Hot Forging Steel Composition for MnS False Pattern Suppression

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Solution Overview

Problem

Hot forging steels with high sulfur content improve machinability but often result in false patterns during magnetic particle testing due to non-magnetic MnS inclusions, leading to incorrect defect detection and increased inspection workload.

Innovation Solution

A hot forging steel composition with controlled sulfur content and refined MnS inclusions, achieved through a specific range of elements like Bi, which refines the dendrite structure and disperses MnS, reducing the size and number of MnS inclusions to prevent false patterns during magnetic particle testing.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If sulfur content is increased to improve machinability, then machinability is improved, but false patterns are generated during magnetic particle testing

Engineering Contradiction:

ImprovemachinabilityVSAvoiddefect detection accuracy

Core Design Contradiction:

Ease of manufactureVSMeasurement precision

Solution Approach 1:

The patent changes the physical and chemical parameters of MnS inclusions by controlling sulfur content (0.01-0.05 mass%) and adding microalloying elements (Ti: 0.01-0.05 mass%, Nb: 0.01-0.05 mass%, V: 0.01-0.05 mass%) to transform MnS from coarse, non-magnetic inclusions that cause false patterns into fine, dispersed inclusions that do not generate significant magnetic leakage flux

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent creates a composite microstructure by combining MnS inclusions with microalloying elements (Ti, Nb, V) to form complex sulfide inclusions (such as TiS, NbS, VS) that have different magnetic properties and distribution characteristics, thereby eliminating false patterns while maintaining machinability

Inventive Principle:

Principle #40Composite materials

2Ease of manufacture

If sulfur content is increased to improve machinability, then machinability is improved, but inspection workload increases due to additional penetrant testing

Engineering Contradiction:

ImprovemachinabilityVSAvoidinspection workload

Core Design Contradiction:

Ease of manufactureVSLoss of time

Solution Approach 1:

The patent converts the harmful effect of sulfur (which causes false patterns) into a beneficial effect by precisely controlling sulfur content and combining it with microalloying elements. This transformation allows the steel to maintain good machinability while the refined MnS inclusions no longer cause false patterns, making magnetic particle testing reliable and eliminating the need for additional penetrant testing

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Ease of manufacture

If MnS inclusions are increased to improve machinability, then machinability is improved, but hot ductility is degraded due to formed FeS

Engineering Contradiction:

ImprovemachinabilityVSAvoidhot ductility

Core Design Contradiction:

Ease of manufactureVSStrength

Solution Approach 1:

The patent changes the composition parameters by limiting sulfur to a low range (0.01-0.05 mass%) and introducing microalloying elements (Ti, Nb, V) that form stable sulfides with higher melting points and better hot workability, thereby preventing FeS formation and maintaining hot ductility while preserving machinability through controlled MnS dispersion

Inventive Principle:

Principle #35Parameter changes

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 enhances machinability while minimizing false patterns in magnetic particle testing, ensuring accurate defect detection and reducing inspection workload, thereby improving the reliability of hot forging processes.

Implementation Method 1

a magnetic leakage flux is generated in a surface defect part of a hot forged product by magnetizing the hot forged product, and a magnetic particle pattern is formed by causing magnetic particle to be adsorbed to a place where a significant magnetic leakage flux is generated

Methodology Applied

Scientific EffectMagnetic leakage flux: Magnetic Field

Implementation Method 2

a magnetic particle pattern is formed by causing magnetic particle to be adsorbed to a place where a significant magnetic leakage flux is generated

Methodology Applied

Scientific EffectMagnetic particle adsorption: Adsorption

Implementation Method 3

refines the dendrite structure and disperses MnS, reducing the size and number of MnS inclusions

Methodology Applied

Scientific EffectDendrite refinement: Crystallisation

Data Source

PatentEP3366799B1Steel for hot forging and hot forged product

Publication Date: 2021.03.24 NIPPON STEEL CORPORATION

AI summary

There is provided a hot forging steel including, by mass%, C: more than 0.30% and less than 0.60%, Si: 0.10% to 0.90%, Mn: 0.50% to 2.00%, S: 0.010% to 0.100%, Cr: 0.01% to 1.00%, Al: more than 0.005% and 0.100% or less, N: 0.0030% to 0.0200%, Bi: more than 0.0001% and 0.0050% or less, Ti: 0% or more and less than 0.040%, V: 0% to 0.30%, Ca: 0% to 0.0040%, Pb: 0% to 0.40%, and a remainder including Fe and impurities, in which P and O in the impurities are respectively P: 0.050% or less and O: 0.0050% or less, an expression d+3σ<20 is satisfied, and a presence density of MnS having an equivalent circle diameter of smaller than 2.0 µm is 300 pieces/mm2 or more in a cross section parallel to a rolling direction of a steel.