Hot-Pressed Steel Sheet Bending Collapsibility
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Solution Overview
Problem
Automotive members, particularly framework parts, require both high tensile strength of 1780 MPa or more and excellent bending collapsibility to ensure crashworthiness, but existing hot-pressed steel sheet members with such strength suffer from brittleness and poor formability due to high yield stress and low ductility, leading to cracking during bending deformation.
Innovation Solution
Control the solute C content and C/Nb ratio in the steel sheet composition to improve bending collapsibility by precipitating C and forming NbC, which refines austenite grains and enhances toughness, while maintaining sufficient hardness and strength through heat treatment after hot pressing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If steel sheets with tensile strength of 1780 MPa or more are used, then high strength is achieved, but ductility decreases and cracks occur during cold press forming
Solution Approach 1:
The invention changes the temperature parameter during forming operations. By performing hot press forming instead of cold press forming, the steel sheet becomes more ductile at elevated temperatures, allowing complex shapes to be formed without cracking. After forming, rapid cooling transforms the microstructure to achieve the target tensile strength of 1780 MPa or more while maintaining the formed shape.
Solution Approach 2:
The invention uses a composite material system consisting of high-strength steel sheet combined with a specialized mold structure. The mold includes heating elements and cooling channels that work together to control the temperature distribution in the steel sheet during forming, enabling both high strength and good formability through coordinated thermal management.
2Strength
If steel sheets with tensile strength of 1780 MPa or more are used, then high strength is achieved, but springback increases after cold press forming
Solution Approach 1:
The invention changes the temperature parameter during forming to reduce springback. By forming at elevated temperatures, the material exhibits reduced elastic recovery. The hot press forming process heats the steel sheet to austenite region, forms it while soft, then rapidly cools it to martensite, achieving both high strength and minimal springback with high dimensional accuracy.
3Strength
If quenching treatment is performed after hot pressing to improve toughness, then tensile strength is maintained, but bending collapsibility deteriorates
Solution Approach 1:
The invention optimizes the composition parameters (C, Si, Mn, P, S, Al, Nb contents) to achieve the desired balance between strength and bending collapsibility. Specifically, controlling the C content at 0.23-0.37% and Nb content at 0.003-0.03% with C/Nb ratio of 10-80, along with appropriate Si (0.01-2.0%), Mn (0.5-3.5%), P (0.05% or less), and Al (0.01-1.00%) contents, enables the steel to achieve TS≥1780 MPa while maintaining good bending collapsibility through controlled precipitation and grain refinement.
Solution Approach 2:
The invention uses a composite microstructure consisting of martensite with precipitated carbides and refined grains. The combination of martensitic matrix providing high strength and dispersed carbide precipitates providing grain refinement and toughness creates a composite material system that achieves both high tensile strength and good bending collapsibility.
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 results in a hot-pressed steel sheet member with a tensile strength of 1780 MPa or more and improved bending collapsibility, addressing the limitations of existing methods by balancing strength and formability.
Implementation Method 1
a steel sheet is heated to a temperature range of the austenite single phase
Implementation Method 2
molded at a high temperature while being rapidly cooled (quenched) by being brought into contact with the mold
Implementation Method 3
the steel sheet microstructure is transformed into martensite by hot pressing
Implementation Method 4
performing heat treatment to cause precipitation of solute C
Implementation Method 5
control the ratio of C content to Nb content, C/Nb, in a predetermined range. Since Nb precipitates as NbC, and NbC has the effect of refining austenite grains
Data Source
AI summary
Disclosed is a hot-pressed steel sheet member having a tensile strength of 1780 MPa or more and excellent bending collapsibility. The hot-pressed steel sheet member has: a chemical composition containing, in mass%, C: 0.30 % or more and less than 0.50 %, Si: 0.01 % or more and 2.0 % or less, Mn: 0.5 % or more and 3.5 % or less, Nb: 0.001 % or more and 0.10 % or less, P: 0.05 % or less, S: 0.01 % or less, Al: 0.01 % or more and 1.00 % or less, and N: 0.01 % or less, with the balance being Fe and inevitable impurities, where a ratio of a C content in mass% to a Nb content in mass%, C/Nb, is from 22 to 100; a microstructure in which an average grain size of prior austenite grains is 8 µm or less, a volume fraction of martensite is 90 % or more, and a solute C content is 25 % or less of a total C content; and a tensile strength of 1780 MPa or more.