Cold-Rolled Steel Sheet Partitioning for Strength-Formability Balance
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current steel sheets used in automotive manufacturing, such as DP and TRIP steels, face challenges in achieving a balance of high strength, ductility, and formability, particularly in meeting specific mechanical property criteria like yield strength, tensile strength, uniform elongation, total elongation, and hole expansion ratio, while maintaining good weldability and formability.
Innovation Solution
A cold-rolled and heat-treated steel sheet with a specific composition and microstructure, including a carbon content of 0.10% to 0.25%, manganese between 3.5% and 6.0%, and controlled annealing processes to achieve a microstructure of 10% to 45% ferrite, 8% to 30% retained austenite, and optimal cementite and martensite fractions, which enhances strength, ductility, and weldability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If sheets are cooled to quenching temperature below Ms transformation point and then heated to partitioning temperature, then tensile strength is improved, but total elongation and formability deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling the partitioning temperature (350-500°C) and holding time (3-1000 seconds) to achieve optimal microstructure. By adjusting these parameters, the steel sheet achieves a balance between high tensile strength (1200-1600 MPa) and adequate total elongation (≥14%), resolving the contradiction between strength and formability
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite (for strength), retained austenite (for ductility and elongation), and optionally bainite and/or ferrite. This composite structure at the micro level allows the material to simultaneously exhibit high strength and good formability, overcoming the trade-off between these properties
2Strength
If yield strength and tensile strength are increased to reduce automotive weight, then fuel efficiency is improved, but ductility and stretch flangeability deteriorate
Solution Approach 1:
The patent controls the partitioning temperature range (350-500°C) and holding time (3-1000 seconds) to achieve optimal carbon redistribution. This results in high yield strength (1000-1300 MPa) while maintaining uniform elongation (≥10%) and total elongation (≥14%), thereby preserving stretch flangeability despite high strength
Solution Approach 2:
The martensite-retained austenite composite microstructure provides high yield strength through martensite while retained austenite (8-30% by area) contributes to ductility and stretch flangeability through TRIP effect during deformation, resolving the contradiction between strength and formability
3Strength
If carbon content is increased to improve strength, then yield strength and tensile strength are improved, but total elongation and hole expansion ratio deteriorate
Solution Approach 1:
The patent specifies carbon content in the range of 0.10-0.25% and controls partitioning temperature (350-500°C) and holding time (3-1000 seconds) to achieve optimal carbon redistribution. This results in high yield strength (1000-1300 MPa) while maintaining total elongation (≥14%) and hole expansion ratio (≥20%), resolving the contradiction between strength and ductility
4Strength
If manganese content is increased to improve strength and retained austenite stability, then tensile strength and total elongation are improved, but manufacturing complexity increases
Solution Approach 1:
The patent specifies manganese content in the range of 3.5-6.0% and controls partitioning temperature (350-500°C) and holding time (3-1000 seconds) to achieve optimal microstructure. This results in high tensile strength (1200-1600 MPa) and total elongation (≥14%) while using a conventional quenching and partitioning process, avoiding excessive manufacturing complexity
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 solution achieves yield strengths between 1000 MPa and 1300 MPa, tensile strengths between 1200 MPa and 1600 MPa, uniform elongation of at least 10%, total elongation of at least 14%, and a hole expansion ratio of at least 20%, along with excellent weldability and formability, surpassing the limitations of existing methods.
Implementation Method 1
the sheets are cooled from an annealing temperature, down to a quenching temperature below the Ms transformation point
Implementation Method 2
thereafter heated to a partitioning temperature and maintained at this temperature for a given time
Implementation Method 3
resistance spot welded joint
Data Source
AI summary
High strength and high formability steel sheet and manufacturing method Cold-rolled and heat-treated steel sheet, having a composition comprising, by weight percent: n0.10%≤C≤0.25%, 3.5%≤Mn≤6.0%, 0.5%≤Si≤2.0%, 0.3%≤Al≤1.2%, with Si+Al≥0.8%, 0.10%≤Mo≤0.50%, S≤0.010%, P≤0.020%, N≤0.008%, said cold-rolled steel sheet having a microstructure consisting of, in surface fraction: between 10% and 45% of ferrite, having an average grain size of at most 1.3 mm, the product of the surface fraction of ferrite by the average grain size of the ferrite being of at most 35 mm%, between 8% and 30% of retained austenite, said retained austenite having an Mn content higher than 1.1*Mn%, Mn% designating the Mn content of the steel, at most 8% of fresh martensite, at most 2.5% of cementite and partitioned martensite.