Low Yield Steel Plate Strain Aging Resistance
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Solution Overview
Problem
Existing methods for manufacturing steel plates with low yield ratio, high strength, and high toughness for linepipes face challenges such as increased productivity and material costs, decreased toughness in weld heat affected zones, and strain ageing issues, particularly at lower temperatures.
Innovation Solution
A manufacturing process involving controlled rolling, accelerated cooling, and reheating to form a microstructure with a polygonal ferrite phase and island martensite, stabilized by appropriate Mn content and controlled rolling conditions, achieving a duplex-phase structure with a low yield ratio and high ductility without significant strength loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If additional heat treatment processes (quenching and tempering) are performed to achieve low yield ratio and high strength, then the mechanical properties are improved, but productivity decreases and manufacturing cost increases
Solution Approach 1:
The invention merges the rolling process and heat treatment process into a single integrated controlled rolling process. By controlling the rolling temperature to be at or below the Ar3 transformation point and applying specific rolling reduction rates, the desired microstructure (ferrite phase with dispersed hard phases) is formed directly during rolling, eliminating the need for separate quenching and tempering operations.
Solution Approach 2:
The invention performs preliminary action by controlling the rolling temperature and reduction rate in advance to pre-form the desired microstructure before final cooling. By finishing rolling at or below the Ar3 transformation point and applying controlled cooling rates, the microstructure is prepared in advance to achieve the target yield ratio and strength properties without additional heat treatment.
2Strength
If alloying chemical elements (Cu, Ni, Mo) are added to achieve low yield ratio and high strength, then the mechanical properties are improved, but material cost increases
Solution Approach 1:
The invention changes the physical and processing parameters (rolling temperature at or below Ar3, rolling reduction rate of 5-20%, cooling rate of 10-50°C/s) to achieve the desired microstructure and mechanical properties through process control rather than through adding alloying elements. This parameter-based approach replaces the need for Cu, Ni, and Mo additions.
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 process enables the production of steel plates with a low yield ratio, high strength, and excellent strain ageing resistance, maintaining toughness across the weld heat affected zone and base metal, even at lower temperatures, thus enhancing productivity and economic efficiency.
Implementation Method 1
quenching starting from a temperature range for forming a dual phase consisting of a ferrite phase and an austenite phase
Implementation Method 2
as an intermediate treatment between quenching and tempering, quenching starting from a temperature range for forming a dual phase
Implementation Method 3
quenching (hereinafter, also referred to as Q) and tempering (hereinafter, also referred to as T)
Data Source
Figure 1
AI summary
A steel plate having a low yield ratio and high strength excellent in terms of strain ageing resistance of an API 5L X70 grade or less, a method for manufacturing the steel plate and a high strength welded steel pipe made of the steel plate are provided. A high strength steel plate having a low yield ratio, the steel plate having a chemical composition containing, by mass%, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a metallographic structure including a bainite phase and island martensite, and further including a polygonal ferrite in surface portions within 5 mm from the upper and lower surfaces, wherein the area fraction of the island martensite is 3% or more and 15% or less, wherein the equivalent circle diameter of the island martensite is 3.0 µm or less, wherein the area fraction of the polygonal ferrite in the surface portions is 10% or more and less than 80%, and wherein the remainder of the metallographic structure consists of a bainite phase, a hardness variation in the thickness direction of ΔHV30 or less in terms of Vickers hardness, a hardness variation in the width direction of ΔHV30 or less in terms of Vickers hardness, a maximum hardness in the surface portions of the steel plate of HV230 or less in terms of Vickers hardness and a yield ratio of 85% or less and an elongation of 22% or more in a full-thickness tensile test using a test piece having a shape in accordance with GOST standards.