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

Ultra-high-strength steel plate with high R value and manufacturing method thereof Technical Field

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

[0002] The r-value represents the steel plate's resistance to thinning during the drawing process. For high-formability deep-drawing steel plates with tensile strengths below 350 MPa (such as interstitial-free steels), the r-value is a key technical indicator. A high r-value indicates superior drawability.

[0003] However, the existing high-strength steel plates are not used for deep drawing, and the r-value of the steel plates and the performance represented by it are not paid attention to.

[0004] For example, Chinese patent publication number CN101768695A, published on July 7, 2010, and titled "1000 MPa-grade Ti microalloyed ultrafine-grained cold-rolled dual-phase steel and its preparation process," discloses a chemical composition of 0.03-0.2% C, 0.2-0.8% Si, 1.2-2.0% Mn, 0.03-0.15% Ti, ≤0.02% P, ≤0.015% S, 0.02-0.15% Al, with the remainder consisting of Fe and unavoidable impurities. After hot and cold rolling, and then intercritical annealing at a cooling rate of less than 50°C / s, a cold-rolled dual-phase steel with a strength exceeding 980 MPa is obtained. This high-strength steel does not address the r-value of the steel plate.

[0005] For example, Chinese patent publication number CN101363099A, published on February 11, 2009, and titled "A Cold-Rolled Dual-Phase Steel Plate with a Tensile Strength of 1000 MPa 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 760-820°C at a cooling rate of 40-50°C / s and overaged at 240-320°C for 180-300s. This high-strength steel does not address the r-value of the plate.

[0006] Xiong Ziliu et al. introduced a duplex steel with a tensile strength of 980 MPa in "Forming Performance of High-Strength Dual-Phase Steel" (Metal Heat Treatment, Vol. 46, No. 5, 2021), but its r value is lower than 0.9.

[0007] Ge Delong et al. introduced 1000-1400MPa martensitic steel in "Maximum Bending Angle of 1000MPa Ultra-High Strength Martensitic Steel Based on Three-Point Bending" (Journal of Plasticity Engineering, Vol. 21, No. 4, 2014), whose r value is less than 0.7.

[0008] Summary of the Invention

[0009] One of the objectives of the present invention is to provide a high-r-value ultra-high-strength steel plate. This steel plate has ultra-high strength and a high r-value. The high-r-value ultra-high-strength steel plate undergoes less thickness reduction during deformation, thereby exhibiting better formability regardless of global tensile deformation or local deformation.

[0010] In the high-r-value ultra-high-strength steel plates described herein, the r-value represents the steel plate's resistance to thickness reduction during the drawing process. Steel plates with high r-values ​​are less likely to reduce thickness during tensile deformation, thus being less susceptible to fracture. For ultra-high-strength steel plates, a high r-value reduces localized thickness reduction during severe local deformation, slowing the necking process and thus improving the steel plate's formability.

[0011] In order to achieve the above-mentioned objectives, the present invention proposes a high-r-value ultra-high-strength steel plate, which includes an upper surface layer, an intermediate layer and a lower surface layer in the thickness direction; wherein the main body of the microstructure of the upper surface layer and the lower surface layer is ferrite, the carbon content of the upper surface layer and the lower surface layer is ≤0.025%, and the mass percentage of the chemical elements in the upper surface layer and the lower surface layer respectively satisfies: Ti-3.42N-3.98C ≥ 0; the microstructure of the intermediate layer includes ferrite and tempered martensite.

[0012] One of the core points of the present invention is that a high r-value is achieved by ensuring that the mass percentages of chemical elements in the upper and lower surface layers respectively satisfy Ti-3.42N-3.98C ≥ 0, and the carbon content of the upper and lower surface layers is ≤ 0.025%. At the same time, the high strength of the steel plate as a whole is mainly guaranteed by the structure of the intermediate layer. In some embodiments, the carbon content of the upper and lower surface layers is 0.02-0.025%. The mass percentages of chemical elements in the upper and lower surface layers respectively satisfy: Ti-3.42N-3.98C ≥ 0.060, preferably ≥ 0.068. In some embodiments, the mass percentages of chemical elements in the upper and lower surface layers respectively satisfy: 0.16 ≥ Ti-3.42N-3.98C ≥ 0.06.

[0013] Furthermore, in the high r-value ultra-high strength steel plate of the present invention, the average grain diameter of the ferrite in the upper and lower surface layers is ≤15 microns. In some embodiments, in the high r-value ultra-high strength steel plate of the present invention, the average grain diameter of the ferrite in the upper and lower surface layers may be ≤10 microns. In some embodiments, in the high r-value ultra-high strength steel plate of the present invention, the average grain diameter of the ferrite in the upper and lower surface layers is 6-10 microns.

[0014] Furthermore, in the high r-value ultra-high strength steel plate of the present invention, the volume ratio of ferrite in the upper and lower surface layers is ≥ 97%. In some embodiments, the volume ratio of ferrite in the upper and lower surface layers is 97-99%.

[0015] Furthermore, in the high r-value ultra-high strength steel plate of the present invention, the microstructure of the intermediate layer may further include bainite.

[0016] Furthermore, in the high r-value ultra-high-strength steel plate of the present invention, regardless of whether the intermediate layer contains bainite, the volume ratio of tempered martensite + bainite is ≥ 40%. In some embodiments, the intermediate layer contains ferrite and at least one of tempered martensite and bainite. In some embodiments, the volume ratio of ferrite in the intermediate layer is 55-60%, preferably 57-60%. In some embodiments, the volume ratio of tempered martensite in the intermediate layer is 40-45%, preferably 40-43%. In some embodiments, the volume ratio of bainite in the intermediate layer is 0-5%, preferably 0-3%.

[0017] Furthermore, in the high r-value ultra-high strength steel plate described in the present invention, the ferrite of the upper surface layer and the lower surface layer contains carbides, and the types of the carbides are Ti(C, N) and Nb(C, N).

[0018] Furthermore, in the high r-value ultra-high strength steel plate of the present invention, the thickness of the upper surface layer and the lower surface layer are both 100-200 microns.

[0019] Furthermore, in the high r-value ultra-high strength steel plate of the present invention, the sum of the thickness of the upper surface layer and the lower surface layer does not exceed 40% of the total thickness of the steel plate.

[0020] Furthermore, the thickness of the high r-value ultra-high strength steel plate of the present invention is 0.8-2.5 mm, preferably 1.2-2.0 mm.

[0021] Furthermore, the high r-value ultra-high strength steel plate of the present invention contains Fe and unavoidable impurity elements, and further contains the following chemical elements in the following mass percentages:

[0022] 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%.

[0023] Furthermore, in the high r-value ultra-high strength steel plate of the present invention, the mass percentage of each chemical element is:

[0024] 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%; the balance is Fe and unavoidable impurities.

[0025] In some embodiments, the design principles of the chemical elements of the high r-value ultra-high strength steel plate of the present invention are specifically as follows:

[0026] C: In the high-r-value, high-strength steel plates described herein, C can improve steel strength by affecting martensite hardness. If the carbon content in the steel is too low, the strength after quenching will be low; however, a higher carbon content will increase the strength after quenching. However, excessive carbon content is detrimental to achieving a high r-value. Therefore, the C content in the high-r-value, ultra-high-strength steel plates described herein is controlled between 0.08% and 0.20%.

[0027] Si: In the high-r-value, high-strength steel plates described herein, Si plays a role in solid solution strengthening, inhibiting Fe3C precipitation, and promoting the formation of retained austenite. For this reason, the Si content is controlled within a range of 0.01-1.7%. In some embodiments, the Si content is 0.4-1.7%.

[0028] Mn: Mn is the main element that improves the hardenability of steel. The Mn content matches the cooling capacity of the cooling method used. When the Mn content is too low, the steel plate cannot be hardened, and thus cannot obtain high strength. If the Mn content is too high, it is not conducive to the carbon equivalent. Therefore, in the high r value ultra-high strength steel plate described in the present invention, the Mn element content is controlled between 0.7-2.7%. In some embodiments, the Mn content is 1.0-2.7%.

[0029] Ti: In the high-r-value, high-strength steel plates described herein, the Ti content is closely related to the r-value. Too low a Ti content is insufficient to fully anchor the carbon and nitrogen in the upper and lower surface layers, resulting in a low r-value. Too high a Ti content results in an excess of Ti. Therefore, in the high-r-value, ultra-high-strength steel plates described herein, the Ti content is controlled between 0.16% and 0.26%.

[0030] Nb: In the high-r-value, high-strength steel plates described herein, Nb helps fix carbon and nitrogen, forming Nb(C,N) and ensuring that neither carbon nor nitrogen remains in solid solution in the upper and lower surface regions. Therefore, the Nb content in the high-r-value, ultra-high-strength steel plates described herein is controlled between 0.015% and 0.05%.

[0031] Al: Its main function in the present invention is deoxidation during the smelting process, and its content can range from 0.02 to 0.06%.

[0032] N: In the present invention, N is not conducive to forming a composition ratio of Ti-3.42N-3.98C ≥ 0 in the upper and lower surface layers, and is not conducive to forming a state without interstitial atoms locally, so it is necessary to control N ≤ 0.005%.

[0033] In the present invention, unavoidable impurities are primarily S and P. Their contents are desirably as low as possible, if process conditions permit. In the high-r-value, ultra-high-strength steel plate of the present invention, the P content is ≤ 0.015%. In the high-r-value, ultra-high-strength steel plate of the present invention, the S content is ≤ 0.003%.

[0034] It can be understood that the basic composition of the high-r-value, high-strength steel plate described in the present invention is: C: 0.08-0.20%; Si: 0.01-1.7%; Mn: 0.7-2.7%. This basic composition provides the foundation for the steel plate's strength, but it does not guarantee a high r-value. To achieve a high r-value, the steel plate described in the present invention also contains: Ti: 0.16-0.26%, Nb: 0.015-0.05%, and N ≤ 0.005%.

[0035] Furthermore, the high r-value ultra-high-strength steel plate of the present invention further comprises at least one of Cr, Mo, and B, wherein B is ≤ 0.005% and Cr + Mo is ≤ 0.5%. In some embodiments, when present, the B content does not exceed 0.005%, the Cr content does not exceed 0.3%, and the Mo content does not exceed 0.2%.

[0036] In the appropriate amount of high r-value ultra-high strength steel plate described in the present invention, the alloying elements Cr, Mo, and B can effectively cooperate with manganese, thereby improving the hardenability, so as to match the hardenability and rapid cooling capacity of the steel, and further avoid the inability to harden or the carbon equivalent being too high.

[0037] Furthermore, the tensile strength of the high r value ultra-high strength steel plate of the present invention is ≥980MPa, and its r value r 90≥1.2, the microhardness of the upper and lower surface layers HV ≤ 130, and the microhardness of the intermediate layer HV ≥ 330. Preferably, the tensile strength of the high r-value ultra-high-strength steel plate of the present invention is ≥ 1020 MPa. In some embodiments, the tensile strength of the high r-value ultra-high-strength steel plate of the present invention can be 980-1100 MPa, preferably 980-1060 MPa.

[0038] In some embodiments, the r value of the high r value ultra-high strength steel plate of the present invention is 90 It is 1.2-1.4.

[0039] In some embodiments, the microhardness HV of the upper surface layer and the lower surface layer of the high r-value ultra-high strength steel plate of the present invention are independently 100-130.

[0040] In some embodiments, the microhardness HV of the middle layer of the high r-value ultra-high strength steel plate of the present invention is 330-380.

[0041] In some embodiments, the elongation of the high r-value ultra-high strength steel plate of the present invention is ≥12%, such as 12-16% or 13-15%.

[0042] In some embodiments, the yield strength of the high r-value ultra-high strength steel plate of the present invention is ≥570 MPa, such as 570-660 MPa.

[0043] Accordingly, another object of the present invention is to provide a method for manufacturing a high r-value ultra-high strength steel plate, which can produce a high r-value ultra-high strength steel plate by optimizing the process.

[0044] In order to achieve the above object, the present invention also proposes a method for manufacturing a high r-value ultra-high strength steel plate as described above, comprising the steps of: smelting and casting; hot rolling; cold rolling after pickling; annealing; wherein the annealing step comprises:

[0045] High-temperature humidification and decarburization process: spray water vapor in the annealing furnace to increase humidity, and at the same time control the steel plate heating temperature to 880-950℃ for 100-300s, and control the dew point in the annealing furnace to be above 0℃;

[0046] Ordinary slow cooling process: the steel plate is cooled from the high temperature and humidification temperature to between 730-770℃ at a cooling rate of 3-10℃ / s;

[0047] Quasi-static slow cooling process: The steel plate is slowly cooled from 730-770℃ to 680-720℃, the cooling temperature range is controlled at 40-60℃, the cooling rate is 0.03-0.1℃ / s, and the slow cooling process is ensured to last for more than 500s;

[0048] Rapid cooling process: The steel plate is rapidly cooled to below 300℃ at a cooling rate of ≥50℃ / s;

[0049] Overaging process: the steel plate is tempered at 240-320℃ for 150-500s.

[0050] The processes used in the manufacturing method of the present invention, especially the unique annealing process, are crucial for obtaining a high r-value for the ultra-high strength steel plate of the present invention.

[0051] The decarburization method involves spraying water vapor within the furnace to increase humidity. At high temperatures, the following reactions occur, promoting decarburization of the steel sheet: C + H2O = CO + H2; C + 2H2O = CO2 + 2H2. The steel sheet is heated to a temperature between 880°C and 950°C and maintained for 100-300 seconds. The amount of water used for humidification is controlled based on the weight of the steel strip passing through the furnace and the thickness of the desired decarburization layer, ensuring the dew point within the annealing furnace is above 0°C. In some embodiments, the dew point within the annealing furnace is between 0°C and 20°C. This high-temperature humidification decarburization process creates a preliminary decarburization layer with a thickness of 100-200 microns on the upper and lower surfaces of the steel sheet, respectively (i.e., both the upper and lower surfaces are decarburized layers). During this process, if the steel sheet heating temperature is too low, insufficient decarburization will occur; if the steel sheet heating temperature is too high, excessive grain growth will occur. Therefore, controlling the steel sheet heating temperature between 880°C and 950°C during the high-temperature humidification decarburization process is beneficial for promoting decarburization of the steel sheet. The reason for selecting the dew point in the furnace to be above 0°C is that if the dew point in the furnace is low, the decarburization reaction of the steel plate will be slow, which is not enough to obtain a decarburization layer of sufficient thickness.

[0052] After the above-mentioned high-temperature humidification and decarburization process, the carbon content of the upper and lower surfaces of the steel plate is significantly reduced, and can be reduced to a level of ≤0.05%. At this time, the carbon content of the upper and lower surfaces of the steel plate is still too high, and the austenite is directly cooled and transformed into ferrite, and Fe3C will still precipitate. In order to further reduce the carbon content of the upper and lower surfaces of the steel plate, a quasi-static slow cooling process is further carried out below the Ac1 temperature of the iron-carbon phase diagram. During 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. The temperature range of the entire slow cooling process is controlled at 40-60°C, and the duration is ≥500s. In some embodiments, the slow cooling duration is 500-1500s. Within this temperature range, austenite decomposition occurs first on the upper and lower surfaces of the steel plate that has undergone preliminary decarburization to form ferrite. During the slow cooling process, the carbon in the ferrite diffuses into the austenite in the high-carbon zone (middle layer) that has not undergone transformation, thereby further causing the carbon content of the upper and lower surfaces to decrease. If the slow cooling time is sufficient, the carbon content of the upper and lower surfaces can be reduced to a level close to that of industrial pure iron, so that the upper and lower surface areas of the invented steel meet the requirements of Ti-3.42N-3.98C≥0.

[0053] In the manufacturing method described herein, slow cooling from 730-770°C to 680-720°C is crucial. After decarburization, the decarburized austenite content in the surface layer decreases, while Ac3 increases. Consequently, the austenite to ferrite transformation occurs first during the slow cooling process from 730-770°C to 680-720°C. When the local carbon content in the upper and lower surface layers of the steel plate of the present invention is reduced to ≤0.025%, the Ti, C, and N contents in the steel plate meet the requirement of Ti-3.42N-3.98C ≥ 0, effectively forming an interstitial-free structure in the upper and lower surface layers. This significantly increases the r-value of the upper and lower surface layers, and consequently, the r-value of the steel plate as a whole.

[0054] After slow cooling, the steel plate is rapidly cooled to below 300°C at a cooling rate of ≥50°C / s and tempered at 240-320°C for 150-500s. In some embodiments, the rapid cooling rate is 50-150°C / s. In some embodiments, the termination temperature of the rapid cooling is 240-300°C. During this cooling process, the austenite in the intermediate layer transforms into martensite, which, in some embodiments, may also contain a small amount of bainite and retained austenite, thereby achieving ultra-high strength, for example, a tensile strength of ≥980 MPa.

[0055] In some embodiments, the overaging temperature is 240-300°C.

[0056] The smelting, casting, hot rolling, and cold rolling processes in the manufacturing method described herein can all be conventional processes, and the present invention does not specifically improve these processes. In an exemplary hot rolling step, the steel plate is heated out of the furnace at a temperature of 1220-1280°C, the finishing rolling temperature is 870-930°C, and the coiling temperature is 570-630°C. In an exemplary cold rolling step, the cold rolling reduction is controlled to be 40-65%.

[0057] The high-r-value high-strength steel plate and its manufacturing method of the present invention have the following advantages and beneficial effects compared with the prior art:

[0058] The present invention ensures ultra-high strength by controlling the organizational morphology of the middle layer, and at the same time achieves a high r value by controlling the mass percentage of chemical elements in the upper and lower surface layers to satisfy Ti-3.42N-3.98C≥0, thereby achieving an ultra-high strength steel plate with a high r value.

[0059] From the perspective of composition design, the present invention can substantially achieve a solid solution state without interstitial atoms in the upper and lower surface layers by adding an appropriate amount of Ti and assisting with Nb to fix N and C, thereby greatly improving the r value of the surface layer, which in turn affects the r value of the entire steel plate, so that the r value of the 980MPa cold-rolled high-strength steel plate reaches above 1.2, thereby greatly reducing the thinning rate during the deformation process, thereby obtaining better formability.

[0060] From the perspective of manufacturing process, the high-r-value high-strength steel plate described in the present invention adopts a unique annealing process, undergoing an initial high-temperature humidification decarburization process and a long-term slow cooling austenite to ferrite transformation, so that the carbon content is redistributed, thereby reducing the carbon content of the upper and lower surface layers of the ultra-high-strength steel plate to below 0.025%. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] FIG1 schematically shows an annealing process curve of a method for manufacturing a high-r-value high-strength steel plate according to an embodiment of the present invention. DETAILED DESCRIPTION

[0062] The high r-value high strength steel plate and the manufacturing method thereof of the present invention will be further explained and illustrated below in conjunction with the accompanying drawings and specific embodiments. However, such explanation and illustration do not constitute an undue limitation to the technical solution of the present invention.

[0063] The high r-value high strength steel plates of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2 were prepared by the following steps:

[0064] (1) Smelting: Molten iron is smelted in a converter and further optimized in composition through a refining process;

[0065] (2) Slabs were obtained by continuous casting. The composition and content of the slabs in each embodiment and comparative example are shown in Table 1.

[0066] (3) Hot rolling: conventional hot rolling process is used. Specifically, the steel plate heating furnace temperature is controlled to be 1220-1280°C, the final rolling temperature is 870-930°C, and the coiling temperature is 570-630°C;

[0067] (4) Cold rolling after pickling: control the cold rolling reduction rate to 40-65%;

[0068] (5) Annealing, specifically including:

[0069] High-temperature humidification and decarburization process: spray water vapor in the annealing furnace to increase humidity, and at the same time control the steel plate heating temperature to 880-950℃ for 100-300s, and control the dew point in the annealing furnace to be above 0℃;

[0070] Ordinary slow cooling process: the steel plate is cooled from the high temperature and humidification temperature to between 730-770℃ at a cooling rate of 3-10℃ / s;

[0071] Quasi-static slow cooling process: The steel plate is slowly cooled from 730-770℃ to 680-720℃, the cooling temperature range is controlled at 40-60℃, the cooling rate is 0.03-0.1℃ / s, and the slow cooling process is ensured to last for more than 500s;

[0072] Rapid cooling process: The steel plate is rapidly cooled to below 300℃ at a cooling rate of ≥50℃ / s;

[0073] Overaging process: the steel plate is tempered at 240-320℃ for 150-500s.

[0074] The annealing process parameters of Examples 1-6 and Comparative Example 1 are listed in Table 2-1. Comparative Example 2 did not adopt the annealing process of the present invention, but adopted a conventional quenching + tempering process, and the specific process parameters are listed in Table 2-2.

[0075] Table 1 lists the mass percentages of the chemical elements of Examples 1-6 of the present invention and Comparative Examples 1-2.

[0076] Table 1. (wt%, the balance is Fe and other inevitable impurities except S, P, and N)

[0077] Table 2-1 lists the specific process parameters of the annealing step of the high r-value high strength steel plates of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2.

[0078] Table 2-1

[0079] Table 2-2

[0080] The steel plates of Examples 1-6 and Comparative Examples 1-2 were sampled, and the carbon content and other component contents of the upper and lower surfaces of the steel plates were detected using GDS (glow discharge spectrometer), and their microstructures were observed and detected using a metallographic microscope. The observation and detection results are listed in Table 3.

[0081] Table 3.

[0082] Note: When the Ti-3.42N-3.98C value 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 Ti-3.42N-3.98C value 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 and lower surface layers, respectively.

[0083] In order to verify the effect of the present invention, samples were taken from the steel plates of Examples 1-6 and Comparative Examples 1-2, and their properties were tested. The test results are listed in Table 4.

[0084] Mechanical properties testing involves tensile testing of the steel plate perpendicular to the rolling direction. Testing is performed on a JIS 5# standard plate tensile specimen with a gauge length of 50mm and a gauge area width of 25mm. Strength, elongation, and R90 values ​​are obtained during the test. The R90 value is measured within a tensile strain range of 4-6%.

[0085] The hardness test was performed using a microhardness tester to test the HV hardness with a loading load of 150 g and a loading time of 15 s.

[0086] Table 4 lists the relevant performance parameters of the high r-value high strength steel plates of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2.

[0087] Table 4.

[0088] Note: The upper and lower surface hardness in the table have two columns of data representing the hardness values ​​of the upper surface and lower surface respectively.

[0089] It can be seen from Table 4 that the tensile strength of the high-strength steel plates of Examples 1-6 of the present invention are all ≥985 MPa, and their r values ​​r 90 ≥1.2, the microhardness of the upper and lower surface layers HV≤130, and the microhardness of the middle layer HV≥330, which shows that the high-strength steel plate described in the present invention can have both high strength and high formability.

[0090] The amount of Ti added in Comparative Example 1 does not meet the requirements of the present invention, and even if the process of the present invention is used, a high r value cannot be achieved.

[0091] Although Comparative Example 2 uses the composition of Example 2 of the present invention, it does not use the annealing process of the present invention but uses a conventional quenching and tempering process, and therefore cannot obtain a high r value.

[0092] 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 recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0093] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A high r-value ultra-high strength steel plate, characterized in that: In the thickness direction, it includes an upper surface layer, an intermediate layer and a lower surface layer; the main body of the microstructure of the upper surface layer and the lower surface layer is ferrite, and the carbon content of the upper surface layer and the lower surface layer is ≤0.025%; the mass percentage of the chemical elements in the upper surface layer and the lower surface layer respectively satisfies: Ti-3.42N-3.98C≥0; the microstructure of the intermediate layer includes ferrite and tempered martensite.

2. The high r-value ultra-high strength steel plate according to claim 1, characterized in that: The average grain diameter of the ferrite in the upper surface layer and the lower surface layer is ≤15 microns.

3. The high r-value ultra-high strength steel plate according to claim 1, characterized in that: The volume ratio of ferrite in the upper surface layer and the lower surface layer is ≥97%.

4. The high r-value ultra-high strength steel plate according to claim 1, characterized in that: The microstructure of the intermediate layer also includes bainite.

5. The high r-value ultra-high strength steel plate according to claim 4, characterized in that: The volume phase ratio of tempered martensite + bainite in the intermediate layer is ≥40%; preferably, in the intermediate layer, the volume phase ratio of ferrite is 55-60%, the volume phase ratio of tempered martensite is 40-45%, and the volume phase ratio of bainite is 0-5%.

6. The high r-value ultra-high strength steel plate according to claim 1, characterized in that: The ferrite of the upper surface layer and the lower surface layer contains carbides, and the types of the carbides are Ti(C, N) and Nb(C, N).

7. The high r-value ultra-high strength steel plate according to claim 1, characterized in that: The thickness of the upper surface layer and the lower surface layer are 100-200 microns respectively.

8. The high r-value ultra-high strength steel plate according to claim 1, characterized in that: Its thickness is 0.8-2.5mm.

9. The high r-value ultra-high strength steel plate according to claim 1, characterized in that: The sum of the thickness of the upper and lower surfaces shall not exceed 40% of the total thickness of the steel plate.

10. The high r-value ultra-high strength steel sheet according to claim 1, containing Fe and inevitable impurity elements, characterized in that: It also contains the following chemical elements in the following mass percentages: 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%.

11. The high r-value ultra-high strength steel plate according to claim 10, characterized in that: The mass percentage of each chemical element is: 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%; the balance is Fe and unavoidable impurities.

12. The high r-value ultra-high strength steel plate according to claim 10 or 11, characterized in that: Its chemical elements also contain at least one of Cr, Mo and B; wherein B≤0.005%, Cr+Mo≤0.5%; preferably, B≤0.005%, Cr≤0.3%, Mo≤0.2%.

13. The high r-value ultra-high strength steel plate according to claim 1, characterized in that: Its tensile strength is ≥980MPa, and its r value is 90 ≥1.2, the microhardness of the upper and lower surface layers HV≤130, and the microhardness of the middle layer HV≥330.

14. A method for manufacturing a high r-value ultra-high strength steel plate according to any one of claims 1 to 13, characterized in that: The process includes the following steps: smelting and casting; hot rolling; cold rolling after pickling; annealing; The annealing step comprises: High temperature humidification decarburization process: spray water vapor to humidify the annealing furnace, and at the same time control the steel plate heating temperature to 880-950℃ for 100-300s, and control the dew point in the annealing furnace to be above 0℃; Ordinary slow cooling process: the steel plate is cooled from the high temperature and humidification temperature to between 730-770℃ at a cooling rate of 3-10℃ / s; Quasi-static slow cooling process: the steel plate is slowly cooled from 730-770℃ to 680-720℃, the cooling temperature range is controlled at 40-60℃, the cooling rate is 0.03-0.1℃ / s, and the slow cooling process is ensured to last for more than 500s; Rapid cooling process: The steel plate is rapidly cooled to below 300°C at a cooling rate of ≥50°C / s; Over-aging process: the steel plate is tempered at 240-320℃ for 150-500s.

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

Images