Coated Steel Sheet Composition for Strength-Formability Balance
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Solution Overview
Problem
Current coated steel sheets used in automotive manufacturing lack the necessary combination of high tensile strength, total elongation, and hole expansion ratio to meet the requirements for improved fuel efficiency and environmental conservation, while also maintaining good ductility and formability.
Innovation Solution
A method involving a cold-rolled steel sheet with specific chemical composition and heat treatment processes to achieve a microstructure comprising austenite, tempered martensite, and ferrite, followed by hot-dip coating, which enhances the mechanical properties of the steel sheet.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength of coated steel sheets is increased to at least 1100 MPa, then the fuel efficiency and environmental conservation are improved, but the ductility and formability deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.23%, Mn: 2.0-2.7%, Si: 0.2-1.6%, Al: 0.02-1.2%, Cr: 0.05-0.40%, Nb: 0.01-0.035%, Mo: 0.05-0.10%) and heat treatment parameters (annealing temperature Ta: Ac3-20°C, quenching temperature Qt: 200-270°C, partitioning temperature Pt: 400-480°C, partitioning time Pt: 50-250s) to achieve a microstructure containing 3-15% retained austenite, 30-65% tempered martensite, and 5-35% ferrite, which simultaneously provides high tensile strength (≥1100 MPa) and good ductility (total elongation ≥12%)
Solution Approach 2:
The patent creates a composite microstructure by combining multiple phases (retained austenite, tempered martensite, and ferrite) within the steel sheet. This composite structure allows the material to exhibit both high strength from the tempered martensite and good ductility from the retained austenite, resolving the contradiction between strength and formability
2Ease of operation
If the total elongation is increased to at least 12% to improve formability, then the ductility is improved, but the tensile strength decreases below 1100 MPa
Solution Approach 1:
The patent uses parameter changes to control the partitioning temperature (Pt: 400-480°C) and partitioning time (Pt: 50-250s) to achieve optimal carbon distribution, and controls the quenching temperature (Qt: 200-270°C) to achieve the desired microstructure with 3-15% retained austenite and 30-65% tempered martensite, simultaneously achieving high tensile strength (≥1100 MPa) and high total elongation (≥12%)
Solution Approach 2:
The patent creates a composite microstructure where retained austenite (3-15%) provides ductility and formability (total elongation ≥12%), while tempered martensite (30-65%) provides high strength (≥1100 MPa), and ferrite (5-35%) provides additional ductility, achieving both high strength and high formability simultaneously
3Ease of operation
If the hole expansion ratio is increased to at least 25% to improve stretch flangeability, then the ductility is improved, but the tensile strength decreases below 1100 MPa
Solution Approach 1:
The patent applies parameter changes by controlling the partitioning temperature (Pt: 400-480°C) and partitioning time (Pt: 50-250s) to achieve optimal carbon distribution, and controlling the quenching temperature (Qt: 200-270°C) to achieve the desired microstructure with 3-15% retained austenite and 30-65% tempered martensite, simultaneously achieving high tensile strength (≥1100 MPa) and high hole expansion ratio (≥25%)
Solution Approach 2:
The patent creates a composite microstructure where retained austenite (3-15%) and ferrite (5-35%) provide excellent ductility and stretch flangeability (hole expansion ratio ≥25%), while tempered martensite (30-65%) provides high strength (≥1100 MPa), achieving both high strength and high stretch flangeability simultaneously
4Weight of moving object
If high strength steel sheets are used to reduce automotive weight, then fuel efficiency is improved, but weldability deteriorates due to liquid metal embrittlement
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.23%, Mn: 2.0-2.7%, Si: 0.2-1.6%, Al: 0.02-1.2%, Cr: 0.05-0.40%, Nb: 0.01-0.035%, Mo: 0.05-0.10%) to achieve the desired microstructure while improving weldability by reducing liquid metal embrittlement susceptibility, and controlling heat treatment parameters to achieve high strength (≥1100 MPa) and low weight
Solution Approach 2:
The patent creates a composite microstructure with tempered martensite (30-65%), retained austenite (3-15%), and ferrite (5-35%) that provides high strength for weight reduction while the specific phase composition and distribution reduce susceptibility to liquid metal embrittlement, improving weldability
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 produces coated steel sheets with a tensile strength of at least 1100 MPa, total elongation of at least 12%, and a hole expansion ratio of at least 25%, suitable for use in automotive applications, while reducing the likelihood of liquid metal embrittlement and improving weldability.
Implementation Method 1
annealing the steel sheet at an annealing temperature TA so as to obtain a structure comprising at least 65% of austenite and at most 35% of intercritical ferrite
Implementation Method 2
quenching the sheet from a temperature of at least 600° C. at a cooling rate comprised between 20° C./s and 50° C./s down to a quenching temperature QT between 200° C. and 270° C.
Implementation Method 3
heating the sheet up to a partitioning temperature PT comprised between 400° C. and 480° C. and maintaining the sheet at this partitioning temperature PT for a partitioning time Pt comprised between 50 s and 250 s
Implementation Method 4
hot-dip coating the sheet at a temperature less than 515° C.
Data Source
AI summary
A method for producing a coated steel sheet having a tensile strength TS of at least 1100 MPa, a total elongation TE according to ISO standard 6892-1 of at least 12%, the product TSxTE of the tensile strength by the total elongation being at least 14200 MPa %, and a hole expansion ratio HER according to ISO standard 16630:2009 of at least 25%, the method including the following successive steps:providing a cold-rolled steel sheet, the chemical composition of the steel containing in weight %: 0.15%≤C≤0.23%, 2.0%≤Mn≤2.7%, with C+Mn/10≥0.420%, 0≤Cr≤0.40%, with Mn+Cr≥2.25%, 0.2%≤Si≤1.6%, 0.02%≤Al≤1.2%, with 1.0%≤Si+Al≤2.2%, 0≤Nb≤0.035%, 0≤Mo≤0.1%, the remainder being Fe and unavoidable impurities,annealing the steel sheet at an annealing temperature TA so as to obtain a structure comprising at least 65% of austenite and at most 35% of intercritical ferrite,quenching the sheet from a temperature of at least 600° C. at a cooling rate comprised between 20° C./s and 50° C./s down to a quenching temperature QT between 200° C. and 270° C.,heating the sheet up to a partitioning temperature PT comprised between 400° C. and 480° C. and maintaining the sheet at this partitioning temperature PT for a partitioning time Pt comprised between 50 s and 250 s,hot-dip coating the sheet at a temperature less than 515° C.,cooling the coated sheet down to the room temperature,the steel sheet having a microstructure consisting of, in surface fraction: between 3% and 15% of retained austenite, at least 30% of tempered martensite, at most 5% of fresh martensite, at most 35% of bainite, the sum of the surface fractions of tempered martensite, fresh martensite and bainite being comprised between 55% and 92%, and between 5% and 35% of ferrite.