Hot-Rolled Steel Sheet Q&P Processing for Strength-Ductility Balance
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing manufacturing processes for high strength steels complicate the production route and do not achieve the desired combination of high yield strength, tensile strength, uniform elongation, and hole expansion ratio required for automotive applications, particularly in DP and TRIP steels.
Innovation Solution
A hot rolled and heat-treated steel sheet with specific elemental composition and microstructure, including controlled annealing, quenching, and partitioning processes, to achieve a microstructure of 5-45% ferrite, 25-85% partitioned martensite, 10-30% retained austenite, and low pancaking index, ensuring high ductility and strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If multiple annealing and cold rolling steps are added to control Mn content in retained austenite, then the mechanical properties (high ductility and strength) are improved, but the manufacturing process complexity increases
Solution Approach 1:
The patent applies preliminary action by adding Mn to the steel composition before hot rolling, ensuring the desired Mn content is pre-established in the material. This eliminates the need for subsequent annealing and cold rolling steps that were previously required to control Mn distribution, thereby simplifying the manufacturing process while maintaining the ability to achieve high ductility and strength through the Q&P treatment
Solution Approach 2:
The patent changes the chemical composition parameter by specifying a steel with particular Mn content (3.0-8.0% by weight) and controlled grain size (5-20 μm) before Q&P treatment. This parameter control approach allows the process to achieve the desired microstructure and mechanical properties without requiring multiple intermediate annealing and rolling steps, thus reducing process complexity
2Strength
If high strength steel sheets are produced to meet automotive safety requirements, then the vehicle safety is improved, but the vehicle weight increases
Solution Approach 1:
The patent utilizes phase transitions through the quenching and partitioning (Q&P) heat treatment process. By controlling the transformation of austenite to martensite and retaining some austenite phase, the process achieves high strength (yield strength >950 MPa, tensile strength >1180 MPa) while maintaining good ductility. This enables the production of high-strength steel sheets that can reduce vehicle weight compared to conventional steels, thereby improving fuel efficiency while meeting safety requirements
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite and retained austenite phases through Q&P treatment. This composite structure at the micro level provides both high strength from the martensite and ductility from the retained austenite, enabling the steel to achieve superior mechanical properties that allow for weight reduction in automotive applications
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 strength above 950 MPa, tensile strength above 1180 MPa, uniform elongation above 10%, and hole expansion ratio above 25%, simplifying the manufacturing process and meeting automotive requirements.
Implementation Method 1
the hot rolled steel sheet has to be annealed a first time, cold rolled, and annealed a second time before the quenching and the partitioning steps
Implementation Method 2
quenching the hot-rolled steel sheet to a quenching temperature TQ lower than (Ms-50° C.)
Implementation Method 3
reheating the quenched steel sheet to a partitioning temperature TP comprised between 350° C. and 550° C., and maintaining the quenched steel sheet at said partitioning temperature for a partitioning time comprised between 1 s and 1000 s
Data Source
AI summary
A method for manufacturing a hot rolled and heat-treated steel sheet includes casting to obtain a semi-finished product having a composition comprising, by weight percent: C: 0.12-0.25%, Mn: 3.0-8.0%, Si: 0.7-1.5%, Al: 0.3-1.2%, B: 0.0002-0.004%, S≤0.010%, P≤0.020%, N≤0.008%, and iron; reheating at a Treheat between 1150° C. and 1300° C.; hot rolling the reheated semi-finished product with a finish rolling temperature (Tnr-100° C. to 950° C.), coiling the hot rolled steel sheet at a Tcoil (20° C. and 700° C.) and cooling to obtain a microstructure comprising martensite and bainite, M+B>80%, F<20%, and <20% of the sum of martensite-austenite (M-A) islands and carbides, and having the multiplication of PAGS <1000 μm2 and a pancaking index <5; reheating, quenching, reheating to a partitioning temperature, holding for 1 to 1000 seconds, and cooling.