Steel Sheet Microstructure for Formability and Welding
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Solution Overview
Problem
Conventional high strength steel sheets, such as those in the 980-1180 MPa grade, suffer from low ductility and formability, leading to press cracking during automotive part manufacturing. Additionally, laser welding of these steels results in significant softening of heat-affected zones (HAZ), causing fractures during deformation.
Innovation Solution
A steel sheet with a chemical composition of C: 0.06 to 0.25%, Si: 0.4 to 2.5%, Mn: 1.5 to 3.5%, and controlled microstructure, including 40% or more tempered martensite, 3 to 40% bainitic ferrite with low internal carbides, and 5 to 20% retained austenite, is developed. This microstructure is achieved through a specific heat treatment process involving hot rolling, cold rolling, and annealing, with controlled cooling and heating rates to stabilize retained austenite and reduce massive microstructure formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheets (980-1180 MPa grade) are used to enhance strength, then tensile strength is improved, but ductility and stretch flange formability deteriorate, causing press cracking
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.10-0.25%, Si: 1.00-2.00%, Mn: 1.50-3.00%) and heat treatment parameters (cooling rate: 10°C/s or more, holding temperature: 450-300°C, holding time: 180-600 seconds) to transform the microstructure and achieve both high strength and improved ductility
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (martensite for strength, retained austenite for ductility, bainitic ferrite) within the steel sheet, where each phase contributes different properties that collectively resolve the contradiction between strength and formability
2Strength
If martensite is increased in microstructure to enhance strength, then tensile strength is improved, but HAZ softening increases during laser welding, causing fractures
Solution Approach 1:
The patent applies local quality by creating a balanced microstructure where hard martensite provides strength while softer retained austenite and bainitic ferrite are distributed throughout, ensuring that during laser welding, not all regions soften uniformly, thus preventing preferential fracture in HAZ
Solution Approach 2:
The patent beforehand cushions against HAZ softening by pre-forming a multi-phase microstructure with retained austenite and bainitic ferrite that can absorb and distribute welding heat, preventing catastrophic softening and fracture during subsequent laser welding 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 resulting steel sheet exhibits high ductility, excellent stretch flange formability, and superior laser weldability, with a tensile strength of 980 MPa or higher. This enhances the formability of difficult-to-form parts and reduces the likelihood of HAZ fractures during welding.
Implementation Method 1
the steel sheet includes a steel microstructure including, in area fraction, polygonal ferrite: 10% or less (including 0%), tempered martensite: 40% or more, fresh martensite: 20% or less (including 0%), bainitic ferrite having 20 or less internal carbides per 10 μm2: 3 to 40%, and, in volume fraction, retained austenite: 5 to 20%
Implementation Method 2
a step of causing the steel sheet to reside in a range of temperatures of 340° C. or above and 590° C. or below for 20 seconds or more and 3000 seconds or less while cooling the steel sheet at an average cooling rate CR4 of 0.01 to 5° C./s
Data Source
AI summary
Provided are a steel sheet; a related member; and methods for manufacturing the same. The steel sheet has a chemical composition including, in mass %, C: 0.06 to 0.25%, Si: 0.4 to 2.5%, Mn: 1.5 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol. Al: less than 1.0%, and N: less than 0.015%, the balance being Fe and incidental impurities, the steel sheet being such that the steel sheet includes a steel microstructure including, in area fraction, polygonal ferrite: 10% or less (including 0%), tempered martensite: 40% or more, fresh martensite: 20% or less (including 0%), bainitic ferrite having 20 or less internal carbides per 10 μm2: 3 to 40%, and, in volume fraction, retained austenite: 5 to 20%, and the steel sheet has SC≥0.5/SC≥0.3×100 of 20% or more.