Hot-Pressed Steel Sheet Composition for Strength and Bendability
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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 cracking during bending deformation due to uncontrolled solute C content and decarburization.
Innovation Solution
Control the solute C content through heat treatment after hot pressing and add a predetermined amount of Sb to prevent decarburization, ensuring uniform hardness distribution and improved bending collapsibility.
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 bending collapsibility deteriorates due to brittleness and cracking
Solution Approach 1:
The invention changes the microstructural parameters by controlling solute C content to 2.00 wt% or less and adjusting the ratio of solute C to total C to 5-50%, transforming the martensite from a brittle as-quenched state to a more ductile state with improved bending collapsibility while maintaining high tensile strength of 1780 MPa or more
Solution Approach 2:
The invention applies local quality control by specifically targeting the martensitic regions and modifying their solute C content distribution, creating a microstructure where the martensite blocks have optimized local composition for both strength and ductility, rather than uniformly changing the entire steel sheet properties
2Strength
If quenching treatment is performed after hot pressing to achieve high strength, then tensile strength of 1780 MPa or more is obtained, but bending collapsibility worsens due to retained martensite brittleness
Solution Approach 1:
The invention performs preliminary action by controlling the solute C content during the hot pressing and quenching process itself, rather than requiring subsequent heat treatments. The solute C content is controlled to 2.00 wt% or less before quenching, which pre-determines the martensite properties and eliminates the need for additional heat treatment steps to improve bending collapsibility
3Strength
If steel sheets with high yield stress are used, then high strength is achieved, but dimensional accuracy worsens due to large springback after cold press forming
Solution Approach 1:
The invention utilizes phase transitions by heating the steel sheet to the austenite single phase region before hot pressing, then rapidly cooling it to transform to martensite. This phase transition during hot pressing allows the material to be formed in a soft, ductile state and then locked into the desired shape with high strength, eliminating springback issues associated with cold forming of high-strength steels
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 method achieves a hot-pressed steel sheet member with a tensile strength of 1780 MPa or more and excellent bending collapsibility, addressing the brittleness and cracking issues by managing solute C precipitation and decarburization.
Implementation Method 1
Hot pressing is a molding process in which a steel sheet is heated to a temperature range of the austenite single phase, and molded at a high temperature while being rapidly cooled (quenched) by being brought into contact with the mold
Implementation Method 2
by performing heat treatment to cause precipitation of solute C, it is possible to improve the bending collapsibility while ensuring a certain degree of hardness
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 includes: 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, Sb: 0.001 % or more and 0.020 % 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; 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.