Thin Metal Strip Hot Rolling for Grain Boundary Etching Resistance
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Solution Overview
Problem
Thin metal strips produced by twin roll casting often exhibit cracking and separations along prior austenite grain boundaries after acid etching, which is a result of low friction hot rolling conditions, leading to surface defects and potential failures.
Innovation Solution
A method involving hot rolling with a coefficient of friction equal to or greater than 0.20, either with or without lubrication, to eliminate prior austenite grain boundaries and create elongated surface structures on the metal strips, which are then cooled to form martensitic steel with enhanced mechanical properties and reduced susceptibility to acid etching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If low friction hot rolling is used to reduce roll wear and rolling load, then rolling efficiency is improved, but prior austenite grain boundaries become etched by acid forming cracks and separations
Solution Approach 1:
The invention changes the friction parameter during hot rolling by applying lubrication to achieve a coefficient of friction between 0.15 and 0.35, which optimizes the balance between rolling efficiency and surface integrity. This parameter adjustment prevents excessive grain boundary etching while maintaining productive rolling operations.
Solution Approach 2:
The invention introduces lubrication as an intermediary substance between the rolls and the metal strip during hot rolling. This lubricant layer mediates the friction interaction, reducing roll wear and rolling load while controlling the friction coefficient to prevent harmful grain boundary etching that would occur under low friction conditions.
2Duration of action of stationary object
If lubrication is applied to reduce friction during hot rolling, then roll wear is reduced, but prior austenite grain boundaries are etched by acid forming cracks
Solution Approach 1:
The invention precisely controls the lubrication parameter to achieve a coefficient of friction within the range of 0.15 to 0.35 during hot rolling. This optimized friction level extends roll life through reduced wear while simultaneously preventing the excessive grain boundary etching that causes cracks, thus resolving the contradiction between roll durability and surface integrity.
3Quantity of substance
If acid pickling is performed to remove oxidation scale, then surface cleanliness is improved, but prior austenite grain boundaries are etched forming depressions and cracks
Solution Approach 1:
The invention performs preliminary action by controlling the friction conditions during hot rolling to prevent excessive grain boundary formation and weakening before acid pickling occurs. By optimizing the friction coefficient during rolling, the metal structure is prepared in advance to resist acid etching, reducing the harmful effects of subsequent pickling operations.
Solution Approach 2:
The invention provides beforehand cushioning by applying lubrication during hot rolling to create a protective effect that cushions the metal structure against the harmful impacts of subsequent acid pickling. The controlled friction conditions during rolling prepare the grain structure to withstand acid exposure, preventing excessive etching and crack formation during the pickling process.
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 results in thin metal strips with surfaces primarily free of prior austenite grain boundaries, achieving higher tensile and yield strengths, improved elongation, and resistance to acid etching, while maintaining surface roughness below 4 micrometers.
Implementation Method 1
a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a gap at a nip between the casting rolls... counter rotating the pair of counter-rotatable casting rolls to form metal shells on the casting surfaces of the casting rolls
Implementation Method 2
hot rolling with a coefficient of friction equal to or greater than 0.20... creating opposing hot rolled exterior side surfaces of the thin metal strip primarily free of prior austenite grain boundaries and characterized as having a plurality of elongated surface structure formations formed by shear
Implementation Method 3
hot rolling the thin metal strip using a pair of opposing work rolls... the force applied to the thin metal strip during hot rolling is 600 to 2500 tons
Implementation Method 4
after having been hot rolled and cooled to form a thin metal strip... the thin metal strip, after cooling, is characterized as having a tensile strength of 1100 to 2100 MPa, a yield strength of 900 to 1800 MPa
Data Source
AI summary
Described herein are thin metal strips having hot rolled exterior side surfaces characterized as being primarily or substantially free of all prior austenite grain boundaries, or at least primarily or substantially free of all prior austenite grain boundaries, and including elongated surface structure. As a result, because the prior austenite grain boundaries are not primarily or substantially present, all such prior austenite grain boundaries are not susceptible to grain boundary etching due to acid etching or pickling. In particular examples, the thin metal strips undergo hot rolling performed with a coefficient of friction equal to or greater than 0.20 with or without use of lubrication.