Cold-Heading Wire Rod Composition for Hydrogen-Delayed Fracture Resistance
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Solution Overview
Problem
Existing high-strength steel materials for cold heading are prone to hydrogen-delayed fracturing and require lengthy heat treatments for bainite transformation, increasing manufacturing costs and reducing efficiency.
Innovation Solution
A high-strength wire rod with a specific chemical composition and microstructure, including controlled amounts of C, Si, Mn, Cr, Mo, Ni, and V, is manufactured through finish rolling, cooling, and isothermal heat treatments to form lower bainite, omitting quenching, thereby enhancing resistance to hydrogen-delayed fracturing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel materials are used for cold heading, then tensile strength is improved, but resistance to hydrogen-delayed fracturing deteriorates
Solution Approach 1:
The patent changes the chemical composition parameters by precisely controlling the content ranges of alloying elements (C: 0.20-0.45%, Si: 0.01-0.30%, Mn: 0.30-1.00%, Cr: 0.50-1.50%, Mo: 0.50-1.50%, Ni: 0.50-2.00%, V: 0.01-0.40%) and their relational expressions to achieve a microstructure that provides both high strength and high resistance to hydrogen-delayed fracturing, resolving the contradiction between strength improvement and reliability maintenance
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (martensite, bainite, and pearlite) with specific area percentages, where each phase contributes different properties: martensite provides strength, bainite provides toughness and hydrogen-delayed fracturing resistance, and pearlite provides ductility. This composite microstructure simultaneously achieves high tensile strength and high resistance to hydrogen-delayed fracturing
2Reliability
If heat treatment is performed to transform into bainite, then resistance to hydrogen-delayed fracturing is improved, but manufacturing time increases
Solution Approach 1:
The patent performs preliminary action by controlling the chemical composition and microstructure during the wire rod manufacturing process itself, creating a microstructure that already contains bainite phase (40-80% area percentage) before the cold heading process. This preliminary formation of bainite structure eliminates the need for lengthy post-heat treatment to transform into bainite, thereby reducing manufacturing time while maintaining high resistance to hydrogen-delayed fracturing
Solution Approach 2:
The patent changes the heat treatment parameters by optimizing the cooling rate (0.1-1.0°C/s) and controlling the austenite grain size (30 μm or less) before cooling, which enables bainite transformation to occur during the controlled cooling process rather than requiring prolonged isothermal holding. This parameter optimization achieves bainite formation with reduced heat treatment time
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 achieves a wire rod with tensile strength of 1600 MPa or more and impact toughness of 60 J or more, improving resistance to hydrogen-delayed fracturing and reducing manufacturing time.
Implementation Method 1
Ni is added to activate lower bainite transformation and secure impact toughness
Implementation Method 2
Mo is added to secure hardenability through delayed transformation of pearlite and bainite
Implementation Method 3
V is added to increase strength and refine grains
Implementation Method 4
Si, which is widely known as an element causing solid solution strengthening
Data Source
AI summary
Provided are a wire rod for cold heading having high resistance to delayed facture, a part having high resistance to delayed facture, and methods for manufacturing the wire rod and the part. The wire rod of the present disclosure has a chemical composition including, by wt %, C: 0.3% to 0.5%, Si: 0.01% to 0.3%, Mn: 0.3% to 1.0%, Cr: 0.5% to 1.5%, Mo: 0.5% to 1.5%, Ni: 0.5% to 2.0%, V: 0.01% to 0.4%, and a balance of Fe and other impurities, and the chemical composition satisfies the relational expression 1, wherein the high-strength wire rod has a microstructure including, by area %, 5% to 20% martensite, 0.1% to 1% pearlite, and a balance of bainite.