Hot-Rolled Steel Sheet Composition for Bendability After Quench-Tempering
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Solution Overview
Problem
Existing technologies lack a hot-rolled steel sheet and steel pipe with a hardness difference of less than 15 between the surface and the thickness/4 position, which is essential for high strength, excellent bendability, and a maximum bending angle of 40° or more after heating and quenching-tempering heat treatment.
Innovation Solution
A hot-rolled steel sheet and steel pipe with a composition of 0.20% to 0.3% C, 0.5-1.3% Mn, 0.3% or less Si, and specific relational expressions, resulting in a microstructure of 20-65% ferrite and 35-80% pearlite, and an average prior austenite grain size of 15 μm or more.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheet is used to achieve tensile strength of 1300 MPa or more, then strength is improved, but bendability deteriorates and maximum bending angle becomes less than 40 degrees
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet by strictly controlling carbon content to 0.23% or less, silicon content to 0.01-0.50%, and manganese content to 1.50-3.00%, while maintaining specific Mn/Si ratio relationships. These parameter changes enable achieving tensile strength of 1300 MPa or more while maintaining maximum bending angle of 40 degrees or more, resolving the contradiction between strength and bendability
Solution Approach 2:
The invention controls the microstructure to have specific phase compositions (martensite 5-50%, bainite 10-40%, ferrite 20-60%) and regulates the hardness difference between surface and center portions to be 15 Hv or less. This local quality control ensures uniform bendability throughout the sheet while maintaining high overall strength
2Ease of operation
If silicon content is increased to 0.5% or more to improve bendability through retained austenite formation, then bendability is improved, but welding quality deteriorates due to silicon oxide formation in butt weld zones
Solution Approach 1:
The invention changes the silicon content parameter to a specific range of 0.01-0.50%, which is lower than conventional approaches (0.5% or more). This parameter change, combined with controlled manganese content (1.50-3.00%) and Mn/Si ratio (2.00-6.00), enables achieving maximum bending angle of 40 degrees or more while preventing excessive silicon oxide formation that would harm welding quality
Solution Approach 2:
The invention differentiates the silicon content strategy between bulk material and weld zones. By maintaining silicon at 0.01-0.50% in the bulk material, the invention achieves sufficient bendability through controlled microstructure while minimizing silicon oxide formation in weld zones, thus protecting welding quality
3Strength
If carbon content is increased to achieve high strength of 1300 MPa or more, then strength is improved, but hardness difference between surface and center portions increases, reducing decarburization resistance
Solution Approach 1:
The invention changes the carbon content parameter to 0.23% or less, which is lower than conventional high-strength steels. This parameter change, combined with optimized silicon (0.01-0.50%) and manganese (1.50-3.00%) content, enables achieving tensile strength of 1300 MPa or more while maintaining hardness difference of 15 Hv or less between surface and center portions, thus improving decarburization resistance
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 a hot-rolled steel sheet and steel pipe with high strength of 1300 MPa or more, excellent bendability, and a maximum bending angle of 40° or more after heat treatment, while maintaining a low hardness difference, thus enhancing decarburization resistance and manufacturing efficiency.
Implementation Method 1
after heating and quenching-tempering heat treatment
Data Source
AI summary
A hot rolled steel sheet having excellent cold bendability after heating and quenching-tempering heat treatment, a steel pipe, a member and manufacturing methods therefor are provided. The present invention relates to: a hot rolled steel sheet comprising, by weight %, 0.20% or greater and less than 0.3% of C, 0.5-1.3% of Mn, 0.3% or less of Si (excluding 0%), 0.03% or less of P (including 0%), 0.004% or less of S (including 0%), 0.04% or less of Al (excluding 0%), 0.3% or less of Cr, 0.1-0.4% of Ni, 0.05% of Ti (including 0%), 0.0005-0.0050% of B, 0.01% or less of N (excluding 0%) and the balance of Fe and other impurities, and satisfying relational expressions 1-3; a steel pipe; and a member.