Gradient Microstructure Hot-Pressed Steel for Strength and Bendability
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Solution Overview
Problem
Automotive parts manufactured using existing hot press processes face challenges in achieving both high strength and high bendability, leading to issues such as difficulty in forming and degraded shape fixability due to the contradictory nature of these properties.
Innovation Solution
A hot pressed part with a gradient microstructure that changes in the thickness direction, comprising a hard layer, a soft layer, and a transition layer, is achieved by adjusting the manufacturing conditions of the steel sheet, including dew point, annealing temperature, and forming start temperature, to optimize the chemical composition and microstructure for enhanced strength and bendability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the steel sheet is strengthened to achieve high tensile strength, then the tensile strength increases, but the bendability deteriorates
Solution Approach 1:
The patent applies local quality by creating a gradient microstructure where the steel sheet has different microstructures in different regions: a first microstructure in the surface layer providing high strength, and a second microstructure in the central portion providing good bendability. This allows each region to have properties optimized for its specific function, resolving the contradiction between overall strength and bendability.
Solution Approach 2:
The patent effectively creates a composite material structure by combining two different microstructures within the same steel sheet. The gradient microstructure acts as a composite where the surface layer and central portion have distinct microstructural characteristics, allowing the material to exhibit both high strength and good bendability simultaneously.
2Ease of operation
If cold forming is used to improve bendability, then the bendability improves, but formability deteriorates due to difficulty in forming and degraded shape fixability
Solution Approach 1:
The patent applies parameter changes by controlling the microstructural parameters through specific compositional ranges and heat treatment conditions. By adjusting the chemical composition (C: 0.15-0.30%, Si: 0.01-1.8%, Mn: 1.5-3.0%) and controlling the microstructure formation, the material achieves optimal balance between bendability and formability, resolving the contradiction in manufacturing performance.
3Ease of manufacture
If hot press process is applied to improve formability, then the formability improves, but bendability becomes insufficient
Solution Approach 1:
The patent resolves this contradiction by applying local quality through gradient microstructure control. The surface layer is optimized for strength and formability through its specific microstructure, while the central portion maintains microstructural characteristics that preserve bendability. This localized optimization allows the hot press process to improve formability without sacrificing the bendability achieved through the gradient structure.
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 hot pressed part exhibits a tensile strength of 1100 MPa or more and a bending angle of 70 degrees or more, enabling weight reduction of automotive bodies while maintaining high safety standards.
Implementation Method 1
In the cooling (rapid cooling) process in the tool of press forming, the microstructure of the part undergoes phase transformation from austenite phase to martensite phase
Implementation Method 2
after heating a steel sheet to an austenite region, the steel sheet is conveyed to a press machine
Implementation Method 3
the steel sheet is formed into a part of a desired shape in a tool of press forming, and simultaneously rapid-cooled
Data Source
AI summary
A hot pressed part comprises: a predetermined chemical composition; and a steel microstructure that is a gradient microstructure in which, in a thickness direction, a surface layer is a soft layer, an inside is a hard layer, and a layer between the soft layer and the hard layer is a transition layer.