1180 MPa Plated Steel Sheet With Bendability and HAZ Crack Resistance
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing high strength steel sheets with a tensile strength of 980 MPa face limitations in bendability, intra-steel sheet cracking resistance in spot weld HAZ, and dimensional accuracy, particularly when used in automotive framework structural parts.
Innovation Solution
A high strength steel sheet with a microstructure composed of martensite, ferrite, and retained austenite, controlled chemical composition, and specific manufacturing processes to achieve a tensile strength of 1180 MPa or more, excellent bendability, and high resistance to intra-steel sheet cracking in spot weld HAZ.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength of steel sheets is increased to 1180 MPa or higher, then the strength parameter is improved, but the formability and bendability deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.40%, Si: 0.10-0.60%, Mn: 1.50-3.50%, Ti: 0.020-0.100%) and microstructural parameters (martensite area fraction: 80-99%, ferrite area fraction: 1-20%, retained austenite volume fraction: 3-20%) to achieve a balance between high tensile strength (1180 MPa or higher) and excellent formability, allowing the steel sheet to be formed into parts with bent portions where R/t < 5.0 without significant cracking
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (martensite, ferrite, and retained austenite) within the steel sheet. This multi-phase composite structure combines the high strength of martensite with the ductility and formability contributions from ferrite and retained austenite, enabling the steel to achieve both TS ≥ 1180 MPa and excellent bendability
2Strength
If the yield ratio of steel sheets is increased to improve impact energy absorption, then the strength parameter is improved, but the dimensional accuracy deteriorates due to increased springback
Solution Approach 1:
The patent applies parameter changes by controlling the yield ratio within the specific range of 65-90% through precise adjustment of chemical composition and microstructure. This optimized yield ratio range enables the steel sheet to achieve high impact energy absorption while maintaining dimensional accuracy with reduced springback in formed parts
3Reliability
If the hold time is increased to prevent intra-steel sheet cracking in spot weld HAZ, then the reliability is improved, but the productivity deteriorates
Solution Approach 1:
The patent applies parameter changes by optimizing the chemical composition parameters, particularly Si content (0.10-0.60%) and Ti content (0.020-0.100%), which fundamentally improves the steel sheet's resistance to intra-steel sheet cracking in spot weld HAZ. This compositional optimization enables the use of short hold times (0.1-0.5 seconds) while still preventing LME cracking, thereby maintaining high welding productivity
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 steel sheet achieves high tensile strength, excellent bendability, and improved dimensional accuracy, with reduced cracking in spot weld HAZ, suitable for automotive framework structural parts.
Implementation Method 1
a steel microstructure in which an area fraction of martensite is 80% or more and 99% or less and in which a sum of an area fraction of ferrite and/or a volume fraction of retained austenite is more than 0% and 20% or less
Implementation Method 2
zinc in the galvanized layers diffuses into grain boundaries in the surface layers of the steel sheets, causing liquid metal embrittlement (LME)
Data Source
AI summary
It is an object to provide a high strength steel sheet of 1180 MPa or more and a high strength coated steel sheet of 1180 MPa or more that have excellent bendability and high resistance to intra-steel sheet cracking in a spot weld HAZ and can be used for the production of parts with high dimensional accuracy, to provide methods for producing the steel sheets, and to provide a member. A high strength steel sheet has a prescribed chemical composition. An effective Ti mole fraction (xTi,eff) determined by xTi,eff = xTi - xN - xS (each element symbol represents the mole fraction of the element in the steel) satisfies 0.001 or more. The high strength steel sheet has, at a position 1/4 of the thickness, a steel microstructure in which the area fraction of martensite is 80% or more and 99% or less and in which the sum of the area fraction of ferrite and/or the volume fraction of retained austenite is more than 0% and 20% or less. At a position 5 µm from the surface of the steel sheet in the thickness direction, the concentration of Si is 0.10% or more and 0.60% or less. The number density of MnS inclusions present in Mn-segregated regions on the surface of the steel sheet is 5.0 pieces/mm2 or less. The standard deviation of the Vickers hardness of the surface of the steel sheet is 15 or less, and the hardness variation frequency per 1100 µm in the sheet width direction on the surface of the steel sheet is 7 or less.