Steel Sheet Microstructure Control for Ductility
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Solution Overview
Problem
Current steel sheets with high tensile strength, such as those in the 980 to 1180 MPa range, face challenges in achieving both high ductility and stretch-flangeability due to issues like cracking during pressing processes, particularly in forming complex components like center pillars, where known TRIP steel and Q & P processes fall short in providing sufficient uniform deformation and microstructure stability.
Innovation Solution
A two-step cooling process is employed to form a microstructure with stable retained γ and reduced massive microstructure, involving the formation of upper bainite and lower bainite, along with tempered martensite, and controlled carbon partitioning to enhance ductility and stretch-flangeability, achieved through specific heat treatment conditions including holding at 450°C for 14 to 200 seconds and rapid cooling to 315°C, followed by slow cooling and reheating to 400°C to stabilize the microstructure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheets (980 to 1180 MPa) are applied to automobile components, then weight reduction is achieved, but ductility and stretch-flangeability decrease causing cracking during pressing processes
Solution Approach 1:
The patent applies parameter changes by precisely controlling the heat treatment parameters (cooling rates of 10-100°C/s, holding temperatures of 450-550°C for 1-10 minutes) to transform the microstructure and achieve both high strength and improved ductility. The composition parameters (C: 0.10-0.30%, Si: 1.00-2.00%, Mn: 1.50-3.00%) are also optimized to enable TRIP effect.
Solution Approach 2:
The patent utilizes phase transitions by controlling the transformation of austenite to martensite and bainite during heat treatment. The TRIP (transformation-induced plasticity) effect is harnessed where retained austenite transforms to martensite during forming, providing dynamic strengthening and improved ductility simultaneously.
2Reliability
If TRIP steel with retained γ is developed to improve ductility, then uniform elongation increases, but stretch-flangeability remains insufficient for difficult-to-form components
Solution Approach 1:
The patent applies local quality by creating different microstructural regions with specific properties: retained austenite regions for TRIP effect and ductility, bainitic ferrite regions for strength, and controlling their spatial distribution to achieve both uniform elongation and stretch-flangeability simultaneously.
Solution Approach 2:
The patent uses composite materials by combining multiple phases (retained austenite, bainitic ferrite, martensite) in a controlled microstructure. This composite microstructure leverages the advantages of each phase: austenite for TRIP effect, bainitic ferrite for strength, and martensite for hardness, achieving balanced formability.
3Reliability
If Q & P process is used to form microstructure with ferrite, tempered martensite, and retained austenite, then ductility improves, but breakage occurs in difficult-to-form components due to insufficient uniform deformation
Solution Approach 1:
The patent applies preliminary action by performing austenite formation at high temperature (800-950°C) before cooling, ensuring uniform austenite distribution throughout the microstructure. This preliminary homogeneous austenite structure serves as a stable foundation that prevents breakage during subsequent cooling and forming operations.
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 approach results in steel sheets with significantly improved ductility and stretch-flangeability, meeting the requirements for high tensile strength while preventing cracking during forming, and can be applied to various applications like automobiles and household appliances.
Implementation Method 1
cooling the steel to a temperature in the range of 450°C to 300°C at a cooling rate of 10°C/s or more, and holding the steel for 180 to 600 seconds such that controlling retained austenite to 5% by area percentage or more, bainitic ferrite to 60% by area percentage or more
Implementation Method 2
This utilizes the principle of Quenching & Partitioning (Q & P, quenching and partitioning of carbon from martensite to austenite), which includes in a cooling process once cooling to a temperature range between a martensite transformation start temperature (Ms point) and a martensite transformation finish temperature (Mf point) and then reheating and holding to stabilize retained γ
Implementation Method 3
annealing steel containing C: 0.10% to 0.45%, S: 0.5% to 1.8%, and Mn: 0.5% to 3.0% and holding the steel in the range of 350°C to 500°C for 1 to 30 minutes to form retained γ
Data Source
AI summary
A steel sheet having a specified chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure comprising ferrite: 5% or less, and at least one of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ: 95% to 100%, and retained γ: 5% to 20%. Retained γUB has a specified area percentage SγUB, retained γLB has a specified distribution number NγLB, and at least one of (i) fresh martensite has a specified equivalent circular grain diameter and aspect ratio and (ii) retained γ grains has a specified equivalent circular grain diameter and aspect ratio.