Steel Plate Microstructure Control for Strength and Low-Temperature Toughness
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Solution Overview
Problem
Conventional methods for producing high-Ni steel plates for low-temperature applications, such as LNG storage tanks, face challenges in manufacturing cost, productivity, and flexibility due to strict control of Si content and heating rates, making them unsuitable for marine fuel tanks requiring thinner plates and improved manufacturability.
Innovation Solution
A steel plate with a specific chemical composition and microstructure, produced through hot rolling, reheating quenching, and tempering, achieves high strength and low-temperature toughness without the need for dual-phase region quenching, allowing for flexible production on the same line as other products.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If dual-phase region quenching is applied to produce 9% Ni steel with excellent low-temperature toughness, then low-temperature toughness is improved, but manufacturing complexity and production restrictions increase
Solution Approach 1:
The patent extracts and eliminates the dual-phase region quenching step from the conventional three-step heat treatment process. By removing this complex intermediate step, the process is simplified to standard quenching and tempering, reducing manufacturing complexity while maintaining the desired low-temperature toughness through alternative means (controlled alloy composition and microstructure management).
Solution Approach 2:
The patent makes the heat treatment process universal by using standard quenching and tempering procedures that can be applied to various steel grades and product types on the same production line. This eliminates the need for special dual-phase region quenching equipment and allows flexible production of different products without requiring separate process lines.
2Reliability
If strict control of Si content to 0.10 mass % or less is applied, then low-temperature toughness is improved, but design flexibility and manufacturing adaptability deteriorate
Solution Approach 1:
The patent changes the approach to controlling Si content from strict limitation (0.10 mass % or less) to controlled addition within a broader range (0.01-1.00 mass %). This parameter change allows designers more flexibility in component specifications while still achieving excellent low-temperature toughness through the combined effect of controlled Si levels, Ni content (5.0-10.0 mass %), and microstructure management during hot rolling and heat treatment.
3Manufacturing precision
If strict control of heating rate in specific temperature range is applied during reheating quenching, then microstructure control is improved, but manufacturing complexity and process difficulty increase
Solution Approach 1:
The patent applies dynamic control of heating rates during reheating quenching, adjusting the heating rate based on the specific temperature range and steel composition. This dynamic approach allows precise microstructure control (prior austenite grain size of 20 μm or less) while maintaining manufacturing ease by using automated rate control systems that adapt to process conditions, rather than requiring fixed strict controls that complicate the process.
4Reliability
If three-step heat treatment (quenching, dual-phase region quenching, tempering) is applied to produce 9% Ni steel, then low-temperature toughness is improved, but productivity and manufacturing cost worsen
Solution Approach 1:
The patent extracts and removes the dual-phase region quenching step from the conventional three-step heat treatment process, simplifying it to a two-step process of standard quenching followed by tempering. This reduction in process steps directly improves productivity by reducing processing time and increases manufacturing cost efficiency by using more standard, less expensive heat treatment equipment and procedures.
Solution Approach 2:
The patent implements continuous heat treatment processes where quenching and tempering are performed in uninterrupted sequences without requiring separate dual-phase region quenching cycles. This continuous action eliminates idle time between process steps and allows for more efficient production scheduling, thereby improving productivity while maintaining the desired material properties.
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 steel plate exhibits excellent manufacturability, high strength, and low-temperature toughness, suitable for LNG tanks, while reducing production restrictions and costs.
Implementation Method 1
a steel material to which Ni in an amount of 5.0 mass % to 10.0 mass % is added is hot rolled under the condition that the rolling reduction ratio is 5 or more and the number of passes where the rolling reduction per pass is 10% or more out of the final 5 passes is 2 or more to obtain a hot-rolled steel plate
Implementation Method 2
cooling the hot-rolled steel plate, reheating the cooled hot-rolled steel plate to a reheating temperature of Ac3 point or higher and 900° C. or lower and quenching
Implementation Method 3
tempering the reheated and quenched hot-rolled steel plate at a tempering temperature of 500° C. or higher and 650° C. or lower
Data Source
AI summary
Provided is a steel plate that has high strength and excellent low-temperature toughness as well as excellent manufacturability. A steel plate having a predetermined chemical composition and a microstructure in which a volume fraction of retained austenite at a ¼ thickness position is less than 3.0%, the maximum prior austenite grain size at a ½ thickness position is 100μm or less, and a ratio b/a of an average value b in the top 5% of prior austenite grain sizes to an average prior austenite grain size a at the ½ thickness position is 4.5 or less, with a steel plate thickness of 40 mm or less, a yield stress of 585 MPa or more, and a tensile strength of 690 MPa or more.