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KR-20260068134-A - High-bendability press-hardened steel parts and methods for manufacturing the same

KR20260068134AKR 20260068134 AKR20260068134 AKR 20260068134AKR-20260068134-A

Abstract

The present invention relates to a high-flexibility press-hardened steel part, which has a composition comprising, in weight%, C: 0.05 - 0.14%, Mn: 1.4 - 2.5%, Si: 0.1 - 1.5%, Al: 0.02 - 0.1%, Cr: 0.01 - 1.0%, B: 0.0005 - 0.004%, Ti: 0.01 - 0.1%, P ≤ 0.020%, S ≤ 0.010%, N ≤ 0.02%, and optionally, in weight%, one or more of the following elements: Ni ≤ 0.4%, Mo ≤ 0.40%, Nb ≤ 0.08%, Ca ≤ 0.1%, and the remainder of the composition is iron and unavoidable impurities from smelting. The press-hardened steel part comprises a bulk having a microstructure in which the surface fraction is 85% or more martensite and the remainder is bainite, an interdiffusion layer with a thickness of th inter , and an aluminum-based coating layer with a thickness of th coat , wherein the ratio of th inter / (th inter + th coat ) is 70% or more.

Inventors

  • 필리뽀 끌레망

Assignees

  • 아르셀러미탈

Dates

Publication Date
20260513
Application Date
20241127
Priority Date
20231201

Claims (8)

  1. As a press-hardened steel part manufactured from coated and re-rolled steel sheet, in weight% C : 0.05 - 0.14 % Mn : 1.4 - 2.5 % Si : 0.1 - 1.5 % Al : 0.02 - 0.1 % Cr : 0.01 - 1.0 % B: 0.0005 - 0.004 % Ti: 0.01 - 0.1% P ≤ 0.020% S ≤ 0.010% Includes N ≤ 0.02%, and Also optionally, in weight percent, the following elements: Ni ≤ 0.4% Mo ≤ 0.40% Nb ≤ 0.08% Ca ≤ 0.1% Having a composition including one or more of the following, The remainder of the composition consists of iron and inevitable impurities resulting from smelting, and The above steel part is continuous from the bulk of the steel part to the surface, - Bulk having a microstructure in which, by surface fraction, martensite is 85% or more and the remainder is optionally bainite, - Interdiffusion layer having thickness, - Includes an aluminum-based coating layer having a thickness of th coat , and - Press-hardened steel part having a ratio between the interdiffusion layer thickness (th inter) and the sum of the coating layer thickness and the interdiffusion layer thickness (th inter + th coat ) of 70% or more.
  2. A press-hardened steel part according to claim 1, wherein the manganese content of the press-hardened steel part is 1.4% to 1.9%.
  3. A press-hardened steel part according to claims 1 and 2, wherein the silicon content of the press-hardened steel part is 0.1% to 1%.
  4. A press-hardened steel part according to any one of claims 1 to 3, wherein the bending angle of the press-hardened steel part is 85° or more.
  5. A press-hardened steel part according to any one of claims 1 to 4, wherein the yield strength (YS) of the press-hardened steel part is 830 MPa or higher.
  6. A press-hardened steel part according to any one of claims 1 to 5, wherein the tensile strength (TS) and yield strength (YS) of the press-hardened steel part satisfy (TS + YS ) / (%C*%Mn) > 17500, and %C and %Mn are nominal carbon and manganese values expressed in weight%.
  7. A method for manufacturing a press-hardened steel part according to claim 1, comprising the following consecutive steps: - The step of providing an aluminum-based coated steel sheet having a chemical composition according to claim 1, - A step of re-rolling at least a portion of the coated steel plate at a reduction rate of 20 to 60% to obtain a coated and re-rolled steel plate, - A step of cutting the above coated and re-rolled steel plate into a predetermined shape to obtain a steel blank, - A step of heating the steel blank to a temperature T1 higher than Ac3, and maintaining it at the temperature T1 for a holding time t1 of 10 seconds to 900 seconds to obtain a heated steel blank, - The step of transferring the above heated steel blank to a forming press, - A step of hot forming the above heated steel blank in the above forming press to obtain a formed part, - Step of die-quenching the above-mentioned molded part A method for manufacturing press-hardened steel parts, comprising
  8. In claim 7, the aluminum-based coated steel sheet undergoes the following consecutive steps: - A step of casting steel having the composition according to paragraph 1 to obtain a semi-finished product, - A step of heating the above semi-finished product to a temperature of 1100℃ to 1300℃, - A step of hot rolling the heated semi-finished product at a finishing hot rolling temperature of 800℃ to 950℃, - A step of coiling a hot-rolled steel plate at a coiling temperature of less than 670℃, - The step of pickling the above steel plate, - A step of cold rolling the steel plate with a reduction rate of 20% to 80%, - A step of heating the steel plate to a temperature T H of 700℃ to 900℃ and maintaining it at the temperature T H for a holding time t H of 10 seconds to 600 seconds, - A step of coating the above steel plate with an aluminum-based coating, - Step of cooling the above aluminum-coated steel plate to room temperature A method for manufacturing press-hardened steel parts provided according to

Description

High-bendability press-hardened steel parts and methods for manufacturing the same The present invention relates to a high-strength press-hardened steel part having high bendability. High-strength press-hardened parts can be used as structural elements of automobiles for intrusion prevention or energy absorption functions. In this type of application, it is desirable to manufacture steel parts that combine high mechanical strength, high impact resistance, and good corrosion resistance. Furthermore, one of the major challenges of the automotive industry is to reduce the weight of vehicles to improve fuel efficiency in terms of global environmental conservation, without neglecting safety requirements, including those in harsh environments. This weight reduction can be achieved thanks to the use of steel parts with a martensitic or bainite/martensite microstructure. Publication WO2017006159 relates to a press-hardened steel part characterized by a bending angle greater than 75°, a thickness of 0.8 to 4 mm, a yield stress (YS) of 700 to 950 MPa, a tensile stress (TS) of 950 to 1200 MPa, and high ductility. Nevertheless, to improve properties such as bendability, the steel part must contain at least 5% of self-tempered martensite and bainite obtained by a two-stage cooling process of the steel. Additionally, to improve bendability, the average size of TiN must be controlled and limited to the near-surface region. Such control of TiN to achieve high bendability can be cumbersome. Publication WO2022129994 relates to a coated steel sheet and a high-strength press-hardened steel part having good bending properties, with a bending angle exceeding 70° and a yield strength (YS) of 1000 MPa or more. To ensure excellent bending properties of the steel part, the steel sheet is heated in an atmosphere where moisture is injected and the dew point is controlled, so that a decarburized layer covered by a ferrite layer is formed on the upper surface of the steel sheet. After forming, this decarburized layer helps to improve bendability, and bendability is improved when the decarburized layer of the steel sheet includes such an upper ferrite layer. This moisture injection process requires equipment specifically designed for moisture injection. Therefore, the objective of the present invention is to solve the aforementioned problems and provide a press-hardened steel part that has high bendability, a bending angle of 85° or more, a high yield strength (YS) of 830 MPa or more, and can be easily processed in a normal process path. Preferably, the yield strength is 880 MPa or higher. Preferably, the tensile strength TS (MPa) and YS satisfy (TS + YS) / (%C*%Mn) > 17500, where %C and %Mn are the nominal carbon and manganese values. The object of the present invention is achieved by providing a steel part according to claim 1. The steel part may also include features according to any one of claims 2 to 6. Another object is achieved by providing a method according to any one of claims 7 to 8. Now, the present invention will be described in detail and shown by example without introducing limitations. Now, the composition of a press-hardened steel part according to the present invention is described, and the content is expressed in weight% (wt. %). According to the present invention, the carbon content is 0.05% to 0.14% to ensure satisfactory strength. If the carbon content exceeds 0.14%, the weldability and bendability of the steel may decrease. If the carbon content is less than 0.05%, the tensile strength becomes too low. Preferably, the carbon content is 0.05% to 0.10%, more preferably 0.06% to 0.10%. The manganese content is 1.4% to 2.5%. If added in excess of 2.5%, the risk of center segregation increases, impairing bendability. At less than 1.4%, the hardenability of the steel decreases, and the tensile strength and yield strength become too low. In a preferred embodiment of the present invention, the manganese content is 1.4% to 1.9%. Preferably, the manganese content is 1.5% to 1.9%, and more preferably 1.5% to 1.8%. According to the present invention, the silicon content is 0.1% to 1.5%. Silicon is an element that participates in solid solution hardening. Silicon is added to limit carbide formation. If it exceeds 1.5%, silicon is detrimental to toughness. In addition, silicon oxide may form on the surface, which worsens the plating properties of the steel and may reduce the weldability of steel sheets and steel parts. In a preferred embodiment of the present invention, the silicon content is 0.1% to 1%. Preferably, the silicon content is 0.1% to 0.8%, more preferably 0.1% to 0.7%, and even more preferably 0.1% to 0.6%. Since aluminum is a very effective element for deoxidizing liquid steel during elaboration, the aluminum content is 0.02% to 0.1%. Aluminum can protect boron if the titanium content is insufficient. The aluminum content is less than 0.1% to avoid ferrite formation and oxidation problems during press hardening. Pr