Steel sheet for manufacturing press hardened parts, press hardened part having a combination of high strength and crash ductility, and manufacturing methods thereof
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Solution Overview
Problem
Existing steel sheets for press hardened parts lack a combination of high tensile strength, ductility, and weldability, particularly in automotive applications, and often require costly coatings or processes that compromise mechanical properties.
Innovation Solution
A steel sheet with a specific composition and microstructure, including controlled amounts of elements like C, Mn, Si, Al, Cr, Mo, Ti, B, Nb, N, S, and P, along with a metallic coating, that allows for high yield strength, tensile strength, and ductility, and enables weldability without compromising mechanical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional steel sheets are used for press hardening to achieve high tensile strength, then tensile strength is improved (TS ≥ 1500 MPa), but ductility deteriorates (total elongation < 6%)
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet, specifically controlling carbon content at 0.23-0.38% (higher than conventional), manganese at 1.50-3.50%, and adding microalloying elements (Ti: 0.020-0.080%, Nb: 0.010-0.050%, V: 0.020-0.100%), along with precise control of impurities (B: 0.0005-0.0050%, S: 0.0010-0.0050%, P: 0.0010-0.0100%). These parameter changes enable the steel to achieve both high tensile strength (≥1500 MPa) and improved ductility (total elongation ≥6%) after press hardening.
Solution Approach 2:
The invention creates a composite microstructure consisting of martensite as the primary phase with dispersed fine precipitates of titanium carbide, niobium carbonitride, and vanadium carbide. This composite structure at the microlevel provides both the strength from martensite and the ductility from the precipitate reinforcement, resolving the contradiction between strength and ductility.
2Strength
If high carbon content steel is used to increase tensile strength, then strength is improved, but weldability deteriorates
Solution Approach 1:
The invention carefully balances the carbon content at 0.23-0.38% (higher than conventional for strength) while simultaneously controlling the carbon equivalent through precise control of alloying elements and impurities. The addition of microalloying elements (Ti, Nb, V) and controlled impurity levels (B, S, P) modifies the hardenability and weldability parameters, enabling the high-strength steel to maintain adequate weldability despite elevated carbon content.
3Strength
If conventional press hardening processes are used to achieve high strength, then tensile strength is improved, but ductility in bent and welded zones deteriorates
Solution Approach 1:
The invention performs preliminary microalloying with Ti, Nb, and V elements before press hardening, which creates fine precipitates that will remain stable during the subsequent heating and cooling cycles. This preliminary action ensures that the microstructure is pre-configured to maintain ductility in bent and welded zones after the hardening process, rather than attempting to correct ductility issues afterward.
Solution Approach 2:
The invention creates a composite microstructure with martensite matrix and fine dispersed precipitates of titanium carbide, niobium carbonitride, and vanadium carbide. This composite structure provides crack resistance and maintains ductility in critical zones (bent and welded areas) while achieving high overall tensile strength, as the precipitates act as obstacles to crack propagation and maintain structural integrity under stress.
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 press hardened parts with yield strength of at least 1000 MPa, tensile strength between 1300 and 1600 MPa, fracture strain over 0.50, and bending angle over 60°, with improved weldability and corrosion resistance, maintaining mechanical integrity across weld zones.
Implementation Method 1
a steel sheet for hot forming to produce a part with a martensitic microstructure
Implementation Method 2
Holding the part in the tooling after forming has been performed makes it possible to achieve a rapid cooling that leads to the formation of hardened microstructures
Data Source
AI summary
A steel sheet for the manufacture of a press hardened part is provided, having a composition of: 0.15%≤C≤0.22%, 3.5%≤Mn<4.2%, 0.001%≤Si≤1.5%, 0.020%≤Al≤0.9%, 0.001%≤Cr≤1%, 0.001%≤Mo≤0.3%, 0.001%≤Ti≤0.040%, 0.0003%≤B≤0.004%, 0.001%≤Nb≤0.060%, 0.001%≤N≤0.009%, 0.0005%≤S≤0.003%, 0.001%≤P≤0.020%. A microstructure has less than 50% ferrite, 1% to 20% retained austenite, cementite, such that the surface density of cementite particles larger than 60 nm is lower than 107/mm2, and a complement of bainite and/or martensite, the retained austenite having an average Mn content of at least 1.1*Mn %. Press-hardened steel part obtained by hot forming the steel sheet, and manufacturing methods thereof.