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EP-4737603-A1 - ULTRAHIGH-STRENGTH STEEL PLATE HAVING HIGH R VALUE AND MANUFACTURING METHOD THEREFOR

EP4737603A1EP 4737603 A1EP4737603 A1EP 4737603A1EP-4737603-A1

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.

Inventors

  • ZHU, XIAODONG
  • XUE, Peng
  • LI, WEI

Assignees

  • BAOSHAN IRON & STEEL CO., LTD.

Dates

Publication Date
20260506
Application Date
20240628

Claims (15)

  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.

Description

Technical Field 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 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. 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. 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. 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. 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. 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 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. 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. 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. 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 chemica