Steel Sheet Hot Forming Suppresses Ferritic Transformation
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Solution Overview
Problem
Current hot forming techniques face challenges in suppressing strain-induced ferritic transformation during high strain forming, leading to non-uniform hardness and anisotropy in toughness, particularly in complex-shaped components like those requiring burring, which affects the dimensional accuracy and toughness of high-strength steel sheets.
Innovation Solution
A steel sheet with a controlled chemical composition, including specific ranges of C, Si, Mn, Cr, B, P, S, N, Ni, Cu, Ti, and Nb, along with a cleanliness level and Mn segregation degree, is developed to suppress strain-induced ferritic transformation, ensuring uniform hardness and improved toughness during hot forming.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheet with tensile strength of 780 MPa or higher is used, then strength is improved, but press formability deteriorates
Solution Approach 1:
The patent applies temperature parameter changes to resolve the contradiction between strength and press formability. By heating the steel sheet to the austenite region (Ac3 transformation point or higher) before forming, the material becomes soft and formable. After forming, rapid cooling transforms the austenite to martensite, achieving high strength (1100 MPa or higher). This temporal separation of formability (at high temperature) and strength (after cooling) resolves the contradiction.
2Ease of manufacture
If hot pressing is applied to complex-shaped members requiring high strain forming, then formability is improved, but strain-induced ferritic transformation occurs causing non-uniform hardness
Solution Approach 1:
The patent controls the temperature parameter during hot pressing to prevent ferritic transformation. By maintaining the steel sheet temperature at the Ac3 transformation point or higher throughout the forming process, the austenite phase is stabilized even under high strain conditions. This prevents strain-induced ferritic transformation and ensures uniform hardness distribution in the final martensitic structure after cooling.
Solution Approach 2:
The patent applies preliminary heating to transform the steel sheet into the austenite phase before forming begins. This preliminary phase transformation creates a uniform austenitic structure that is resistant to strain-induced ferritic transformation during subsequent high strain forming. The preliminary action of heating and phase transformation prepares the material to maintain hardness uniformity throughout the forming process.
3Manufacturing precision
If hot forming temperature is increased to suppress strain-induced ferritic transformation, then hardness uniformity is improved, but productivity decreases and manufacturing cost increases
Solution Approach 1:
The patent optimizes the temperature parameter to the minimum effective level (Ac3 transformation point or higher) rather than using excessively high temperatures. This allows suppression of strain-induced ferritic transformation while minimizing energy consumption and cycle time. The precise control of temperature at the transformation point balances hardness uniformity with productivity and cost-effectiveness.
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 solution achieves stable hardness distribution and low anisotropy in toughness, making the steel sheet suitable for complex-shaped components like vehicle underbody members, with enhanced productivity and cost-effectiveness by preventing ferritic transformation during high strain forming.
Implementation Method 1
a structure mainly including martensite can be obtained after the quenching
Implementation Method 2
strain-induced ferritic transformation in the formed portion is suppressed
Data Source
AI summary
In a steel sheet, a cleanliness of a metal structure is 0.08% or less, α which is an Mn segregation degree is 1.6 or less, and a difference ΔHv between a low strain formed portion that undergoes a plastic strain of 5% or less and a high strain formed portion that undergoes a plastic strain of 20% or higher in a hot forming in average hardness after the hot forming is 40 or less.