Heat-Treated Steel Material with High Dislocation Density
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Solution Overview
Problem
Current steel materials with high tensile strength of 1.800 GPa or more struggle to achieve both excellent toughness and weldability, as existing methods either compromise on strength or fail to provide sufficient ductility and formability.
Innovation Solution
A heat-treated steel material with specific compositions of C and Mn, along with other elements, is developed, which includes a chemical composition and manufacturing process that enhances dislocation density and substructure refinement, thereby achieving the desired strength, toughness, and weldability by controlling the heating and cooling rates and microstructure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of the steel sheet is increased to improve fuel efficiency and crashworthiness, then the tensile strength is improved, but the ductility and press formability deteriorate
Solution Approach 1:
The patent applies parameter changes by heating the steel sheet to the austenite region (Ac3 transformation point or higher) before forming, then rapidly cooling it to obtain martensite microstructure. This thermal parameter change temporarily improves ductility during heating, enabling formability, while the final cooled state achieves high strength (1800 MPa or more) with the martensitic structure.
2Strength
If conventional hot stamping method is used to improve formability and strength, then the tensile strength and formability are improved, but the toughness and weldability remain insufficient
Solution Approach 1:
The patent precisely controls chemical composition parameters (C: 0.15-0.40%, Mn: 1.50-3.50%, Cr: 0.50-2.00%, Ti: 0.010-0.050%, B: 0.0005-0.0050%) and thermal processing parameters (heating to Ac3+50℃ to Ac3+200℃, holding for 5-30 minutes, rapid cooling) to achieve a balance between strength (1800 MPa or more), toughness, and weldability, which conventional hot stamping cannot simultaneously achieve.
3Strength
If other conventional techniques are used to achieve high strength, then the tensile strength is improved, but sufficient toughness and weldability cannot be obtained
Solution Approach 1:
The patent creates a composite microstructure within the steel by combining multiple alloying elements (C, Mn, Cr, Ti, B) in specific proportions, where each element contributes different strengthening mechanisms (solid solution strengthening, precipitation hardening, grain boundary strengthening) to achieve a balanced combination of strength (1800 MPa or more), toughness, and weldability that cannot be achieved by single-element strengthening.
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 steel material achieves a tensile strength of 1.800 GPa or more while maintaining excellent toughness and weldability, overcoming the limitations of previous methods by leveraging the strengthening effects of C and Mn through dislocation and grain refinement mechanisms.
Implementation Method 1
Transformation from austenite to martensite is accompanied by expansion, so that in accordance with martensite transformation, strain (transformation strain) is introduced into surrounding non-transformed austenite
Implementation Method 2
heating a steel sheet to a temperature zone of not less than an Ac3 point nor more than 'the Ac3 point+200°C.'
Implementation Method 3
cooling the steel sheet from the temperature zone to an Ms point at a rate equal to or more than an upper critical cooling rate; and next, cooling the steel sheet from the Ms point to 100°C at an average cooling rate of 50°C/s or more
Data Source
AI summary
A heat-treated steel material is provided having strength of 1.800 GPa or more. The heat-treated steel material includes a chemical composition represented by, in mass %: C: 0.05% to 0.30%; Mn: 2.0% to 10.0%; Cr: 0.01% to 1.00%; Ti: 0.010% to 0.100%; B: 0.0010% to 0.0100%; Si: 0.08% or less; P: 0.050% or less; S: 0.0500% or less; N: 0.0100% or less; Ni: 0% to 2.0%; each of Cu, Mo, and V: 0% to 1.0%; each of Al and Nb: 0% to 1.00%; and the balance: Fe and impurities. “4612×[C]+102×[Mn]+605≥1800” is satisfied where [C] denotes a C content and [Mn] denotes a Mn content. The heat-treated steel material includes a microstructure in which 90 volume % or more is formed of martensite, and a dislocation density in the martensite is equal to or more than 9.0×1015 m−2.