Multiphase Steel with Internal Oxidation for Automotive Parts
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Solution Overview
Problem
Current high-strength steels face a challenge in achieving a balance between high tensile strength and ductility, with existing Advanced High Strength Steels (AHSS) often sacrificing ductility for increased strength, and lacking sufficient yield strength and hole expansion performance needed for complex automotive parts.
Innovation Solution
A method for producing hot dip coated multiphase steel with a specific chemical composition and processing steps that result in a steel with tensile strength above 980 MPa, yield strength above 500 MPa, total elongation above 8%, and hole expansion superior to 20%, while being compatible with continuous annealing galvanizing lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the ultimate tensile strength is increased, then the strength is improved, but the ductility decreases
Solution Approach 1:
The invention creates a composite microstructure containing multiple phases (martensite, bainite, and ferrite) within the steel. The martensite provides high strength, while the bainite and ferrite phases provide ductility and toughness. This multi-phase composite structure allows the steel to achieve both high ultimate tensile strength (≥1100 MPa) and adequate ductility (total elongation ≥10%), resolving the contradiction between strength and ductility.
Solution Approach 2:
The invention applies local quality by creating regions with different microstructural characteristics. Specifically, it controls the distribution and morphology of soft phases (bainite and ferrite) within the martensitic matrix, ensuring that these ductile phases are strategically positioned to provide local ductility while maintaining overall high strength. The coefficient of variation of the circle-equivalent diameter of soft phases is controlled to be ≤1.0, ensuring uniform local properties throughout the material.
2Ease of operation
If the yield strength and hole expansion performance are improved, then the formability is improved, but the complexity of the part design increases
Solution Approach 1:
The invention changes the material parameters by precisely controlling the chemical composition (C: 0.15-0.25%, Si: 1.2-2%, Mn: 1.8-3.0%, and other elements within specified ranges) and processing parameters (continuous annealing temperature, cooling rate, and phase transformation conditions). These parameter changes enable the steel to achieve superior yield strength and hole expansion performance, improving formability without requiring more complex part designs.
3Strength
If the tensile strength is increased to above 1100 MPa, then the strength is improved, but the total elongation decreases below typical levels
Solution Approach 1:
The invention employs a composite microstructure with martensite (≥50% area) providing high strength (≥1100 MPa) and soft phases (bainitic ferrite ≥15% area and polygonal ferrite ≥5% area) providing ductility. The controlled distribution and size uniformity of these soft phases (coefficient of variation ≤1.0) ensure that the steel achieves both high tensile strength and adequate total elongation, resolving the contradiction between strength and elongation.
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 steel with enhanced mechanical properties, including high tensile strength, ductility, and improved hole expansion, making it suitable for complex automotive parts without compromising yield strength, thus addressing the limitations of existing AHSS.
Implementation Method 1
The metal structure of the steel plate has martensite, and the soft phases of bainitic ferrite and polygonal ferrite
Implementation Method 2
reheating the slab at a temperature Treheat above 1180° C.
Implementation Method 3
cooling the hot rolled steel at conventional cooling rate until a coiling temperature Tcoiling between 500 and 800° C.
Implementation Method 4
hot dip coating the cold rolled steel to obtain coated cold rolled steel
Data Source
AI summary
A method for producing a cold rolled and hot dip coated steel sheet is provided. The method includes casting a steel into a slab, reheating, hot rolling, cooling, coiling, descaling and cold rolling the slab. The cold rolled steel sheet is annealed so a layer of iron oxide forms on the surface with an internal oxidation underneath. The sheet is then heated so the surface is oxidized and the layer of iron oxide is fully reduced to obtain an internally oxidized depth between 200 nm and 100 μm which includes one or more of Si, Mn, Al, Ti containing oxides. The sheet is then hot dip coated and cooled.