Hot-Press Steel Sheet Composition for Crack-Resistant Bendability
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Solution Overview
Problem
Existing steel sheets for hot press forming face challenges in achieving high bendability while maintaining ultra-high strength, often resulting in increased susceptibility to crack formation and poor formability due to micro-segregation and non-uniform microstructures.
Innovation Solution
A steel sheet composition with specific ranges of C, Si, Mn, Al, Ti, Cr, P, S, B, Mo, Cu, and Ni, along with a thermal history that includes a painting baking treatment after hot press forming, is used to reduce metallographic non-uniformity and enhance austenite retention, improving bendability and strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the carbon content is increased to achieve ultra-high strength (tensile strength ≥1800 MPa), then the strength is improved, but the bendability deteriorates and susceptibility to crack propagation increases
Solution Approach 1:
The patent optimizes the chemical composition parameters by limiting carbon content to 0.20-0.40 wt% (avoiding excessive carbon that would harm bendability) while precisely controlling ranges of Si (0.50-1.50 wt%), Mn (1.00-2.00 wt%), and other alloying elements. This parameter optimization enables achieving ultra-high strength ≥1800 MPa while maintaining bendability through balanced composition design
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite as the primary phase with controlled amounts of retained austenite and bainite. This composite microstructure achieves ultra-high strength through martensite while the retained austenite (5-20%) provides bendability and crack resistance through TRIP effect, resolving the contradiction between strength and formability
2Ease of manufacture
If conventional steel sheet compositions are used for hot press forming, then the manufacturing process is simple, but micro-segregation and non-uniform microstructures occur, resulting in poor bendability
Solution Approach 1:
The patent introduces specific parameter ranges for alloying elements that prevent micro-segregation during hot press forming. By controlling Si (0.50-1.50 wt%), Mn (1.00-2.00 wt%), Cr (0.10-1.00 wt%), and other elements within precise ranges, the steel achieves uniform microstructure after hot press forming, enabling bendability while maintaining manufacturing simplicity
Solution Approach 2:
The patent achieves homogeneous microstructure by optimizing the chemical composition to prevent segregation during phase transformation. The controlled composition ensures uniform distribution of alloying elements, resulting in consistent martensitic structure with uniform retained austenite distribution, which improves bendability without complicating the manufacturing process
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 proposed steel sheet achieves a marked improvement in bendability and ultra-high strength, suitable for automotive applications, with a tensile strength x bendability balance of 115,000 MPa° or greater, addressing the limitations of previous methods.
Implementation Method 1
a steel sheet blank having a tensile strength of 500 MPa to 800 MPa is heated to a temperature within an austenite temperature range of an Ac3 transformation temperature or higher and is transferred to the press equipped with a cooling device to form the blank and quench the press formed blank (product) in the dies. Therefore, a press formed product ultimately contains martensite or a mixture of martensite and bainite
Implementation Method 2
a steel sheet blank having a tensile strength of 500 MPa to 800 MPa is heated to a temperature within an austenite temperature range of an Ac3 transformation temperature or higher
Implementation Method 3
transferred to the press equipped with a cooling device to form the blank and quench the press formed blank (product) in the dies
Data Source
AI summary
The present invention provides: a steel sheet capable of manufacturing a formed product having superior bendability and ultra-high strength when compared with conventional steel sheets for manufacturing a hot press formed product; the formed product having superior bendability and ultra-high strength by using the same; and a method for manufacturing the same.