Steel Sheet Surface Decarburization for Strength and Bendability
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Solution Overview
Problem
High-strength steel sheets face challenges in achieving both high strength and superior bendability, particularly those with strengths of 980 MPa or more, due to issues with Si and Mn oxidation during annealing, leading to poor plating adhesion and difficulty in forming desired component shapes.
Innovation Solution
A steel sheet with a controlled decarburized layer in the surface layer, where the carbon concentration is managed within specific ranges, combined with a two-stage reduction treatment process to form a decarburized layer and a reduced Fe layer, ensuring high strength and bendability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Si or Mn is added to increase steel sheet strength, then tensile strength is improved, but oxide formation occurs during continuous annealing leading to poor plating adhesion
Solution Approach 1:
The patent applies preliminary oxidation treatment before plating to intentionally form a controlled oxide layer on the steel sheet surface. This preliminary action modifies the surface chemistry in advance, creating a surface layer that promotes zinc wetting and adhesion. The oxidation treatment is performed at 400-750°C in an atmosphere with O2 concentration of 1000 ppm or more, forming a surface layer with specific oxide composition that serves as an intermediate layer between the steel substrate and zinc coating, thereby resolving the plating adhesion problem associated with Si and Mn additions.
2Strength
If high-strength steel sheet with strength of 980 MPa or more is used for weight reduction, then vehicle body weight is reduced, but bendability deteriorates making it difficult to form desired component shapes
Solution Approach 1:
The patent creates a non-uniform carbon concentration distribution through controlled decarburization, where the surface layer has different carbon content than the bulk material. Specifically, the carbon concentration at the surface is reduced to create a softer, more ductile surface layer that facilitates bending, while the bulk material maintains high carbon content for high strength. This local differentiation of material properties allows the steel sheet to exhibit both high strength and good bendability simultaneously.
Solution Approach 2:
The patent changes the carbon concentration parameter in the surface layer through controlled decarburization treatment. By reducing the carbon concentration at the surface (creating a decarburized layer with carbon content lower than the bulk), the material properties at the surface are modified to improve ductility and bendability. This parameter change in the surface region allows high-strength steel to be formed into complex shapes without compromising 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 high tensile strength of 980 MPa or more while maintaining superior bendability, enabling effective weight reduction and component shaping without compromising plating adhesion.
Implementation Method 1
Si and Mn are oxidized even in a reducing atmosphere in which oxidation of Fe does not occur, and oxides of Si and Mn are formed on the outermost surface of a steel sheet
Implementation Method 2
a position where a carbon concentration (mass%) is 50% of a bulk carbon concentration is in a region of 0.20% or more of the sheet thickness from a steel sheet surface
Implementation Method 3
reduction-annealing, in a heating zone, the steel sheet is heated at a heating rate of 0.1°C/sec or more to a temperature of 650 to 900°C in an atmosphere having an H2 concentration of 5 to 30% by volume
Data Source
Figure 1-1~1-2
Figure 2-1~2-2
Figure 3
AI summary
A steel sheet, wherein, in a sheet thickness direction, a position where a carbon concentration (mass%) is 50% of a bulk carbon concentration is in a region of 0.20% or more of the sheet thickness from a steel sheet surface, and a position where a carbon concentration (mass%) is 90% of the bulk carbon concentration is in a region of 8.0% or less of the sheet thickness from the steel sheet surface.