Hot-Stamped Steel Surface Bainite for Hydrogen Embrittlement

Solution Overview

Problem

High-strength steel sheets used in vehicle members face challenges with hydrogen embrittlement cracking due to increased susceptibility as tensile strength rises, which is not adequately addressed by existing technologies like hot stamping with electrolytic zinc-based plated steel sheets.

Innovation Solution

A hot-stamping formed body with a specific chemical composition and microstructure, including a surface layer region with a high bainite area ratio, controlled texture, and deboronization index, enhances hydrogen embrittlement resistance.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Strength

If high-strength steel sheet is used to reduce vehicle body weight, then strength is improved, but hydrogen embrittlement resistance deteriorates

Engineering Contradiction:

Improvetensile strengthVSAvoidhydrogen embrittlement resistance

Core Design Contradiction:

StrengthVSReliability

Solution Approach 1:

The patent applies local quality by creating a dual-phase microstructure where the base material consists of martensite with high strength, while the surface layer region (0-10μm from surface) contains bainite phase that provides hydrogen embrittlement resistance. This local differentiation allows the material to simultaneously achieve high overall strength while maintaining resistance to hydrogen embrittlement at the surface where hydrogen penetration occurs.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent utilizes parameter changes by controlling the chemical composition (specific C, Si, Mn, B content within defined ranges) and processing parameters (heating temperature 800-1000°C, cooling rate 20-100°C/s) to transform the microstructure. The controlled cooling rate and heating temperature parameters enable the formation of the desired martensite-bainite dual-phase structure that resolves the contradiction between strength and hydrogen embrittlement resistance.

Inventive Principle:

Principle #35Parameter changes

2Strength

If martensite microstructure with high dislocation density is created to achieve high strength, then strength is improved, but hydrogen embrittlement susceptibility increases

Engineering Contradiction:

ImprovestrengthVSAvoidhydrogen embrittlement susceptibility

Core Design Contradiction:

StrengthVSObject-affected harmful factors

Solution Approach 1:

The patent applies local quality by creating a dual-phase microstructure where the base material consists of martensite with high strength, while the surface layer region (0-10μm from surface) contains bainite phase that provides hydrogen embrittlement resistance. This local differentiation allows the material to simultaneously achieve high overall strength while maintaining resistance to hydrogen embrittlement at the surface where hydrogen penetration occurs.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent converts the harmful effect of high dislocation density in martensite (which increases hydrogen trapping) by introducing bainite phase in the surface layer. The bainite phase acts as a beneficial counterbalance that reduces hydrogen embrittlement susceptibility while the martensite base maintains the required high strength, effectively converting the harmful high-dislocation structure into a balanced dual-phase system.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Shape

If hot stamping is performed to achieve both formability and strength, then formability is improved, but hydrogen embrittlement resistance deteriorates

Engineering Contradiction:

ImproveformabilityVSAvoidhydrogen embrittlement resistance

Core Design Contradiction:

ShapeVSReliability

Solution Approach 1:

The patent utilizes parameter changes by controlling the chemical composition (specific C, Si, Mn, B content within defined ranges) and processing parameters (heating temperature 800-1000°C, cooling rate 20-100°C/s) to transform the microstructure. The controlled cooling rate and heating temperature parameters enable the formation of the desired martensite-bainite dual-phase structure that resolves the contradiction between strength and hydrogen embrittlement resistance.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent applies preliminary action by pre-establishing the chemical composition parameters (C: 0.26-0.45%, Si: 0.01-3.00%, Mn: 0.10-0.60%, B: 0.0005-0.0200%) before the hot stamping process. This preliminary compositional design ensures that during subsequent heating and cooling, the material will naturally form the desired dual-phase microstructure with appropriate surface layer characteristics, preventing hydrogen embrittlement before it can occur.

Inventive Principle:

Principle #10Preliminary action

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-stamping formed body with high strength and improved resistance to hydrogen embrittlement, maintaining structural integrity under stress.

Implementation Method 1

press forming is performed after a steel sheet is heated up to a high temperature of an austenite range where the steel sheet softens

Methodology Applied

Scientific EffectPhase transition (austenite formation): Phase Change

Implementation Method 2

performing hardening of the steel sheet in a die at the same time as press working

Methodology Applied

Scientific EffectPhase transition (martensite formation): Phase Change

Data Source

PatentUS20250108424A1Hot-stamping formed body

Publication Date: 2025.04.03 NIPPON STEEL CORPORATION

AI summary

This hot-stamping formed body has a predetermined chemical composition, in a surface layer region, an area ratio of bainite of more than 10%, a maximum value of pole density of a texture of 4.0 or less, and a deboronization index of 0.05 or more.