Press-Hardening Steel Composition With Two-Stage Cooling Balance
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Solution Overview
Problem
Current press hardening processes for steel parts in automotive applications face challenges in achieving a balance of high mechanical strength, impact resistance, corrosion resistance, dimensional accuracy, and weldability, while also maintaining sufficient ductility and avoiding costly high-nickel or high-chromium compositions.
Innovation Solution
A steel composition with specific chemical ranges (0.062%≤C≤0.095%, 1.4%≤Mn≤1.9%, 0.2%≤Si≤0.5%, 0.020%≤Al≤0.070%, 0.02%≤Cr≤0.1%) and a microstructure comprising predominantly martensite with controlled cooling rates and deformation levels, allowing for press hardening with yield stress between 700 and 950 MPa, tensile stress between 950 and 1200 MPa, and a bending angle greater than 75°, along with a metallic coating for enhanced corrosion resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high carbon content steel (C>0.1%) is used to achieve high tensile strength (>1500 MPa), then tensile strength is improved, but total elongation deteriorates (lower than 6%)
Solution Approach 1:
The patent changes the chemical composition parameters by reducing carbon content to 0.040-0.100% (below conventional levels) while precisely controlling alloying elements (Mn: 1.00-2.00%, Si: 0.10-0.50%, Cr: 0.50-2.00%, Mo: 0.10-0.50%, B: 0.0005-0.0050%) to achieve both high tensile strength (≥1500 MPa) and improved total elongation (≥6%) after press hardening
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite as the primary phase with controlled amounts of retained austenite and bainite, achieved through specific alloy composition and heat treatment, resulting in a material that combines high strength with enhanced ductility
2Strength
If high nickel content (0.5-1.8% Ni) is added to achieve high tensile strength (>1000 MPa) and elongation (>10%), then mechanical properties are improved, but manufacturing cost deteriorates
Solution Approach 1:
The patent replaces expensive nickel (typically 0.5-1.8% in conventional steels) with cheaper alternative alloying elements, specifically using Mn (1.00-2.00%), Cr (0.50-2.00%), Mo (0.10-0.50%), and B (0.0005-0.0050%) to achieve the same hardenability and mechanical properties at lower cost
Solution Approach 2:
The patent merges the functions of multiple alloying elements (Mn for hardenability, Cr for strength, Mo for tempering resistance, B for grain boundary strengthening) to collectively replace the role of nickel, achieving cost reduction while maintaining or improving mechanical properties
3Strength
If high chromium content (1.00-2.00% Cr) is used to achieve high tensile strength (>1200 MPa) and elongation (>12%), then mechanical properties are improved, but manufacturing cost deteriorates
Solution Approach 1:
The patent optimizes chromium content to a balanced range (0.50-2.00%, preferably 0.80-1.50%) and combines it with precise control of other elements (Mn, Si, Mo, B) to achieve the required mechanical properties with minimized total alloy content, reducing manufacturing cost while maintaining high strength and ductility
4Strength
If press hardening is used to achieve high mechanical strength and dimensional accuracy, then strength is improved, but ductility in deformed zones deteriorates
Solution Approach 1:
The patent performs preliminary microstructure optimization through controlled alloying (especially B for grain boundary strength, and Mn/Cr for hardenability) before press hardening, creating a microstructure that can withstand the deformation process without losing ductility in heavily deformed zones
Solution Approach 2:
The patent creates a composite microstructure with martensite (for strength) combined with retained austenite and bainite (for ductility), where the softer phases act as ductility reservoirs that can transform during deformation (TRIP effect), maintaining ductility in heavily deformed zones even after press hardening
5Strength
If press hardening is used to achieve high mechanical strength, then strength is improved, but weldability deteriorates
Solution Approach 1:
The patent reduces carbon content to 0.040-0.100% (lower than conventional high-strength steels) to improve weldability by reducing carbon equivalent and susceptibility to cold cracking, while compensating for strength through optimized alloying elements (Mn, Cr, Mo, B) and controlled microstructure to maintain tensile strength ≥1500 MPa
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 provides steel parts with improved mechanical properties, high ductility, and weldability, including Laser welding compatibility, while reducing material costs by avoiding high-nickel or high-chromium compositions and ensuring structural integrity in vehicle applications.
Implementation Method 1
A hardened steel part is obtained by heating a steel blank to a temperature at which the steel is transformed into austenite and then hot formed in a press. The blank is simultaneously rapidly cooled in the press tool and held so to prevent distortion thus obtaining a martensitic and/or bainitic microstructure.
Implementation Method 2
The blank is simultaneously rapidly cooled in the press tool and held so to prevent distortion thus obtaining a martensitic and/or bainitic microstructure.
Implementation Method 3
A hardened steel part is obtained by heating a steel blank to a temperature at which the steel is transformed into austenite and then hot formed in a press.
Data Source
AI summary
A process for manufacturing a press hardened steel part is provided. The steel of the part has a chemical composition including, in weight: 0.062%≤C≤0.095%, 1.4%≤Mn≤1.9%, 0.2%≤Si≤0.5%, 0.020%≤Al≤0.070%, 0.02%≤Cr≤0.1%, wherein: 1.5%≤(C+Mn+Si+Cr)≤2.7%, 0.040%≤Nb≤0.060%, 3.4×N≤Ti≤8×N wherein: 0.044%≤(Nb+Ti)≤0.090%, 0.0005≤B≤0.004%, 0.001%≤N≤0.009%, 0.0005%≤S≤0.003%, 0.001%≤P≤0.020%, optionally: 0.0001%≤Ca≤0.003%, and the remainder being Fe and unavoidable impurities. The process includes hot forming the heated blank in the forming press so as to obtain a formed part and cooling the formed part at a cooling rate CR1 between 40 and 360°C/s in a temperature range from 750 to 450°C. and at a cooling rate CR2 between 15 to 150°C/s in a temperature range from 450°C to 250°C. wherein CR2<CR1.