Steel Sheet Texture Control for Strength and Stretch Formability
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Solution Overview
Problem
Current steel sheets with high tensile strength struggle to achieve excellent stretch formability due to issues with martensite crystallographic texture integration and orientation, which affects their isotropic deformation and formability.
Innovation Solution
A steel sheet with a specific chemical composition and production process that optimizes hot rolling, annealing, and cold rolling conditions to reduce the integration degree of certain crystal orientations, thereby improving martensite texture randomness and enhancing stretch formability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If martensite structure is utilized to achieve high strength, then tensile strength is improved, but stretch formability deteriorates due to martensite accumulation in specific orientations
Solution Approach 1:
The invention changes the crystallographic texture parameters of austenite by controlling hot rolling reduction ratio and temperature, which subsequently changes the martensite orientation distribution after transformation, achieving both high strength and good formability
Solution Approach 2:
The invention performs preliminary control of austenite crystallographic texture during hot rolling before martensite transformation occurs, preventing the formation of harmful orientation patterns in the final martensite structure
2Strength
If austenite crystallographic texture in copper orientation and brass orientation is formed, then martensite strength is improved, but crystallographic texture integration in martensite increases leading to poor formability
Solution Approach 1:
The invention changes the hot rolling parameters (reduction ratio and temperature) to control the austenite texture parameters, transforming the texture from harmful copper/brass orientations to more favorable patterns that reduce martensite orientation integration
3Strength
If DP steel composition is optimized for strength, then tensile strength reaches 550-1100 MPa, but stretch formability remains insufficient due to orientation integration issues
Solution Approach 1:
The invention changes the processing parameters (hot rolling reduction ratio to 20-40% and temperature to 850-950°C) to control the crystallographic texture evolution, achieving a balance between strength and formability that composition optimization alone cannot provide
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 approach results in a steel sheet with high strength and excellent stretch formability, achieving a tensile strength of 550 to 1100 MPa and total elongation of 10% or more, while maintaining a strong strength-ductility balance.
Implementation Method 1
a high strength is achieved by utilizing martensite structure; however, martensite is sometimes accumulated in a specific orientation. This is caused by the formation of an austenite crystallographic texture and, specifically, the formation of an austenite crystallographic texture in the orientations referred to as 'copper orientation' and 'brass orientation' leads to the formation of a crystallographic texture in martensite that is generated when austenite is cooled.
Implementation Method 2
a structure which is composed of a ferrite phase having a volume ratio of 70 to 97% and the remainder being a low-temperature transformed phase mainly constituted by a bainite phase is formed
Implementation Method 3
an intermediate annealing is performed on a cold-rolled sheet
Data Source
AI summary
Provided are: a steel sheet having a high strength and excellent stretch formability; and a method of producing the same. The steel sheet has prescribed chemical composition and structure, in which the integration degree of the (111)<112> orientation of ferrite is 3.0 or higher, and the integration degree of the (252)<2-11> orientation of martensite and tempered martensite is 5.0 or lower. The method of producing the steel sheet includes: the step of continuously casting a molten steel having the prescribed chemical composition, and performing 5 to 40% rolling reduction at a temperature of 800° C. to lower than 1,200° C. in a period after the continuous casting until cooling to room temperature; the hot rolling step of performing hot rolling with a finishing temperature of 650 to 950° C.; the step of coiling the resulting hot-rolled steel sheet at a coiling temperature of 400 to 700° C.; the step of retaining the hot-rolled steel sheet at (coiling start temperature+20° C. to 100° C.) for 5 to 300 minutes; the step of cold rolling the hot-rolled steel sheet at a rolling reduction ratio of 10.0 to 90.0%; and the step of annealing the resulting cold-rolled steel sheet at 700 to 900° C.