Steel Wire for Non-Heat-Treatment Bolts
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Solution Overview
Problem
High-strength bolts without quenching or tempering treatments face challenges in achieving tensile strengths of 1200 MPa or more while maintaining delayed fracture resistance and cold forgeability, as existing methods either increase manufacturing costs or reduce forging die life.
Innovation Solution
A high-strength steel wire with a two-phase ferrite and perlite structure, controlled chemical composition, and specific microstructural adjustments, including a perlite area rate between 40% and 80% and lamellar spacing of 250 nm or less, combined with appropriate cold wire drawing and isothermal transformation processes, to achieve both high strength and delayed fracture resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If quenching and tempering treatments are applied to achieve high tensile strength, then tensile strength is improved, but manufacturing cost and process complexity increase
Solution Approach 1:
The invention extracts and removes the quenching and tempering heat treatment steps from the conventional bolt manufacturing process. By achieving high strength (1200 MPa or more) through cold wire drawing alone, the patent eliminates the need for complex thermal processing equipment and multiple treatment stages, thereby simplifying the overall manufacturing process while maintaining high tensile strength
Solution Approach 2:
The invention performs strength enhancement in advance during the cold wire drawing process rather than after bolt formation. By pre-hardening the steel wire through controlled cold drawing with total area reduction of 50-80%, the material achieves high strength before being formed into the final bolt shape, eliminating the need for subsequent heat treatment operations
2Strength
If high strength wire drawing is applied to achieve 1200 MPa tensile strength, then tensile strength is improved, but cold forgeability deteriorates
Solution Approach 1:
The invention changes the microstructural parameters of the steel wire by controlling the cold wire drawing process to achieve a specific perlite area rate (40-80%) and lamellar spacing (250 nm or less). This parameter control allows the material to achieve high strength while maintaining adequate cold forgeability for bolt formation
Solution Approach 2:
The invention creates local quality differentiation within the steel wire microstructure by controlling the distribution and morphology of perlite colonies. The refined perlite structure with specific area rate and lamellar spacing provides localized strength enhancement while maintaining overall ductility and forgeability needed for cold forming operations
3Strength
If high strength wire drawing with high perlite proportion is applied to achieve high strength, then tensile strength is improved, but forging die life is shortened
Solution Approach 1:
The invention optimizes the perlite area rate to a specific range (40-80%) rather than maximizing it to 100%. This parameter optimization achieves high strength (1200 MPa or more) while avoiding excessive deformation resistance that would shorten forging die life. The controlled perlite proportion balances strength and formability
Solution Approach 2:
The invention creates a composite microstructure consisting of ferrite and perlite phases with controlled proportions and morphologies. This composite structure combines the strength benefits of perlite with the ductility and formability of ferrite, achieving high strength while maintaining adequate cold forgeability for extended die life
4Reliability
If alloying elements are added to improve delayed fracture resistance, then delayed fracture resistance is improved, but manufacturing cost increases
Solution Approach 1:
The invention converts the harmful effect of cold wire drawing (which can create microcracks and reduce delayed fracture resistance) into a beneficial process. By carefully controlling the drawing parameters to achieve refined perlite structure with 250 nm or less lamellar spacing, the cold drawing process itself becomes a means of improving delayed fracture resistance through microstructural refinement, eliminating the need for additional alloying elements
Solution Approach 2:
The invention replaces expensive alloying elements (such as Cr, Mo, B) with a cost-effective cold wire drawing process. The mechanical processing method achieves delayed fracture resistance through microstructural control rather than chemical composition modification, significantly reducing material costs while maintaining reliability
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 enables the production of bolts with tensile strengths of 1200 MPa or more, improved cold forgeability, and enhanced delayed fracture resistance, reducing manufacturing costs and extending forging die life.
Implementation Method 1
the kind of its microstructure, the presence proportion between species therein, and the form thereof are appropriately adjusted
Implementation Method 2
The non heat-treatment bolts are required to ensure a target strength by work hardening during the wire drawing
Implementation Method 3
cause an interface between cementite and ferrite in the perlite structure to trap hydrogen in the steel, thereby improving the steel in delayed fracture resistance
Implementation Method 4
the relationship among the total area reduction rate of raw steel therefor when the steel is subjected to cold wire drawing, the total area reduction rate when the steel is subjected to diameter reduction, and the Ceq (carbon equivalent) thereof
Data Source
AI summary
A steel wire for high-strength bolts is used for a non heat-treatment bolt with an excellent cold forgeability for which quenching and tempering steps have been omitted after bolt formation, and which has a tensile strength of 1200 MPa or more and an excellent delayed fracture resistance. The steel wire includes C, Si, Mn, P, S, Cr, Al, N, and B, at least one selected from the group consisting of Ti, V, and Nb with the balance consisting of iron and inevitable impurities. The steel wire has a microstructure wherein ferrite and perlite have a total area rate of 98% or more, perlite lamellar spacing is 250 nm or less, and an area rate of the perlite is more than 40%, and 80% or less. The steel wire has a tensile strength of 1300 MPa or less.