Cold-Rolled Steel Sheet Microstructure for Stable Flow Stress
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Solution Overview
Problem
High strength cold-rolled steel sheets primarily composed of martensite and/or tempered martensite exhibit a decrease in flow stress when a load is reapplied after a period of rest, due to variations in dislocation density and mobility across the sheet thickness.
Innovation Solution
A cold-rolled steel sheet with a specific chemical composition and microstructure, including a ratio of dislocation density and hardness across different thickness layers, along with a manufacturing process that includes hot rolling, cold rolling, annealing, heat treatment, and skin pass rolling, to stabilize dislocation density and prevent flow stress reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheet is used to reduce vehicle weight and increase strength, then tensile strength is improved, but flow stress decreases when load is reapplied after rest period
Solution Approach 1:
The patent applies local quality by creating different dislocation density distributions at different positions through the sheet thickness. Specifically, the dislocation density at the surface (20 μm from surface) is controlled to be 0.80 or more times the dislocation density at the center portion (t/4 depth), establishing a non-uniform dislocation structure that prevents flow stress decrease during repeated loading.
2Strength
If martensite and tempered martensite structure is used to achieve high strength, then tensile strength is improved, but flow stress decreases when load is reapplied after rest period
Solution Approach 1:
The patent applies parameter changes by precisely controlling the dislocation density parameter across the sheet thickness. The dislocation density ratio between surface and center portions is maintained within a specific range (0.80 or more), and the microstructure is controlled to have 90.0% or more martensite and tempered martensite in the t/4 portion, preventing flow stress decrease during repeated loading.
3Strength
If dislocation density varies across sheet thickness, then high strength is achieved, but flow stress decreases when load is reapplied after rest period
Solution Approach 1:
The patent applies local quality by intentionally creating non-uniform dislocation density distribution through the sheet thickness. The dislocation density at the surface (20 μm from surface) is controlled to be 0.80 or more times the dislocation density at the center portion (t/4 depth), establishing a specific gradient structure that prevents flow stress decrease during repeated loading while maintaining high strength.
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 suppresses the decrease in flow stress when a load is reapplied after a rest period, maintaining high tensile strength and ensuring consistent performance.
Implementation Method 1
an annealing process of heating the cold-rolled steel sheet to an annealing temperature of higher than Ac3° C., holding the cold-rolled steel sheet at the annealing temperature, and cooling the cold-rolled steel sheet after the holding to a cooling stop temperature
Implementation Method 2
a heat treatment process of heating the cold-rolled steel sheet after the annealing process to a temperature range of 200° C. to 350° C. and holding the cold-rolled steel sheet in the temperature range
Data Source
AI summary
This cold-rolled steel sheet has a predetermined chemical composition, in which, in a case where a range of ⅛ to ⅜ of a sheet thickness from a surface in a sheet thickness direction is defined as a t/4 portion and a range of 20 μm from the surface in the sheet thickness direction is defined as a surface layer portion, a microstructure at the t/4 portion includes, by volume percentage, 0% or more and 10.0% or less of retained austenite and 90.0% or more and 100% or less of one or two of martensite and tempered martensite, a ratio of a dislocation density of the surface layer portion to a dislocation density of the t/4 portion is 0.80 or more, a ratio of a hardness of the surface layer portion to a hardness of the t/4 portion is 0.90 or more, and a tensile strength of the cold-rolled steel sheet is 1,310 MPa or more.