High-r-value ultra-high-strength steel sheet and method for manufacturing the same.

Abstract

The present invention discloses a high-r-value ultra-high-strength steel sheet, which comprises an upper surface layer, an intermediate layer, and a lower surface layer in the thickness direction, wherein the microstructure of the upper and lower surface layers is mainly ferrite, the carbon content of both the upper and lower surface layers is ≤0.025%, and the mass percentage content of chemical elements in the upper and lower surface layers satisfies Ti-3.42N-3.98C≧0, respectively, and the microstructure of the intermediate layer is at least one of tempered martensite and bainite. Accordingly, the present invention further discloses a method for manufacturing a high-r-value ultra-high-strength steel sheet. The steel sheet according to the present invention has ultra-high strength while also having a high r-value, and because the thinning of the sheet thickness during the deformation process is less significant, the high-r-value ultra-high-strength steel sheet exhibits good formability in both overall tensile deformation and local deformation.

Description

Technical Field

[0001] The present invention relates to a steel sheet and a method for manufacturing the same, and particularly to a high-strength steel sheet and a method for manufacturing the same.

Background Art

[0002] The r value indicates the height of the resistance ability against the thinning of the sheet thickness during the drawing forming deformation process of the steel sheet. In a high formability deep drawing steel sheet (for example, interstitial free steel) with a tensile strength of 350 MPa or less, the r value is one of the important technical indicators. The higher the r value, the better the drawing forming performance of the steel sheet.

[0003] However, since existing high-tensile steel sheets are not used for deep drawing, the r value of the steel sheet and the performance indicated thereby have not been noticed.

[0004] For example, in a Chinese patent document with a publication number of CN101768695A, a publication date of July 7, 2010, and a title of "1000 MPa grade Ti micro-alloyed ultrafine grain cold-rolled dual-phase steel and its manufacturing process", the following was disclosed: its chemical composition is 0.03 to 0.2% C, 0.2 to 0.8% Si, 1.2 to 2.0% Mn, Ti: 0.03 to 0.15%, P ≤ 0.02%, S ≤ 0.015, 0.02 to 0.15% Al, and the balance consists of Fe and inevitable impurities. After hot rolling and cold rolling, annealing is performed in the critical region, and when the cooling rate is less than 50 °C / s, a cold-rolled dual-phase steel of 980 MPa or more can be obtained. It can be seen that this high-strength steel does not consider the r value of the steel sheet.

[0005] For example, in a Chinese patent document with publication number CN101363099A, publication date February 11, 2009, and title "Cold-rolled duplex steel sheet with tensile strength of 1000 MPa and method for manufacturing the same," the following was disclosed: 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%, V: 0.001~0.02%, after hot rolling and cold rolling, the sheet is kept warm at 760~820°C, cooled at a cooling rate of 40~50°C / s, and then subjected to overaging treatment at 240~320°C for 180~300s. The r value of the steel sheet was not taken into consideration.

[0006] In their paper "Forming Performance of High-Strength Duplex Steel" (Metal Heat Treatment, Vol. 46, No. 5, 2021), Kumajiyanagi et al. introduced a duplex steel with a tensile strength of 980 MPa, but its r-value was less than 0.9.

[0007] In "Maximum Bending Angle of 1000MPa-Class Ultra-High-Strength Martensitic Steel Based on Three-Point Bending" (Journal of the Japan Society for Plasticity Engineering, 2014, Vol. 21, No. 4), Ge Delong et al. introduced martensitic steel with a strength of 1000-1400 MPa, but its r-value was less than 0.7. [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] One objective of the present invention is to provide a high-r-value ultra-high-strength steel sheet, which has ultra-high strength while also having a high r-value. Because the thinning of the sheet thickness during the deformation process is less significant, this high-r-value ultra-high-strength steel sheet exhibits good formability in both overall tensile deformation and local deformation. [Means for solving the problem]

[0009] In the high-r-value ultra-high-strength steel sheet according to the present invention, the r-value indicates the high resistance of the steel sheet to thinning of its thickness during the deformation process of drawing. High-r-value steel sheets are less prone to fracture because their thickness does not easily thin during tensile deformation. For ultra-high-strength steel sheets, if severe localized deformation occurs, a high r-value makes localized thinning of the sheet thickness less likely and can delay the necking process, which is advantageous for the formability of the steel sheet.

[0010] To achieve the above objectives, the present invention proposes a high-r-value ultra-high-strength steel sheet, which comprises an upper surface layer, an intermediate layer, and a lower surface layer in the thickness direction, wherein the main component of the microstructure of the upper and lower surface layers is ferrite, the carbon content of both the upper and lower surface layers is ≤0.025%, and the mass percentage content of chemical elements in the upper and lower surface layers satisfies Ti-3.42N-3.98C ≥ 0, respectively, and the microstructure of the intermediate layer comprises ferrite and tempered martensite.

[0011] One of the core aspects of the present invention is to achieve a high r value by ensuring that the mass percentage content of chemical elements in the upper and lower surface layers satisfies Ti-3.42N-3.98C≧0, respectively, and that the carbon content of both the upper and lower surface layers is ≤0.025%. At the same time, the overall high strength of the steel sheet is mainly ensured by the structure of the intermediate layer. In some embodiments, the carbon content of both the upper and lower surface layers is 0.02~0.025%. The mass percentage content of chemical elements in the upper and lower surface layers satisfies Ti-3.42N-3.98C≧0.060, preferably ≥0.068, respectively. In some embodiments, the mass percentage content of chemical elements in the upper and lower surface layers satisfies 0.16≧Ti-3.42N-3.98C≧0.06, respectively.

[0012] Furthermore, in the high-r-value ultra-high-strength steel sheet according to the present invention, the average grain size of ferrite in the upper and lower surface layers is ≤15 micrometers. In some embodiments, in the high-r-value ultra-high-strength steel sheet according to the present invention, the average grain size of ferrite in the upper and lower surface layers may be ≤10 micrometers. In some embodiments, in the high-r-value ultra-high-strength steel sheet according to the present invention, the average grain size of ferrite in the upper and lower surface layers is 6 to 10 micrometers.

[0013] Furthermore, in the high-r-value ultra-high-strength steel sheet according to the present invention, the volume phase ratio of ferrite in the upper and lower surface layers is ≥97%. In some embodiments, the volume phase ratio of ferrite in the upper and lower surface layers is 97-99%.

[0014] Furthermore, in the high-r-value ultra-high-strength steel sheet according to the present invention, the microstructure of the intermediate layer may further contain bainite.

[0015] Furthermore, in the high-r-value ultra-high-strength steel sheet according to the present invention, the volume phase ratio of tempered martensite + bainite is ≥40%, regardless of whether the intermediate layer contains bainite or not. In some embodiments, the intermediate layer contains ferrite and at least one of tempered martensite and bainite. In some embodiments, the volume phase ratio of ferrite in the intermediate layer is 55-60%, preferably 57-60%. In some embodiments, the volume phase ratio of tempered martensite in the intermediate layer is 40-45%, preferably 40-43%. In some embodiments, the volume phase ratio of bainite in the intermediate layer is 0-5%, preferably 0-3%.

[0016] Furthermore, in the high-r-value ultra-high-strength steel sheet according to the present invention, the ferrite in the upper and lower surface layers contains carbides, and the type of carbide is Ti(C,N) or Nb(C,N).

[0017] Furthermore, in the high-r-value ultra-high-strength steel sheet according to the present invention, the thickness of the upper surface layer and the lower surface layer are 100 to 200 micrometers, respectively.

[0018] Furthermore, in the high-r-value ultra-high-strength steel sheet according to the present invention, the sum of the thicknesses of the upper and lower surface layers does not exceed 40% of the total thickness of the steel sheet.

[0019] Furthermore, the thickness of the high-r-value ultra-high-strength steel sheet according to the present invention is 0.8 to 2.5 mm, preferably 1.2 to 2.0 mm.

[0020] Furthermore, the high-r-value ultra-high-strength steel sheet according to the present invention contains Fe and unavoidable impurity elements, and further contains the following chemical elements in the following mass percentages: Contains 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%.

[0021] Furthermore, in the high-r-value ultra-high-strength steel sheet according to the present invention, the mass percentages of each chemical element are: The composition is 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%, with the remainder being Fe and unavoidable impurities.

[0022] In some embodiments, the design principles for each chemical element of the high-r-value ultra-high-strength steel sheet according to the present invention are specifically as follows.

[0023] C: In the high-r-value ultra-high-strength steel sheet according to the present invention, carbon (C) can improve the strength of the steel by influencing the martensitic hardness. If the carbon content in the steel is too low, the strength of the steel after quenching will be low, but the higher the carbon content in the steel, the higher the strength of the steel after quenching. However, excessively high carbon content is detrimental to achieving a high r-value. Therefore, in the high-r-value ultra-high-strength steel sheet according to the present invention, the carbon (C) content is controlled to 0.08-0.20%.

[0024] Si: In the high r-value ultra-high strength steel plate according to the present invention, Si can play roles such as solid solution strengthening, suppression of Fe3C precipitation, and promotion of retained austenite formation. Therefore, in the present invention, the content of Si is controlled to be 0.01 to 1.7%. In some embodiments, the content of Si is 0.4 to 1.7%.

[0025] Mn: Mn is a main element that increases the hardenability of steel, and the content of Mn is compatible with the cooling capacity of the adopted cooling means. If the content of Mn is too low, the steel plate cannot be hardened hard and high strength cannot be obtained. However, if the content of Mn is too high, it is disadvantageous for the carbon equivalent. Therefore, in the high r-value ultra-high strength steel plate according to the present invention, the content of Mn element is controlled to be 0.7 to 2.7%. In some embodiments, the content of Mn is 1.0 to 2.7%.

[0026] Ti: In the high r-value ultra-high strength steel plate according to the present invention, the content of Ti is closely related to the r-value in the present invention. If the content of Ti is too low, it is insufficient to fix all C and N in the upper and lower surface layers, and as a result, the r-value becomes low. However, if the content of Ti is too high, Ti becomes excessive. Therefore, in the high r-value ultra-high strength steel plate according to the present invention, the Ti element is controlled to be 0.16 to 0.26%.

[0027] Nb: In the high r-value ultra-high strength steel plate according to the present invention, Nb plays a role in assisting the immobilization of C and N, and by forming Nb(C,N), it ensures that neither C nor N in the upper and lower surface layer regions is in a solid solution state. Therefore, in the high r-value ultra-high strength steel plate according to the present invention, the Nb element is controlled to be 0.015 to 0.05%.

[0028] Al: Its main role in the present invention is deoxidation in the smelting process, and its content range may be 0.02 to 0.06%.

[0029] N: In this invention, N is unfavorable to forming a composition ratio that satisfies Ti-3.42N-3.98C≧0 in the upper and lower surface layers, and is unfavorable to forming a locally gapless atomic state, therefore it is necessary to control N≦0.005%.

[0030] The unavoidable impurities in this invention are mainly S and P, and as far as the process conditions allow, the lower their content, the better. In the high r-value ultra-high-strength steel sheet according to this invention, the content of element P is ≤0.015%. In the high r-value ultra-high-strength steel sheet according to this invention, the content of element S is ≤0.003%.

[0031] In the high r-value ultra-high-strength steel sheet according to the present invention, the basic composition may be understood as C: 0.08~0.20%; Si: 0.01~1.7%; Mn: 0.7~2.7%. While this basic composition forms the basis of the steel sheet's strength, such a basic composition does not allow for a high r-value to be obtained. In order to obtain a high r-value, the steel sheet according to the present invention further contains Ti: 0.16~0.26%, Nb: 0.015~0.05%, and N≦0.005%.

[0032] Furthermore, in the high-r-value ultra-high-strength steel sheet according to the present invention, the chemical elements further contain at least one of Cr, Mo, and B, provided that B ≤ 0.005% and Cr + Mo ≤ 0.5%. In some embodiments, when present, 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%.

[0033] In the high-r-value ultra-high-strength steel sheet according to the present invention, appropriate amounts of Cr, Mo, and B alloying elements can work efficiently with manganese to improve hardenability. As a result, the hardenability of the steel matches its rapid cooling capacity, further avoiding the inability to harden through quenching or the problem of excessively high carbon equivalent.

[0034] Furthermore, the high r-value ultra-high-strength steel sheet according to the present invention has a tensile strength of ≥980 MPa, and its r-value is r 90The r-value is ≥1.2, the microhardness HV of the upper and lower surface layers is ≤130, and the microhardness HV of the intermediate layer is ≥330. Preferably, the tensile strength of the high r-value ultra-high-strength steel sheet according to the present invention is ≥1020 MPa. In some embodiments, the tensile strength of the high r-value ultra-high-strength steel sheet according to the present invention may be 980 to 1100 MPa, preferably 980 to 1060 MPa.

[0035] In some embodiments, the r value of the high r value ultra-high strength steel sheet according to the present invention 90 It is between 1.2 and 1.4.

[0036] In some embodiments, the microhardness HV of the upper and lower surface layers of the high-r-value ultra-high-strength steel sheet according to the present invention is independently 100 to 130.

[0037] In some embodiments, the microhardness HV of the intermediate layer of the high-r-value ultra-high-strength steel sheet according to the present invention is 330 to 380.

[0038] In some embodiments, the elongation of the high-r-value ultra-high-strength steel sheet according to the present invention is ≥12%, for example, 12-16% or 13-15%.

[0039] In some embodiments, the yield strength of the high-r-value ultra-high-strength steel sheet according to the present invention is ≥ 570 MPa, for example, 570 to 660 MPa.

[0040] Accordingly, another objective of the present invention is to provide a method for manufacturing high-r-value ultra-high-strength steel sheets that can be produced by optimizing the process.

[0041] To achieve the above objective, the present invention proposes a method for manufacturing the above-mentioned high-r-value ultra-high-strength steel sheet, which comprises a smelting and casting process; a hot rolling process; a cold rolling process after pickling; and an annealing process, wherein the annealing process includes the following steps: High-temperature humidified decarburization process: While humidifying the annealing furnace by injecting steam, the heating temperature of the steel plate is controlled to 880-950°C and maintained for 100-300 seconds, and the dew point inside the annealing furnace is controlled to be above 0°C; Typical slow cooling process: The steel plate is cooled from its temperature after high-temperature humidification to 730-770°C at a cooling rate of 3-10°C / s; Quasi-static slow cooling process: The steel plate is slowly cooled from 730-770°C to 680-720°C, the cooling temperature range is controlled to 40-60°C, the cooling rate is set to 0.03-0.1°C / s, and the slow cooling process is ensured to continue for 500 seconds or more. Rapid cooling process: The steel plate is rapidly cooled to below 300°C at a cooling rate of ≥50°C / s; Overaging process: Temper the steel plate at 240-320°C for 150-500 seconds.

[0042] The processes employed in the manufacturing method according to the present invention, particularly the unique annealing process, are extremely important for obtaining a high r value for the ultra-high-strength steel sheet according to the present invention.

[0043] However, the decarburization method involves humidifying the furnace by injecting steam to induce the following reactions at high temperatures, thereby promoting the decarburization reaction of the steel sheet: C + H2O = CO + H2; C + 2H2O = CO2 + 2H2. The heating temperature of the steel sheet is set to 880-950°C and maintained for 100-300 seconds. The amount of humidifying water is controlled based on the weight of the steel strip passing per unit time and the thickness of the layer requiring decarburization, maintaining the dew point in the annealing furnace above 0°C. In some embodiments, the dew point in the annealing furnace is 0-20°C. This high-temperature humidified decarburization process results in the upper and lower surface layers of the steel sheet becoming pre-decarburized layers with a thickness of 100-200 micrometers each (i.e., both the upper and lower surface layers become decarburized layers). In this process, if the heating temperature of the steel sheet is too low, decarburization will be insufficient. However, if the heating temperature is too high, the crystal grains will grow excessively. Therefore, in the high-temperature humidified decarburization process, controlling the heating temperature of the steel sheet to 880-950°C contributes to the decarburization reaction of the steel sheet. On the other hand, the reason for raising the dew point inside the furnace above 0°C is that if the dew point inside the furnace is low, the decarburization reaction of the steel sheet will be slow, and a decarburized layer of sufficient thickness will not be obtained.

[0044] After the high-temperature humidification decarburization process described above, the carbon content of the upper and lower surface layers of the steel sheet is significantly reduced, reaching a level of ≤0.05%. At this point, the carbon content of the upper and lower surface layers of the steel sheet is still too high, causing the austenite to cool directly and transform into ferrite, resulting in the precipitation of Fe3C. To further reduce the carbon content of the upper and lower surface layers of the steel sheet, a quasi-static slow cooling process is carried out below the Ac1 temperature of the iron-carbon phase diagram. In this quasi-static slow cooling process, the steel sheet is slowly cooled from 730-770°C to 680-720°C at a rate of 0.03-0.1°C / s, controlling the temperature range of the entire slow cooling process to 40-60°C and the duration to ≥500 s. In some embodiments, the slow cooling duration is set to 500-1500 s. Within the specified temperature range, in the upper and lower surface layers of the partially decarburized steel sheet, austenite decomposition occurs first, and ferrite is formed; during the slow cooling process, carbon in the ferrite diffuses into the untransformed high-carbon region (intermediate layer) of austenite, thereby further reducing the carbon content of the upper and lower surface layers; if the slow cooling time is sufficient, the carbon content of the upper and lower surface layers can be reduced to a level close to that of industrial pure iron, and the upper and lower surface regions of the steel according to the present invention will then satisfy the condition Ti-3.42N-3.98C≧0.

[0045] In the manufacturing method of the present invention, slow cooling from 730-770°C to 680-720°C is extremely important. As the carbon content of austenite in the surface layer decreases and Ac3 increases after decarburization, the austenite-to-ferrite transformation occurs first during the slow cooling process from 730-770°C to 680-720°C. In the steel sheet according to the present invention, when the local carbon content of the upper and lower surface layers decreases to ≤0.025%, the Ti, C, and N content in the steel sheet becomes Ti-3.42N-3.98C≧0. This substantially forms a gapless atomic structure in the upper and lower surface layers, significantly improving the r value of the upper and lower surface layers, and consequently improving the r value of the entire steel sheet.

[0046] After slow cooling is complete, the steel sheet is rapidly cooled to below 300°C at a cooling rate of ≥50°C / s, and tempered at 240-320°C for 150-500 seconds. In some embodiments, the rapid cooling rate is 50-150°C / s. In some embodiments, the rapid cooling termination temperature is 240-300°C. During this cooling process, the austenite in the intermediate layer transforms into a martensite structure, but in some embodiments, it may also contain small amounts of bainite and retained austenite, thereby achieving ultra-high strength, for example, a tensile strength of ≥980 MPa.

[0047] In some embodiments, the overaging temperature is set to 240-300°C.

[0048] The smelting, casting, hot rolling, and cold rolling processes in the manufacturing method according to the present invention can all be conventional processes, and the present invention does not particularly improve upon these processes. In an exemplary hot rolling process, the temperature at which the steel sheet is removed from the heating furnace is 1220 to 1280°C, the rolling completion temperature is 870 to 930°C, and the coiling temperature is 570 to 630°C. In an exemplary cold rolling process, the cold rolling reduction ratio is controlled to 40 to 65%. [Effects of the Invention]

[0049] The high-r-value ultra-high-strength steel sheet and its manufacturing method according to the present invention have the following advantages and beneficial effects compared to the prior art:

[0050] This invention ensures ultra-high strength by controlling the microstructure of the intermediate layer, while simultaneously achieving a high r value by controlling the mass percentage content of chemical elements in the upper and lower surface layers to satisfy the condition Ti-3.42N-3.98C≧0, respectively. As a result, the ultra-high-strength steel sheet has achieved a high r value.

[0051] From a compositional design perspective, the present invention, by adding an appropriate amount of Ti and assisting in the immobilization of N and C with Nb, can substantially achieve a solid solution state of gapless atoms in the upper and lower surface layers. This significantly improves the r value of the surface layer, which in turn affects the r value of the entire steel sheet, resulting in an r value of 1.2 or higher for a 980 MPa cold-rolled high-strength steel sheet. As a result, the rate of thinning during the deformation process is greatly reduced, and superior formability is obtained.

[0052] From a manufacturing process perspective, the high-r-value ultra-high-strength steel sheet according to the present invention employs a unique annealing process that involves a basic high-temperature humidification decarburization process and austenite-to-ferrite transformation through prolonged slow cooling, thereby redistributing the carbon content. This makes it possible to reduce the carbon content of the upper and lower surface layers of the ultra-high-strength steel sheet to 0.025% or less. [Brief explanation of the drawing]

[0053] [Figure 1] Figure 1 conceptually shows the annealing process curve in one embodiment of the method for manufacturing high-r-value ultra-high-strength steel sheets according to the present invention. [Modes for carrying out the invention]

[0054] The following interpretation and explanation will be based on the drawings and specific embodiments, and will further describe the high-r-value ultra-high-strength steel sheet and its manufacturing method according to the present invention, but this interpretation and explanation will not unduly limit the technical solutions of the present invention.

[0055] The high-r-value ultra-high-strength steel sheets used in Examples 1-6 and the comparative steel sheets used in Comparative Examples 1-2 were manufactured using the following process: (1) Smelting: Molten iron was smelted in a converter, and its composition was further optimized through the smelting process; (2) Slabs were obtained by continuous casting, and the component content of the slabs for each example and comparative example is shown in Table 1; (3) Hot rolling: A conventional hot rolling process was adopted, specifically, the temperature at which the steel sheet was removed from the heating furnace was set to 1220-1280°C, the rolling completion temperature to 870-930°C, and the coiling temperature to 570-630°C; (4) Cold rolling after pickling: The reduction ratio during cold rolling was controlled to 40-65%; (5) Annealing, specifically including the following process: High-temperature humidified decarburization process: While humidifying the annealing furnace by injecting steam, the heating temperature of the steel plate was controlled to 880-950°C and held for 100-300 seconds, and the dew point inside the annealing furnace was controlled to be above 0°C. Typical slow cooling process: The steel plate was cooled from its temperature after high-temperature humidification to 730-770°C at a cooling rate of 3-10°C / s; Quasi-static slow cooling process: The steel plate was slowly cooled from 730-770°C to 680-720°C, the cooling temperature range was controlled to 40-60°C, the cooling rate was set to 0.03-0.1°C / s, and the slow cooling process was ensured to continue for 500 seconds or more. Rapid cooling process: The steel plate was rapidly cooled to below 300°C at a cooling rate of ≥50°C / s; Overaging process: The steel plate was tempered at 240-320°C for 150-500 seconds.

[0056] However, the annealing process parameters for Examples 1-6 and Comparative Example 1 are shown in Table 2-1. Comparative Example 2 employed a conventional quenching + tempering process instead of the annealing process of the present invention, so its specific process parameters are shown in Table 2-2.

[0057] The mass percentage formulations of the chemical elements in Examples 1-6 and Comparative Examples 1-2 of the present invention are shown in Table 1.

[0058] [Table 1]

[0059] Table 2-1 shows the specific process parameters for the annealing process of the high-r-value ultra-high-strength steel sheets used in Examples 1-6 and the comparative steel sheets used in Comparative Examples 1-2.

[0060] [Table 2-1]

[0061] [Table 2-2]

[0062] Samples were taken from the steel sheets of Examples 1-6 and Comparative Examples 1-2. The carbon content and other component content of the upper and lower surface layers of the steel sheets were measured using a GDS (glow discharge emission spectrometer), and the microstructure was observed and measured using a metallurgical microscope. The results of the observation and measurement are shown in Table 3.

[0063] [Table 3]

[0064] To verify the effectiveness of the present invention, samples were taken from steel plates according to Examples 1-6 and Comparative Examples 1-2, and their performance was measured. The measurement results are shown in Table 4. However, For measuring the mechanical performance, tensile tests were performed along a direction perpendicular to the rolling direction of the steel plate. A JIS 5# standard plate-shaped tensile sample with a scale length of 50 mm and a scale area width of 25 mm was measured using a tensile testing machine. During the measurement process, data on strength, elongation, and r90 value were obtained, and a tensile strain range of 4-6% was defined as the numerical range for the r90 value. For hardness measurement, a microhardness tester was used to measure the HV hardness, with a load of 150g and a loading time of 15s.

[0065] The relevant performance parameters for the high-r-value ultra-high-strength steel sheets used in Examples 1-6 and the comparative steel sheets used in Comparative Examples 1-2 are shown in Table 4.

[0066] [Table 4]

[0067] As can be seen from Table 4, the high r-value ultra-high-strength steel sheets according to Examples 1 to 6 of the present invention all have a tensile strength of ≥980 MPa, and their r-value is r 90Since the microhardness HV is ≥ 1.2, the microhardness HV of the upper and lower surface layers is ≤ 130, and the microhardness HV of the intermediate layer is ≥ 330, the high-strength steel sheet according to the present invention can have both high strength and high formability.

[0068] Since the amount of Ti added in Comparative Example 1 did not satisfy the requirements of the present invention, a high r value could not be obtained even if the process of the present invention was adopted.

[0069] Comparative Example 2 adopted the composition of Example 2 of the present invention, but because it used a conventional quenching and tempering process instead of the annealing process of the present invention, a high r value could not be obtained.

[0070] Furthermore, the combinations of technical features in this application are not limited to the combinations described in the claims or the specific embodiments, and all technical features described in this application can be freely combined or combined in any form, as long as they do not contradict each other.

[0071] Furthermore, it should be noted that the embodiments described above are merely specific examples of the present invention. The present invention is not limited to the above embodiments, and it is clear that any similar changes or modifications that a person skilled in the art can directly derive from the disclosure of the present invention or readily conceive are also covered within the scope of the present invention.

Claims

1. A high-r-value ultra-high-strength steel sheet characterized by comprising an upper surface layer, an intermediate layer, and a lower surface layer in the thickness direction, wherein the main component of the microstructure of the upper and lower surface layers is ferrite, the carbon content of both the upper and lower surface layers is ≤0.025%, and the mass percentage content of chemical elements in the upper and lower surface layers satisfies the condition that Ti-3.42N-3.98C ≥ 0, respectively, and the microstructure of the intermediate layer contains ferrite and tempered martensite.

2. The high-r-value ultra-high-strength steel sheet according to claim 1, characterized in that the average crystal grain size of ferrite in the upper and lower surface layers is ≤ 15 micrometers.

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

4. The high-r-value ultra-high-strength steel sheet according to claim 1, characterized in that the microstructure of the intermediate layer further contains bainite.

5. The high r-value ultra-high-strength steel sheet according to claim 4, characterized in that the volume phase ratio of tempered martensite + bainite in the intermediate layer is ≥ 40%; preferably, the volume phase ratio of ferrite in the intermediate layer 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 sheet according to claim 1, characterized in that the ferrite in the upper and lower surface layers contains carbides, and the type of carbide is Ti(C,N) or Nb(C,N).

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

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

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

10. It contains Fe and unavoidable impurity elements, and further 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%, contained A high-r-value ultra-high-strength steel plate according to claim 1, characterized in that...

11. The mass percentages of each chemical element are: The composition is 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%, with the remainder being Fe and unavoidable impurities. A high-r-value ultra-high-strength steel plate according to claim 10, characterized in that...

12. The high r-value ultra-high-strength steel sheet according to claim 10 or 11, wherein the chemical elements further contain at least one of Cr, Mo, and B, provided that B ≤ 0.005% and Cr + Mo ≤ 0.5%; preferably, B ≤ 0.005%, Cr ≤ 0.3%, and Mo ≤ 0.2%.

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

14. A method for manufacturing a high-r-value ultra-high-strength steel sheet according to any one of claims 1 to 13, comprising a smelting and casting process; a hot rolling process; a cold rolling process after pickling; and an annealing process, wherein the annealing process includes the following steps: High-temperature humidified decarburization process: While humidifying the annealing furnace by injecting steam, the heating temperature of the steel plate is controlled to 880-950°C and maintained for 100-300 seconds, and the dew point inside the annealing furnace is controlled to be above 0°C; Typical slow cooling process: The steel plate is cooled from its temperature after high-temperature humidification to 730-770°C at a cooling rate of 3-10°C / s; Quasi-static slow cooling process: The steel plate is slowly cooled from 730-770°C to 680-720°C, the cooling temperature range is controlled to 40-60°C, the cooling rate is set to 0.03-0.1°C / s, and the slow cooling process is ensured to continue for 500 seconds or more. Rapid cooling process: The steel plate is rapidly cooled to below 300°C at a cooling rate of ≥50°C / s; Overaging process: Temper the steel plate at 240-320°C for 150-500 seconds.

15. The manufacturing method of claim 14 is characterized by having one or more of the following features: (1) In the hot rolling process, the temperature at which the steel sheet is removed from the heating furnace is controlled to 1220 to 1280°C, the rolling completion temperature is controlled to 870 to 930°C, and the winding temperature is controlled to 570 to 630°C; (2) The reduction ratio of the cold rolling shall be 40 to 65%; (3) The dew point inside the annealing furnace shall be 0 to 20°C; (4) Continue the slow cooling for 500 to 1500 seconds; (5) The rapid cooling rate shall be 50 to 150°C / s; and (6) The tempering temperature shall be 240 to 300°C.

Images