Bake-Hardenable Steel Sheet Composition for Formability
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Solution Overview
Problem
Conventional bake-hardenable cold-rolled steel sheets face challenges in achieving a balance between high tensile strength, bake-hardenability, cold aging resistance, deep-drawability, and reduced planar anisotropy, with existing methods often compromising on formability and surface defects.
Innovation Solution
A bake-hardenable high-strength cold-rolled steel sheet with specific chemical compositions and manufacturing processes, including controlled cold rolling reduction ratios and annealing temperatures, to achieve tensile strength between 300 MPa to 450 MPa, excellent bake hardenability, cold aging resistance, and reduced planar anisotropy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the amount of solid-solution elements (C and N) is increased to enhance bake hardenability, then the bake hardenability improves, but the formability deteriorates due to cold aging
Solution Approach 1:
The invention precisely controls the parameters of solid-solution elements (C: 0.0010-0.0040%, N: 0.0010-0.0050%) and the ratio of Mn to P (1.6-45) to achieve optimal bake hardenability while preventing excessive cold aging. This parameter optimization resolves the contradiction by finding the precise balance point where BH property is enhanced without compromising formability.
Solution Approach 2:
The invention creates a composite microstructure through controlled addition of multiple elements (Mn, P, Nb, Mo, Al) that work synergistically. The combination of solid-solution strengthening elements (Mn, P) with microstructure control elements (Nb, Mo) and deoxidation elements (Al) produces a composite effect that simultaneously achieves high bake hardenability and acceptable formability by distributing the functional roles across multiple components.
2Strength
If Mn and P are added to enhance the strength of the steel, then the tensile strength increases, but the deep-drawability deteriorates due to changes in r value and planar anisotropy
Solution Approach 1:
The invention optimizes the parameters of Mn (0.1-0.8%) and P (0.01-0.07%) within specific ranges and controls their ratio (Mn/P: 1.6-45) to achieve the desired balance between strength and deep-drawability. By precisely adjusting these parameters, the invention enhances tensile strength while minimizing the negative impact on r value and planar anisotropy.
Solution Approach 2:
The invention applies local quality by controlling the distribution and concentration of Mn and P elements to achieve different properties in different aspects. The controlled addition ensures that strength enhancement is achieved through solid-solution strengthening while the impact on deep-drawability is minimized through optimized element distribution and interaction with other alloying elements.
3Reliability
If Mo is added to increase the cold aging resistance, then the cold aging resistance improves, but the deep-drawability is further affected due to interaction with Mn and P
Solution Approach 1:
The invention creates a composite alloying system where Mo (0.005-0.050%) works synergistically with Mn and P. The controlled combination of these elements achieves cold aging resistance enhancement while the interaction effects are managed through optimized ratios, preventing excessive impact on deep-drawability. The composite effect of multiple elements distributes the functional responsibilities.
Solution Approach 2:
The invention controls the parameter of Mo addition within a specific range (0.005-0.050%) and considers its interaction with Mn and P levels. By optimizing these parameters together, the invention achieves improved cold aging resistance while minimizing the combined effect on deep-drawability properties such as r value and planar anisotropy.
4Reliability
If AlN dispersion is used to make the microstructure finer for grain boundary carbon, then the bake hardenability is improved, but the grain growth and recrystallization are inhibited
Solution Approach 1:
The invention optimizes the parameter of Al addition (0.01-0.08%) and controls the formation of AlN dispersoids to achieve fine microstructure for grain boundary carbon. By precisely controlling these parameters, the invention enhances bake hardenability while managing the impact on grain growth and recrystallization behavior during processing.
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 effectively enhances the steel sheet's tensile strength, bake hardenability, cold aging resistance, and deep-drawability while minimizing planar anisotropy, thereby improving its overall performance for automotive applications.
Implementation Method 1
The baked hardening utilizes a sort of strain aging in which dislocation occurring during deformation is fixed by carbon in solid solution or nitrogen in solid solution, which are interstitial elements solid solved in steel.
Implementation Method 2
performing continuous annealing in a temperature range of 770° C. to 820° C.
Data Source
AI summary
The present invention provides a bake-hardenable high-strength cold-rolled steel sheet having excellent bake hardenability, cold aging resistance, and deep-drawability, and reduced planar anisotropy, containing chemical components in % by mass of: C: 0.0010% to 0.0040%, Si: 0.005% to 0.05%, Mn: 0.1% to 0.8%, P: 0.01% to 0.07%, S: 0.001% to 0.01%, Al: 0.01% to 0.08%, N: 0.0010% to 0.0050%, Nb: 0.002% to 0.020%, and Mo: 0.005% to 0.050%, a value of [Mn %]/[P %] being in the range of 1.6 to 45, where [Mn %] is an amount of Mn and [P %] is an amount of P, an amount of C in solid solution obtained from [C %]−(12/93)×[Nb %] being in the range of 0.0005% to 0.0025%, where [C %] is an amount of C and [Nb %] is an amount of Nb, with a balance including Fe and inevitable impurities, wherein the bake-hardenable high-strength cold-rolled steel sheet satisfies the following Equation (1), where X(222), X(110), and X(200) represent ratios of integrated intensity of X-ray diffraction of {222} plane, {110} plane, and {200} plane, respectively, being parallel to a plane located at a depth of ¼ plate thickness measured from the surface of the steel sheet, and the bake-hardenable high-strength cold-rolled steel sheet has tensile strength in the range of 300 MPa to 450 MPa.