Deboronized Steel Sheet for High Strength and Bendability
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Solution Overview
Problem
Existing high strength steel sheets used in automobile parts face challenges in maintaining bendability after plastic strain is introduced, leading to potential fracture during collisions, with insufficient focus on improving bendability after plastic working.
Innovation Solution
The development of a steel sheet with a specific chemical composition and microstructure, including a deboronized surface layer, combined with a controlled manufacturing process to enhance tensile strength and bendability, involves forming a suitable deboronized layer at the surface part to inhibit crack formation and propagation during bending deformation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheet is used to increase tensile strength, then strength is improved, but bendability after plastic working deteriorates
Solution Approach 1:
The patent applies local quality by creating a surface layer with different chemical composition and microstructure from the base material. The surface layer has reduced boron content (0.0003-0.0020 mass%) compared to the base material (0.0005-0.0050 mass% boron), and contains 3-12 mass% silicon to form a soft structure that improves bendability after plastic working, while the base material maintains high strength through its microstructure (martensite 5-20%, bainite 70-95%).
Solution Approach 2:
The patent changes chemical composition parameters by controlling boron content in the surface layer to be lower than in the base material, and by controlling silicon content (3-12 mass%) in the surface layer. It also controls microstructure parameters by adjusting the proportions of martensite, bainite, and other phases through heat treatment parameters (cooling rates, holding temperatures) to achieve the desired balance between strength and post-forming bendability.
2Strength
If boron is added to improve tensile strength, then strength is improved, but bendability after plastic working deteriorates due to crack formation
Solution Approach 1:
The patent applies the extraction principle by removing boron from the surface layer, creating a boron-depleted zone (0.0003-0.0020 mass% boron) compared to the boron-containing base material (0.0005-0.0050 mass% boron). This extraction of boron from the surface prevents crack initiation and propagation during bending operations, while the base material retains boron for maintaining high tensile strength.
Solution Approach 2:
The patent creates local quality differences by having the surface layer with reduced boron content and increased silicon content (3-12 mass%), forming a soft structure that is resistant to crack formation. The base material maintains its boron content for strength, creating a gradient structure where the surface layer protects against cracking while the base material provides overall 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 steel sheet achieves improved tensile strength and enhanced bendability after plastic working, reducing the risk of fracture during collisions by controlling the distribution of boron and carbon concentrations through a precise manufacturing process.
Implementation Method 1
B is contained at the steel sheet surface layer part mainly in a precipitated state and inside of the steel sheet mainly in a solid solution state
Implementation Method 2
B is contained at the steel sheet surface layer part mainly in a precipitated state and inside of the steel sheet mainly in a solid solution state
Data Source
Figure 1
AI summary
The present invention provides a steel sheet having improved bendability after plastic working and excellent tensile strength, and also provides a manufacturing method therefor. A steel sheet according to the present invention is characterized by exhibiting a tensile strength of 1180 MPa or more, and having a prescribed chemical composition and steel structure, wherein the surface layer portion of the steel sheet has a deboronized layer where the luminescence intensities B30, B140, and B150 of B, as measured in the depth direction from the steel sheet surface by high-frequency glow discharge optical emission spectrometry, at the depth positions of 30 µm, 140 µm, and 150 µm from the steel sheet surface satisfy B30/B150<0.90 and 0.90≤B140/B150≤1.10, and in the surface layer portion of the steel sheet, the luminescence intensities C30, C140, and C150 of C, as measured in the depth direction from the steel sheet surface by high-frequency glow discharge optical emission spectrometry, at the depth positions of 30 µm, 140 µm, and 150 µm from the steel sheet surface satisfy C30/C150≤0.50 and 0.90≤C140/C150≤1.10.