METHOD FOR MANUFACTURED A HIGH-STRENGTH STEEL SHEET WITH IMPROVED FORMING ABILITY, AND THE SHEET THUS OBTAINED

MA40198AActive Publication Date: 2017-05-10ARCELORMITTAL SA
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Patent Information

Application Number
MA40198
Authority / Receiving Office
MA · MA
Patent Type
Applications
Current Assignee / Owner
Priority Date
2015-07-03
Filing Date
2015-07-03
Publication Date
2017-05-10
Estimated Expiration
2035-07-03

AI Technical Summary

Technical Problem

Existing methods for manufacturing high-strength steel sheets with improved formability are inadequate, particularly in achieving the desired mechanical properties and formability at elevated temperatures between 300°C and 500°C, and there is a need for a process that can maintain these properties during cooling to ambient temperature.

Method used

A manufacturing process involving hot rolling followed by controlled cooling to maintain the steel sheet at a temperature above 300°C for more than 10 seconds before cooling to ambient temperature, ensuring the development of improved mechanical properties and formability.

Benefits of technology

The process enhances the mechanical properties and formability of high-strength steel sheets by maintaining them at elevated temperatures, resulting in improved performance characteristics.

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Abstract

A process for manufacturing a high-strength steel plate exhibiting improved segregation with respect to the chemical composition of the steel, in percentages by weight: 0.1% ≤ c ≤ 0.4%, 4.2% ≤ mn ≤ 8%, 1% ≤ si ≤ 3%, 0.2% ≤ mo ≤ 0.5%, the remainder being composed of fe and unavoidable impurities, the process comprising the steps of annealing a rolled plate made of said steel by quenching it at an annealing temperature above the transformation temperature ac3 of the steel, quenching the plate by cooling it to a quenching temperature qt between the transformation points ms and mf in order to obtain a structure containing at least 65% martensite and at least 20% residual austenite,The sum of the ferrite and bainite contents being less than 10%, heat the sheet to a superheat temperature (pt) between 300°C and 500°C and maintain it at this temperature for a duration (pt) greater than 10 seconds, then cool the sheet to room temperature. Sheet obtained.
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Description

between 300°C and 500°C and maintain at this temperature for a duration greater than 10 seconds, then cool the sheet to room temperature. Sheet obtained.

Claims

DEMANDS 1. A process for manufacturing a high-strength steel having improved machinability, according to which the chemical composition of the steel contains, in: percentages by weight 0.4% 0.1% 5 / 0 5.5 >Mn> 5 / 0 4.5 5 / 0 3; Si; 5 / 0 1 5 / 0 0.5; Mo; 5 / 0 0.2 the remainder being Fe and unavoidable impurities, the process comprising the steps of: - annealing a rolled sheet made of said steel by immersing it at an annealing temperature AT above the transformation point AC3 of the steel, - quenching the sheet by bringing it back to a quenching temperature QT between the transformation points Ms and Mf of the steel in order to obtain a final structure containing at least residual austenite, the sum of ferrite 5 / 0 of martensite and at least 10 5 / 0 minus 50 ,5 / 0 10 and bainite being less than heating the sheet to a survival temperature PT between 300°C and -500°C and maintaining it at said temperature for a time Pt ​​greater than 10 s, and - bringing the sheet back to ambient temperature.

2. A method according to claim 1, characterized in that the chemical composition of the steel is such that 5 / 0 0.25; c; 5 / 0 0.15 5 / 0 1.8> Si; 5 / 0 1.4 5 / 0 0.35; Mo; 5 / 0 0.2 is greater than 78O٥C and less than 95O٥C, and in that the annealing temperature A2 quenching temperature is between 13O٥C and 18O٥C, and the aging time is between 100 s and 600 s.

3. A method according to any one of claims 1 and 2, characterized in that the sheet metal is brought back to the quenching temperature QT so that the final structure satisfies one or more of the following conditions: ,5 / 5 65 - the martensite content is at least ,5 / 0 20 the residual austenite content is at least - - the sum of ferrite and bainite is less than 5%.

4. A method according to any one of claims 1 to 3, characterized in that the sheet metal is further coated.

5. The method according to claim 4, characterized in that the sheet metal is coated by hot immersion coating with or without alloy formation, the coating being carried out before the sheet metal is brought back to ambient temperature.

6. High-strength steel sheet made of steel whose climatic composition contains, in percentages by weight: Q.15 / o <C<Q.4° / o 4.5% <Mn<5,5% 1%؛Si3؛% 0.2 ؛ 5 / 5 Mo 5 / 5 0.5 ة the remainder being Fe and unavoidable impurities, the steel having a structure containing more than 50 µ / µ of martensite, more than 10 µ / µ of residual austenite, less than 10 µ / µ of the sum of ferrite and bainite, and in which there is no central segregation when observed under an optical microscope.

7. High-strength steel sheet according to claim 6, characterized in that the chemical composition of the sheet is such that: 0.15; 5 / 0 c 5 / 5 0.25; 1.4; 5 / 5 If 5 / 5 1.8; 0.2; 5 / 5Mo5 / 0 0.35; 8. High strength steel sheet according to claims 6 and 7, characterized in that the yield strength is greater than or equal to 1000 MPa, the tensile strength is greater than or equal to 1300 MPa, the uniform elongation UE is greater than or equal to 10 5 / 5, the total elongation is greater than or equal to 13%, and the hole expansion rate HER is greater than or equal to 15 5 / 5.

9. High-strength steel sheet according to any one of claims 6 to 8, characterized in that the structure satisfies one or more of the following conditions: - the martensite content is at least 65% - the residual austenite content is at least 2% - the sum of ferrite and bainite is less than 5%.

10. High-strength steel sheet according to any one of claims 6 to 9, characterized in that at least one face of the sheet is coated.