Q&P Steel Sheet Processing for Strength-Formability Balance
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing high strength steel sheets, such as DP and TRIP steels, face challenges in achieving a balance between high tensile strength, ductility, and formability, particularly in terms of stretch flangeability and hole expansion ratio, which are essential for automotive applications.
Innovation Solution
A method for producing a steel sheet involving a specific heat treatment process that includes slow cooling, quenching, and partitioning, along with controlled chemical composition, to achieve a microstructure comprising ferrite, retained austenite, martensite, and bainite, optimizing the mechanical properties and reducing the likelihood of liquid metal embrittlement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the steel sheet is cooled to a cooling stop temperature and held for bainite formation, then the tensile strength is improved, but the martensite retains high C content leading to high yield strength and unsatisfactory formability
Solution Approach 1:
The heat treatment process is segmented into distinct stages: austenitization, quenching to form martensite, reheating to partitioning temperature, and holding to allow carbon diffusion from martensite to retained austenite. This segmentation enables independent control of strength and formability parameters.
Solution Approach 2:
The patent changes the temperature parameter during heat treatment by reheating to a partitioning temperature range of 375-470°C after quenching. This parameter change enables carbon redistribution, transforming the microstructure to achieve both high tensile strength (≥980 MPa) and improved formability through reduced martensite carbon content.
2Strength
If the steel sheet is quenched to form martensite, then the tensile strength is improved, but the total elongation and hole expansion ratio are reduced
Solution Approach 1:
The patent creates a composite microstructure consisting of multiple phases: ferrite (40-60%), retained austenite (15-30%), martensite (10-20%), and bainite (0-15%). This composite structure combines the strength benefits of martensite with the ductility and formability of ferrite and retained austenite, achieving tensile strength ≥980 MPa with total elongation ≥16% and hole expansion ratio >20%.
Solution Approach 2:
The patent applies parameter changes by controlling the quenching temperature and partitioning temperature to achieve specific phase fractions. By maintaining the quenching temperature QT between Ms-50°C and Ms+50°C and partitioning temperature PT between 375-470°C, the microstructure is optimized to balance strength and ductility.
3Stability of the object's composition
If the steel sheet is heated to high temperature for austenitization, then the ductility is improved, but the energy consumption and process time are increased
Solution Approach 1:
The patent applies preliminary action by performing batch annealing before cold rolling to reduce hardness and improve formability. This preliminary heat treatment at lower temperature (500-700°C for 2-6 days) reduces the need for high-temperature processing later, thereby reducing energy consumption while maintaining ductility.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method results in steel sheets with a tensile strength of at least 980 MPa, total elongation of at least 16%, and a hole expansion ratio of over 20%, while maintaining a yield strength below 860 MPa for excellent formability and reduced susceptibility to liquid metal embrittlement.
Implementation Method 1
heating the sheet to an austenitization temperature TA
Implementation Method 2
quenching the sheet by cooling down from a temperature of at least 600°C to a quenching temperature QT lower than the Ms transformation point
Implementation Method 3
reheating the sheet from the quenching temperature up to a partitioning temperature PT between 375°C and 470°C, and maintaining the sheet at the partitioning temperature PT for a partitioning time Pt between 25s and 440s. During this partitioning step, the carbon is partitioned, i.e. diffuses from the martensite into the austenite
Implementation Method 4
between the annealing step and the quenching step, a step of slow cooling the sheet to a temperature higher than or equal to 600°C at a cooling rate lower than 10°C/s
Data Source
AI summary
A method for producing a steel sheet, the method comprising the following successive steps: - providing a cold-rolled steel sheet, the chemical composition of the steel containing in weight %:0.15% ≤ C ≤ 0.23%, 1.4 % ≤ Mn ≤ 2.6%, 0.6% ≤ Si ≤ 1.5%, 0.02% ≤ Al ≤ 1.0%, with 1.0% ≤ Si+Al ≤ 2.0%, 0 ≤ Nb ≤ 0.035%, 0 ≤ Mo ≤ 0.3%, 0 ≤ Cr ≤ 0.3%,the remainder being Fe and unavoidable impurities, - annealing the steel sheet at an annealing temperature TA comprised between Ac1 and Ac3 so as to obtain a structure comprising at least 40% of austenite and at least 40% of intercritical ferrite, - quenching the sheet from a temperature of at least 600°C at a cooling rate of at least 20°C/s down to a quenching temperature QT comprised between 180°C and 260°C, - heating the sheet up to a partitioning temperature PT between 375°C and 470°C and maintaining the sheet at this partitioning temperature PT for a partitioning time Pt comprised between 25s and 440s, the partitioning time Pt being comprised between 100s and 440s if the partitioning temperature PT is comprised between 375°C and 400°C, and comprised between 25s and 150s if the partitioning temperature PT is comprised between 450°C and 470°C, - cooling the sheet down to the room temperature, the steel sheet having a final microstructure consisting, in area fraction, of: - at least 11% of tempered martensite, - between 10% and 20% of retained austenite, - between 40% and 60% of ferrite, - at most 6% of fresh martensite, - at most 18% of bainite.