Bainitic Steel for Cold-Forged Bolts
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Solution Overview
Problem
There is a need for a steel for bolts that can achieve low deformation resistance during cold forging in bolt head forming and high product yield without thermal refining, specifically for non-heat-treated bolts with a strength classification of 8.8 as specified in JIS B1051.
Innovation Solution
The steel composition includes C: 0.18% to 0.24%, Si: 0.10% to 0.22%, Mn: 0.60% to 1.00%, Al: 0.010% to 0.050%, Cr: 0.65% to 0.95%, Ti: 0.010% to 0.050%, B: 0.0015% to 0.0050%, N: 0.0050% to 0.0100%, P: 0.025% or less, S: 0.025% or less, Cu: 0.20% or less, and Ni: 0.30% or less, with a bainitic microstructure present in an area ratio of 95% or more, and prior austenite grains with a grain size number of 6 or more, to achieve a Bauschinger effect and suppress strength variation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the steel composition is optimized for high strength (C: 0.18% to 0.24%, Cr: 0.65% to 0.95%, etc.), then the tensile strength reaches 800 MPa or more suitable for 8.8 class bolts, but the cold workability deteriorates requiring annealing treatment before cold forging
Solution Approach 1:
The invention changes the chemical composition parameters by strictly controlling C (0.18-0.24%), Cr (0.65-0.95%), Mn (0.60-1.00%), and other elements within specific ranges to achieve the desired balance between strength and cold workability without requiring annealing treatment
Solution Approach 2:
The invention creates a composite microstructure consisting of bainite (90-100% area ratio) with specific prior austenite grain size (5-15 μm), combining the strength characteristics of bainite with the ductility provided by controlled grain refinement
2Ease of manufacture
If annealing treatment is applied to improve cold workability, then the steel becomes softer and more formable, but the manufacturing process complexity increases and the advantage of omitting thermal refining is lost
Solution Approach 1:
The invention performs preliminary action by optimizing the chemical composition and controlling the microstructure (bainite with specific grain size) during steel production, so that the steel inherently possesses the necessary cold workability without requiring subsequent annealing treatment
Solution Approach 2:
The invention extracts and eliminates the annealing treatment step from the manufacturing process by incorporating all necessary microstructural control measures into the steel composition and production process itself
3Reliability
If controlled cooling after hot rolling is applied to obtain bainitic microstructure, then the toughness is improved, but the austenite crystal grains become coarsened during preheating resulting in cracking during cold working
Solution Approach 1:
The invention changes the parameter of prior austenite grain size to a specific range (5-15 μm) through controlled chemical composition and processing, achieving both toughness and cracking resistance during cold working
Solution Approach 2:
The invention applies local quality by ensuring uniform bainitic microstructure with specific grain size characteristics throughout the steel, creating consistent mechanical properties and cracking resistance in all regions
4Ease of manufacture
If the steel strength is reduced to improve workability, then the cold workability improves, but the tensile strength falls below the 800 MPa requirement for 8.8 class bolts
Solution Approach 1:
The invention creates a composite microstructure of bainite with controlled prior austenite grain size that combines high strength (≥800 MPa tensile strength) with good workability, achieving both requirements simultaneously
Solution Approach 2:
The invention optimizes multiple parameters including C (0.18-0.24%), Cr (0.65-0.95%), prior austenite grain size (5-15 μm), and bainite area ratio (90-100%) to achieve the optimal balance between strength and workability
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 provides a steel for bolts with high product yield and low deformation resistance during cold forging, suitable for non-heat-treated bolts with a strength classification of 8.8, effectively suppressing cracking and maintaining good drawability.
Implementation Method 1
obtain a Bauschinger effect and suppress strength variation
Data Source
AI summary
Disclosed is a non-heat-treated steel that has low deformation resistance during cold forging in bolt head forming and excellent product yield, and that can be manufactured without the need to perform heat treatment for controlling strength variation. The disclosed steel has a chemical composition containing C: 0.18-0.24%, Si: 0.10-0.22%, Mn: 0.60-1.00%, Al: 0.010-0.050%, Cr: 0.65-0.95%, Ti: 0.010-0.050%, B: 0.0015-0.0050%, N: 0.0050-0.0100%, P: 0.025% or less inclusive of 0, S: 0.025% or less inclusive of 0, Cu: 0.20% or less inclusive of 0, and Ni: 0.30% or less inclusive of 0, in a range satisfying: 0.45≤C+Si/24+Mn/6+Ni/40+Cr/5≤0.60 and N≤0.519Al+0.292Ti, with the balance being Fe and inevitable impurities; and a microstructure in which bainite is present in an area ratio of 95% or more, where the microstructure contains prior austenite grains with a grain size number of 6 or more, and strength variation is 100 MPa or less.