Ni-Steel Plate Toughness via Parameter H Control
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Solution Overview
Problem
High-Ni steel plates used in LNG storage tanks experience a deterioration in low-temperature toughness when subjected to large plastic strains due to process-induced transformation of residual austenite to martensite, and existing methods to maintain toughness are inefficient with high phosphorus content, which also reduces productivity.
Innovation Solution
A method for producing Ni-containing steel plates with excellent low-temperature toughness after plastic strain by controlling a parameter H, which involves specific heating and holding times during intermediate and tempering treatments, allowing for optimal diffusion of C and Ni into residual austenite, thereby maintaining stability and preventing transformation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the P content is restricted to 0.001% by mass or less to improve low-temperature toughness, then the low-temperature toughness is improved, but the productivity of steel plates deteriorates
Solution Approach 1:
The invention changes the parameter of phosphorus content from the conventional strict limit of 0.001% or less to a relaxed range of 0.001% to 0.005% by mass, combined with optimized heat treatment parameters (tempering temperature of 500-650°C and holding time of 10-60 minutes) to maintain low-temperature toughness while improving productivity
Solution Approach 2:
The invention performs preliminary heat treatment (tempering) after quenching to stabilize the martensitic structure and prevent brittle fracture before the steel plate is put into service, thereby maintaining toughness even with higher phosphorus content
2Adaptability or versatility
If a large plastic strain is applied to the steel plate to improve formability, then the formability is improved, but the residual austenite transforms to martensite and low-temperature toughness deteriorates
Solution Approach 1:
The invention performs preliminary tempering treatment to stabilize the martensitic structure formed during quenching, creating a pre-stabilized microstructure that can accommodate subsequent plastic deformation without causing process-induced transformation of residual austenite
Solution Approach 2:
The invention utilizes controlled phase transition during tempering treatment to transform unstable martensite into a more stable structure, and to control the stability of residual austenite so that it does not transform during subsequent plastic deformation
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 ensures a brittle fracture rate of 5% or less, maintaining excellent strain aging properties even with phosphorus content exceeding 0.001% by mass, enhancing the steel's toughness and stability without compromising productivity.
Implementation Method 1
optimal diffusion of C and Ni into residual austenite
Implementation Method 2
process-induced transformation to become martensite
Data Source
Figure 1
AI summary
The method for production according to the embodiment of the present invention includes the steps of, in sequence: quenching a steel including a predetermined chemical composition, with the balance being Fe and inevitable impurities, from a quenching temperature of 800°C or higher and 820°C or lower after hot-rolling the steel; applying an intermediate heat treatment to the quenched steel by holding the steel at a heating temperature of 690°C or higher and 710°C or lower and then cooling the steel at an average cooling rate of 5°C/sec or more until a cooling end temperature of 200°C or lower; and tempering the steel at a tempering temperature of 570°C or higher and 600°C or lower, wherein a parameter H defined by a heating and holding time during the intermediate heat treatment, and a heating and holding time during the tempering is set at 1.73 × 10-6 or more and 1.96 × 10-6 or less.