Hot Press-Formed Steel Microstructure for Strength and Bendability
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Solution Overview
Problem
Hot press-formed parts for automobiles face challenges in achieving both high strength and sufficient ductility, as existing methods often result in inadequate bendability due to insufficient residual austenite content and unstable bainitic transformation, leading to potential cracking during collisions.
Innovation Solution
A high-strength hot press-formed part is developed by adjusting the composition and structure to include a composite structure of tempered martensite, residual austenite, and bainite, with specific proportions and pole density control, ensuring excellent strength, ductility, and bendability through a process involving heating, reheating, and controlled cooling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strengthening is promoted to achieve high strength, then tensile strength is improved, but bainitic transformation is delayed and residual austenite generation time increases, significantly impairing productivity
Solution Approach 1:
The invention changes the temperature parameter during heat treatment by reheating to 350-450°C after initial cooling to Ac3 point or higher. This temperature parameter change accelerates bainitic transformation and stabilizes residual austenite, achieving both high strength (tensile strength 1320 MPa or more) and acceptable productivity without excessive retention time
Solution Approach 2:
The invention employs periodic action through a two-stage heat treatment process: first heating to austenite zone and cooling, then reheating to 350-450°C for retention. This periodic thermal action ensures complete transformation to composite structure containing martensite, bainite, and stable residual austenite, achieving high strength while controlling total processing time
2Strength
If high-strengthening is promoted to achieve high strength, then tensile strength is improved, but ductility deteriorates due to insufficient residual austenite, causing cracking during collision
Solution Approach 1:
The invention changes the temperature parameter by reheating to 350-450°C after initial cooling, which stabilizes residual austenite and prevents its complete transformation to martensite. This parameter change ensures adequate ductility (total elongation 12% or more) while maintaining high strength, preventing cracking during collision
Solution Approach 2:
The invention creates a composite microstructure containing martensite (for strength), bainite (for toughness), and stable residual austenite (for ductility). This composite structure achieves both high tensile strength (1320 MPa or more) and sufficient ductility (total elongation 12% or more), preventing collision-related cracking
3Weight of moving object
If thinning of material is performed to reduce weight, then weight is reduced, but strength becomes insufficient unless high-strengthening is applied
Solution Approach 1:
The invention changes the microstructure through controlled cooling and reheating to 350-450°C, creating a composite structure with martensite, bainite, and stable residual austenite. This parameter change enables achieving high strength (tensile strength 1320 MPa or more) in thinned parts, allowing weight reduction while maintaining sufficient strength
Solution Approach 2:
The invention creates a composite microstructure containing martensite (for strength), bainite (for toughness), and stable residual austenite (for ductility). This composite structure enables thinned parts to achieve both weight reduction and sufficient strength (tensile strength 1320 MPa or more)
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 approach achieves a tensile product of 26,000 MPa·% or greater, with Lankford values of 0.80 or smaller and bending limitations of 2.0 or smaller in both rolling and transverse directions, enhancing collision characteristics and preventing bending cracking.
Implementation Method 1
a steel sheet is press-formed after being heated to a high temperature of an austenite zone
Implementation Method 2
a TRIP steel can include stable residual austenite in its structure even at room temperature by performing bainitic transformation through heat treatment
Implementation Method 3
a steel sheet is press-formed after being heated to a high temperature of an austenite zone
Implementation Method 4
since hardening treatment is performed inside a die at the same time as the press forming is performed, a steel sheet can have high strength
Data Source
Figure 1
AI summary
A hot press-formed part according to an aspect of the present invention contains a predetermined chemical composition; in which a microstructure in a thickness 1/4 portion includes, by unit vol%, tempered martensite: 20% to 90%, bainite: 5% to 75%, and residual austenite: 5% to 25%, and ferrite is limited to 10% or less; and a pole density of an orientation {211}<011> in the thickness 1/4 portion is 3.0 or higher.