Hot-Rolled Steel Sheet Texture Control for Bend Fatigue Resistance

Solution Overview

Problem

Existing high-strength steel sheets used in automobile suspension components suffer from insufficient fatigue properties and impact resistance due to the formation of sharpened recessed parts, such as fine cracks, at the inside bends during bending forming.

Innovation Solution

A hot-rolled steel sheet with a specific chemical composition and metallographic structure is developed, featuring a tensile strength of 880 MPa or more and excellent formability, which includes controlling the rotation angle of specific crystal orientations in the sheet thickness direction to reduce the depth of inside bend recessed parts.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Weight of moving object

If high-strength steel sheets are used to ensure static fracture strength and yield strength while reducing sheet thickness, then weight reduction is achieved, but fatigue properties and impact resistance deteriorate due to formation of sharpened recessed parts at inside bends

Engineering Contradiction:

Improveweight of automobile suspension componentsVSAvoidfatigue properties and impact resistance

Core Design Contradiction:

Weight of moving objectVSReliability

Solution Approach 1:

The invention changes the crystal orientation parameters by controlling the rotation angle of (011) poles in the sheet thickness direction to be within a specific range (10° to 30°). This parameter control modifies the material's response to bending deformation, reducing the formation of sharpened recessed parts while maintaining high strength, thereby improving fatigue properties and impact resistance without sacrificing weight reduction benefits

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The invention applies local quality control by specifically targeting the crystal orientation in the sheet thickness direction (through-thickness direction) rather than uniform orientation control. By controlling the rotation angle of (011) poles locally in the thickness direction, the material exhibits improved resistance to inside bend recession formation at critical locations while maintaining overall high strength properties

Inventive Principle:

Principle #3Local quality

2Strength

If high-strength steel sheets with tensile strength of 880 MPa or more are used, then static fracture strength is improved, but the depth of inside bend recessed parts increases leading to degraded fatigue properties

Engineering Contradiction:

Improvestatic fracture strengthVSAvoiddepth of inside bend recessed parts

Core Design Contradiction:

StrengthVSObject-affected harmful factors

Solution Approach 1:

The invention changes the crystal orientation parameters by controlling the rotation angle of (011) poles in the sheet thickness direction to be within a specific range (10° to 30°). This parameter control modifies the material's response to bending deformation, reducing the formation of sharpened recessed parts while maintaining high strength, thereby improving fatigue properties and impact resistance without sacrificing weight reduction benefits

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The invention applies local quality control by specifically targeting the crystal orientation in the sheet thickness direction (through-thickness direction) rather than uniform orientation control. By controlling the rotation angle of (011) poles locally in the thickness direction, the material exhibits improved resistance to inside bend recession formation at critical locations while maintaining overall high strength properties

Inventive Principle:

Principle #3Local quality

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 proposed solution effectively reduces the depth of inside bend recessed parts, thereby enhancing the fatigue properties and impact resistance of the steel sheets, making them suitable for high-strength applications without compromising formability.

Implementation Method 1

the sharpened recessed part such as a fine crack at the inside bend is not a fine crack and is attributed to unevenness formed by the plastic buckling of the surface layer of the steel sheet toward the outside of the plane in a microscopic region during bending forming

Methodology Applied

Scientific EffectPlastic buckling: Plasticity

Data Source

PatentUS12344913B2Hot-rolled steel sheet and manufacturing method thereof

Publication Date: 2025.07.01 NIPPON STEEL CORPORATION

AI summary

Adopted are a hot-rolled steel sheet having a predetermined chemical composition, in which a region, where a rotation angle between a normal line of a surface and a (011) pole near the normal line becomes 5° or less, is 0.150 or less from the surface in terms of a sheet thickness direction position standardized by a sheet thickness, and a region, where the rotation angle becomes 20° or more, is 0.250 or more from the surface in terms of the sheet thickness direction position standardized by the sheet thickness and a manufacturing method thereof.