: cooling the steel plate from a temperature of high-temperature humidification to between 730-770°C at a cooling rate of 3-10°C / s; a quasi-static slow cooling process: slowly cooling the steel plate from 730-770°C to 680-720°C, controlling the cooling temperature range within 40-60°C, with a cooling rate of 0.03-0.1°C / s, and ensuring the slow cooling process lasts for 500s or longer; a rapid cooling process: rapidly cooling the steel plate to 300°C or lower at a cooling rate of ≥50°C / s; an over-aging process: tempering the steel plate at 240-320°C for 150-500s.

[0041] The processes adopted in the manufacturing method of the present invention, especially the unique annealing process, are crucial for the ultrahigh-strength steel plate of the present invention to achieve a high r value.

[0042] Among them, the method of decarburization is humidification by spraying water vapor in the furnace. At high temperature, the following reactions occur to promote the decarburization reaction of the steel plate: C + H 2 O = CO + H 2 ; C + 2H 2 O = CO 2 + 2H 2 . The heating temperature of the steel plate is between 880-950°C and held for 100-300s. The amount of water for humidification is controlled according to the weight of the steel strip passing through per unit time and the required thickness of the decarburized layer, so as to ensure that the dew point in the annealing furnace is 0°C or higher. In some embodiments, the dew point in the annealing furnace is 0-20°C. This high-temperature humidified decarburization process makes the upper surface layer and the lower surface layer of the steel plate respectively become preliminary decarburized layers with a thickness of 100-200µm (i.e., both the upper surface layer and the lower surface layer are decarburized layers). In this process, if the heating temperature of the steel plate is too low, it will lead to insufficient decarburization; if the heating temperature of the steel plate is too high, it will cause excessive grain growth. Therefore, controlling the heating temperature of the steel plate at 880-950°C during the high-temperature humidification decarburization process is beneficial to the decarburization reaction of the steel plate. The reason for selecting a dew point of 0°C or higher in the furnace is that a low dew point in the furnace results in a slow decarburization reaction of the steel plate, which is insufficient to obtain a decarburized layer with sufficient thickness.

[0043] After the above high-temperature humidification decarburization process, the carbon content in the upper surface layer and lower surface layer of the steel plate decreases significantly, which can drop to a level of ≤ 0.05%. At this point, the carbon content in the upper surface layer and lower surface layer of the steel plate is still too high; if austenite is directly cooled and transformed into ferrite, Fe3C will still precipitate. To further reduce the carbon content in the upper surface layer and lower surface layer of the steel plate, a further quasi-static slow cooling process treatment is performed at the Ac1 or a lower temperature of the iron-carbon phase diagram. In this quasi-static slow cooling process, the steel plate is slowly cooled from 730-770°C to 680-720°C at a rate of 0.03-0.1°C / s, and the temperature range of the entire slow cooling process is controlled within 40-60°C, with a duration of ≥ 500s. In some embodiments, a duration of slow cooling is 500-1500s. Within this temperature range, the austenite in the preliminarily decarburized upper surface layer and the preliminarily decarburized lower surface layer of the steel plate first undergoes decomposition to form ferrite; during the slow cooling process, the carbon in ferrite diffuses into the austenite in the untransformed high-carbon region, middle layer, thereby further promoting the decrease of carbon content in the upper surface layer and lower surface layer. When the slow cooling time is sufficient, the carbon content in the upper surface layer and lower surface layer can be reduced to a level close to that of commercial pure iron, so that the upper surface layer regions and lower surface layer regions of the steel of the present invention meet Ti-3.42N-3.98C ≥ 0.

[0044] In the manufacturing method of the present invention, the slow cooling from 730-770°C to 680-720°C is crucial. After decarburization, the carbon content in the austenite of the surface layer decreases, and Ac3 increases, thus leading to the first occurrence of transformation of austenite → ferrite during the slow cooling process from 730-770°C to 680-720°C. When the local carbon content in the upper surface layer and lower surface layer of the steel plate of the present invention decreases to ≤ 0.025%, the contents of Ti, C, and N in the steel plate can meet the level of Ti-3.42N-3.98C ≥ 0, thereby substantially forming an interstitial-free structure in the upper surface layer and lower surface layer, which greatly improves the r value of the upper surface layer and lower surface layer and further enhances the overall r value of the steel plate.

[0045] After the completion of slow cooling, the steel plate is rapidly cooled to 300°C or lower at a cooling rate of ≥ 50°C / s, and tempered at 240-320°C for 150-500s. In some embodiments, a cooling rate of rapid cooling is 50-150°C / s. In some embodiments, the termination temperature of rapid cooling is 240-300°C. During this cooling process, the austenite in the middle layer transforms into martensite structure; in some embodiments, it may also contain a small amount of bainite and retained austenite structures, thereby achieving ultrahigh strength, such as a tensile strength of ≥ 980MPa.

[0046] In some embodiments, an over-aging temperature is 240-300°C.

[0047] In the manufacturing method of the present invention, the smelting, casting, hot rolling and cold rolling processes can all adopt conventional processes, and the present invention does not involve any special improvements to these processes. In an exemplary hot rolling step, a heating and tapping temperature of the steel plate is 1220-1280°C, a finish rolling temperature is 870-930°C, and a coiling temperature is 570-630°C. In an exemplary cold rolling process, a reduction ratio of the cold rolling is controlled to be 40-65%.

[0048] The high-strength steel plate having a high r value and manufacturing method therefor of the present invention have the following advantages and beneficial effects compared with the prior art: The present invention ensures ultrahigh strength by controlling the structure morphology of the middle layer, and at the same time achieves a high r value by controlling the mass percentages of chemical elements in the upper and lower surface layers to respectively meet Ti-3.42N-3.98C ≥ 0, thereby realizing that the ultrahigh-strength steel plate has a high r value.

[0049] From the perspective of composition design, the present invention can substantially achieve an interstitial-free solid solution state in the upper and lower surface layers by adding an appropriate amount of Ti and assisting in fixing N and C with Nb, thereby greatly improving the r value of the surface layers and further affecting the overall r value of the steel plate. This enables the r value of the 980MPa cold-rolled high-strength steel plate to reach 1.2 or higher, thereby significantly reducing the thinning rate during the deformation process and achieving better formability.

[0050] From the perspective of manufacturing process, the high-strength steel plate having a high r value of the present invention adopts a unique annealing process, and undergoes a preliminary high-temperature humidification decarburization process and long-term slow cooling of transformation of austenite → ferrite to achieve carbon content redistribution, thereby being able to reduce the carbon content in the upper and lower surface layers of the ultrahigh-strength steel plate to 0.025% or lower.Description of the Drawing

[0051] Figure 1 schematically shows an annealing process curve of the manufacturing method for the high-strength steel plate having a high r value of an embodiment of the present invention.Detailed Description

[0052] The high-strength steel plate having a high r value and the manufacturing method therefor of the present invention will be further explained and illustrated below with reference to specific embodiments and drawings. However, such explanations and descriptions do not constitute an improper limitation on the technical solution of the present invention.

[0053] The high-strength steel plates having a high r value of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2 are prepared through the following steps: (1) Smelting: molten iron is smelted in a converter, and its composition is further optimized through a refining process; (2) Continuous casting to obtain slabs: the composition contents of the slabs in each Example and Comparative Example are as shown in Table 1; (3) Hot rolling: a conventional hot rolling process is adopted; specifically, a heating and tapping temperature of the steel plates is controlled at 1220-1280°C, a finish rolling temperature of the steel plates is controlled at 870-930°C, and a coiling temperature of the steel plates is controlled at 570-630°C; (4) Cold rolling after pickling: a reduction ratio of cold rolling is controlled at 40-65%; (5) Annealing, specifically comprises: a high-temperature humidification decarburization process: spraying water vapor in the annealing furnace for humidification, simultaneously controlling the heating temperature of the steel plate to 880-950°C and maintaining it for 100-300s, and controlling the dew point in the annealing furnace to be 0°C or higher; an ordinary slow cooling process: cooling the steel plate from a temperature of high-temperature humidification to between 730-770°C at a cooling rate of 3-10°C / s; a quasi-static slow cooling process: slowly cooling the steel plate from 730-770°C to 680-720°C, controlling the cooling temperature range within 40-60°C, with a cooling rate of 0.03-0.1 °C / s, and ensuring the slow cooling process lasts for 500s or longer; a rapid cooling process: rapidly cooling the steel plate to 300°C or lower at a cooling rate of ≥50°C / s; an over-aging process: tempering the steel plate at 240-320°C for 150-500s.

[0054] Among them, the parameters of annealing process of Examples 1-6 and Comparative Example 1 are listed in Table 2-1. Comparative Example 2 does not adopt the annealing process of the present invention, but instead adopts a conventional quenching + tempering process, with the specific process parameters listed in Table 2-2.

[0055] Table 1 lists the mass percentage ratios of chemical elements in Examples 1-6 and Comparative Examples 1-2 of the present invention. Table 1 (wt%, with a balance of Fe and other inevitable impurities excluding S, P, and N)CSiMnBPSAlNTiNbCrMoEx. 10.080.012.7-0.0120.0020.020.00250.160.015--Ex. 20.120.42-0.0120.00150.040.0020.180.020.2-Ex. 30.1611.50.0020.010.00150.050.0030.20.030.10.1Ex. 40.181.510.0030.010.0010.060.0040.230.040.20.1Ex. 50.21.70.70.0050.010.00080.0350.0050.260.050.30.2Ex. 60.21.70.70.0050.010.00080.0350.0050.260.050.30.2Comp. Ex. 10.120.42-0.0120.00150.040.0020.030.020.2-Comp. Ex. 20.120.42-0.0120.00150.040.0020.180.020.2-

[0056] Table 2-1 lists the parameters of specific process of the annealing step for the high-strength steel plates having a high r value of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2. Table 2-1No.Heating Temperature of Strip Steel of High-te mperature Humidifi cation Decarbu rization (°C)Dew Point of High-temperature Humidification Decarburization in the Furnace (°C)Time of High-temperature Humidified Decarburization (s)Cooling Rate of Ordinary Slow Cooling (°C)Starting Temperature of Quasi-static Slow Cooling (°C)Termination Temperature of Quasi-static Slow Cooling (i.e., Starting Temperature of Rapid Cooling) (°C)Time of Quasi-static Slow Cooling (s)Cooling Rate of Quasi-static Slow Cooling (°C / s)Cooling Rate of Rapid Cooling (°C / s)Termination Temperature of Rapid Cooling (°C)Temperature of Over-aging (°C)Time of Over-aging (s)Ex. 188020170107507005000.150300300150Ex. 29001710077306906000.0770240240200Ex. 39101320057406808000.0890300300500Ex. 493010300375070015000.03100260260350Ex. 59405250676071012000.04120280280400Ex. 69500270577072010000.05150300300300Comp. Ex. 19001712047306856000.0870240240200 Table 2-2 No.Steel GradeTemperature of SoakingDew Point of Soaking Zone, °CTime of SoakingCooling Rate of Slow Cooling (°C)Starting Temperature of Rapid Cooling (°C)Cooling Rate of Rapid Cooling (°C / s)Termination Temperature of Rapid Cooling (°C)Temperature of Over-aging (°C)Time of Over-aging (s)Comp. Ex. 2A2210-351207750100240240200

[0057] Sampling is conducted on the steel plates of Examples 1-6 and Comparative Examples 1-2. The carbon content and contents of other components in the upper and lower surface layers of the steel plates are detected using a GDS (Glow Discharge Spectrometer), and their microstructures are observed and examined with a metallographic microscope. The observed and examined results are listed in Table 3. Table 3No.Total Thickness of Steel Plate (mm)Carbon Content in the Upper and Lower Surface Layers (wt%)Value of Ti-3.42N-3.98C in the Upper and Lower Surface Layers (Calculated Value)Average Diameter of Ferrite Grains in the Upper and Lower Surface Layers (Micrometers)Thickness of the Upper and Lower Surface Layers (µm)Proportion of Ferrite Phase in the Upper and Lower Surface Layers (%)Microstructure Composition of the Middle LayerEx. 11.20.02 / 0.02 10.072 / 0.0688 / 8100 / 10399 / 9957% Ferrite + 40% Tempered Martensite + 3% BainiteEx. 21.40.022 / 0.0 210.094 / 0.097 / 7.1120 / 11798 / 9857% Ferrite + 41% Tempered Martensite + 2% BainiteEx. 31.50.021 / 0.0 220.106 / 0.1028 / 8.2150 / 15398 / 9760% Ferrite + 40% Tempered MartensiteEx. 41.60.023 / 0.0 240.125 / 0.1219 / 8.8170 / 16597 / 9757% Ferrite + 43% Tempered MartensiteEx. 51.80.025 / 0.0 220.143 / 0.1557 / 6.9200 / 18999 / 9858% Ferrite +42% Tempered MartensiteEx. 62.00.025 / 0.0 250.143 / 0.1438 / 8180 / 18499 / 9957% Ferrite +43% MartensiteComp. Ex. 11.60.02 / 0.02 1-0.056 / -0.06 11 / 10.6100 / 9597 / 9760% Ferrite +40% Tempered MartensiteComp. Ex. 21.40.05 / 0.04 9 -0.026 / -0.022 9 / 9.42 / 395 / 9560% Ferrite +40% Tempered MartensiteNote: when the value of Ti-3.42N-3.98C in the upper and lower surface layers is greater than 0, it indicates that there are no interstitial atoms in the upper and lower surface layers; when the value of Ti-3.42N-3.98C is less than 0, it indicates that there are interstitial atoms in the upper and lower surface layers. The two columns of data in the table represent the values of the upper surface layer and the lower surface layer respectively.

[0058] To verify the implementation effect of the present invention, sampling is conducted on the steel plates of Examples 1-6 and Comparative Examples 1-2, their properties are tested, and the test results are listed in Table 4. Specifically: Tensile tests for mechanical property testing are performed along the direction perpendicular to the rolling direction of the steel plates. According to JIS5# standard, plate-shaped tensile specimens with a gauge length of 50 mm and a gauge area width of 25 mm are sampled, and the tests are carried out on a tensile testing machine. Data on strength, elongation, and r90 value can be obtained during the test, and the r90 value is taken within a range of tensile strain of 4-6%.

[0059] For hardness testing, HV hardness is measured using a microhardness tester with a loading load of 150 g and a loading time of 15 s.

[0060] Table 4 lists the parameters of relevant performance of the high-strength steel plates having high r value of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2. Table 4No.σs (MPa)σb (MPa)Elongation (%)r 90 (4-6%)Hardness of the Upper and Lower Surface Layers (HV)Hardness of the Middle Layer (HV)Ex. 1610985141.25100 / 110330Ex. 25701000141.3120 / 115340Ex. 3620990151.2110 / 110350Ex. 46401010131.4130 / 125360Ex. 5650104014.51.35125 / 130370Ex. 66601060141.3120 / 125380Comp. Ex. 16001020140.85 240 / 235 300 Comp. Ex. 2620104513.50.82 300 / 305 310 Note: The two columns of data for the hardness of the upper and lower surface layers in the table represent the values of hardness of the upper surface layer and the lower surface layer respectively.

[0061] It can be seen from Table 4 that a tensile strength of the high-strength steel plates in Examples 1-6 of the present invention is all ≥ 985 MPa, their r value r 90 are ≥ 1.2, their microhardness HV of the upper and lower surface layers are ≤ 130, and their microhardness HV of the middle layer are ≥ 330. This indicates that the high-strength steel plates described in the present invention can possess both high strength and high formability.

[0062] The addition amount of Ti in Comparative Example 1 does not meet the requirements of the present invention; even though it adopts the process of the present invention, it cannot achieve a high r value.

[0063] Although Comparative Example 2 adopts the composition of Example 2 of the present invention, it does not adopt the annealing process of the present invention but instead adopts a conventional quenching and tempering process. Therefore, it also cannot obtain a high r value.

[0064] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0065] It should also be noted that the embodiments listed above are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments. Similar changes or modifications made therewith are directly derived from the contents disclosed in the present invention or can be easily associated with by those skilled in the art and should all fall within the scope of protection of the present invention.

Claims

1. An ultrahigh-strength steel plate having a high r value, comprising, in the thickness direction, an upper surface layer, a middle layer, and a lower surface layer; main bodies of the microstructures of the upper surface layer and the lower surface layer are ferrite, and a carbon content of the upper surface layer and a carbon content of the lower surface layer are both less than or equal to 0.025%; mass percentages of chemical elements in the upper surface layer and in the lower surface layer respectively meet: Ti-3.42N-3.98C ≥ 0; and a microstructure of the middle layer comprises ferrite and tempered martensite.

2. The ultrahigh-strength steel plate having a high r value according to claim 1, wherein an average grain diameter of ferrite in the upper surface layer and the lower surface layer is ≤ 15µm.

3. The ultrahigh-strength steel plate having a high r value according to claim 1, wherein a volume fraction of ferrite in the upper surface layer and the lower surface layer is ≥ 97%.

4. The ultrahigh-strength steel plate having a high r value according to claim 1, wherein the microstructure of the middle layer further comprises bainite.

5. The ultrahigh-strength steel plate having a high r value according to claim 4, wherein a volume fraction of tempered martensite + bainite in the middle layer is ≥ 40%; preferably, in the middle layer, a volume fraction of ferrite is 55-60%, a volume fraction of tempered martensite is 40-45%, and a volume fraction of bainite is 0-5%.

6. The ultrahigh-strength steel plate having a high r value according to claim 1, wherein the ferrite in the upper surface layer and the lower surface layer comprises carbides, and types of the carbides are Ti (C, N) and Nb (C, N).

7. The ultrahigh-strength steel plate having a high r value according to claim 1, wherein thicknesses of the upper surface layer and the lower surface layer are respectively 100-200µm.

8. The ultrahigh-strength steel plate having a high r value according to claim 1, wherein a thickness of the ultrahigh-strength steel plate having a high r value is 0.8-2.5mm.

9. The ultrahigh-strength steel plate having a high r value according to claim 1, wherein a sum of thicknesses of the upper surface layer and the lower surface layer does not exceed 40% of a total thickness of the steel plate.

10. The ultrahigh-strength steel plate having a high r value according to claim 1, comprising Fe and inevitable impurities, wherein it further comprises the following chemical elements with mass percentages as follows: C: 0.08-0.20%; Si: 0.01-1.7%; Mn: 0.7-2.7%; Ti: 0.16-0.26%; Nb: 0.015-0.05%; Al: 0.02-0.06%; and N≤0.005%.

11. The ultrahigh-strength steel plate having a high r value according to claim 10, wherein the mass percentages of each chemical elements are: C: 0.08-0.20%; Si: 0.01-1.7%; Mn: 0.7-2.7%; Ti: 0.16-0.26%; Nb: 0.015-0.05%; Al: 0.02-0.06%; N≤0.005%; and a balance of Fe and inevitable impurities.

12. The ultrahigh-strength steel plate having a high r value according to claim 10 or 11, wherein its chemical elements further comprise at least one of Cr, Mo, and B; wherein B ≤ 0.005%, and Cr + Mo ≤ 0.5%; preferably, B ≤ 0.005%, Cr ≤ 0.3%, and Mo ≤ 0.2%.

13. The ultrahigh-strength steel plate having a high r value according to claim 1, wherein its tensile strength is ≥ 980MPa, its r value r90 is ≥ 1.2, a microhardness HV of the upper surface layer and the lower surface layer is ≤ 130, and a microhardness HV of the middle layer is ≥ 330.

14. A manufacturing method for the ultrahigh-strength steel plate having a high r value according to any one of claims 1 to 13, comprising the steps of: smelting and casting; hot rolling; cold rolling after pickling; and annealing; wherein the annealing step comprises: a high-temperature humidification decarburization process: spraying water vapor in the annealing furnace for humidification, simultaneously controlling the heating temperature of the steel plate to 880-950°C and maintaining it for 100-300s, and controlling the dew point in the annealing furnace to be 0°C or higher; an ordinary slow cooling process: cooling the steel plate from a temperature of high-temperature humidification to between 730-770°C at a cooling rate of 3-10°C / s; a quasi-static slow cooling process: slowly cooling the steel plate from 730-770°C to 680-720°C, controlling the cooling temperature range within 40-60°C, with a cooling rate of 0.03-0.1°C / s, and ensuring the slow cooling process lasts for 500s or longer; a rapid cooling process: rapidly cooling the steel plate to 300°C or lower at a cooling rate of ≥50°C / s; and an over-aging process: tempering the steel plate at 240-320°C for 150-500s.

15. The manufacturing method according to claim 14, wherein the manufacturing method has one or more of the following features: (1) the hot rolling is to control a heating and tapping temperature of the steel plate to be 1220-1280°C, a finish rolling temperature to be 870-930°C, and a coiling temperature to be 570-630°C; (2) a reduction ratio of the cold rolling is 40-65%; (3) a dew point in an annealing furnace is 0-20°C; (4)the slow cooling lasts for 500-1500s; (5) a cooling rate of the rapid cooling is 50-150°C / s; and (6) a tempering temperature is 240-300°C.

Images