Hot-Rolled Steel Sheet Internal Oxide Structure for Faster Pickling
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Solution Overview
Problem
High-strength steel sheets with high Si and Mn content require longer pickling times due to internal oxide layers, leading to reduced productivity and potential cracking during cold rolling, as existing techniques fail to effectively improve picklability without increasing costs or reducing productivity.
Innovation Solution
Control of the Si/Mn ratio and heat treatment conditions after hot rolling and coiling to form an internal oxide layer with a net-like structure, enhancing acid dissolubility and reducing pickling time without altering the steel composition or manufacturing process significantly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the steel sheet contains high Si and Mn content to achieve high strength and ductility, then the strength and ductility are improved, but internal oxide layers are generated which significantly increase pickling time and reduce productivity
Solution Approach 1:
The invention changes the chemical composition parameters by strictly controlling the Si/Mn ratio to be 0.30 or less (while maintaining high Si and Mn content for strength). This parameter change prevents the formation of problematic internal oxide layers that would otherwise require extended pickling times, thus resolving the contradiction between achieving high strength/ductility and maintaining productivity.
2Reliability
If the pickling time is extended to completely remove internal oxide layers, then the picklability is improved, but the productivity significantly lowers
Solution Approach 1:
The invention performs preliminary action by controlling the Si/Mn ratio during steelmaking to prevent the formation of difficult-to-pickle internal oxide structures in the first place. This preventive approach eliminates the need for extended pickling times while ensuring complete removal of oxide layers, thus improving picklability without sacrificing productivity.
3Productivity
If cold rolling is performed without completely removing internal oxide layers, then the productivity is maintained, but cracks occur due to peeling of remaining internal oxide layer causing deterioration of conversion property
Solution Approach 1:
By changing the compositional parameter (Si/Mn ratio ≤ 0.30), the invention modifies the internal oxide layer characteristics so that standard pickling processes can completely remove them. This ensures that cold rolling can proceed without crack formation from peeling oxides, maintaining both productivity and conversion property reliability.
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
Significantly improves picklability and reduces pickling time, maintaining the properties required for high-strength steel sheets while enhancing productivity.
Implementation Method 1
when hot rolling is performed on such a steel material containing a large amount of Si and Mn and then the material is coiled in a coil shape at approximately 550° C. or more, a Si-based oxide is generated at a grain boundary of crystal containing metallic iron as a main parent phase and in a crystal grain, in base iron right below an oxide scale of a steel sheet surface layer portion. The generation of the oxide is so-called internal oxidation
Data Source
AI summary
A Si/Mn ratio of steel material components of a base material is not less than 0.27 nor more than 0.90 in mass ratio, an internal oxide layer having a thickness of not less than 1 μm nor more than 30 μm is provided right below an oxide scale of a steel sheet surface layer portion, and regarding the internal oxide layer, an internal oxide in a crystal grain of the internal oxide layer is an oxide containing Si and having a thickness of not less than 10 nm nor more than 200 nm in a crystal grain in a range of greater than 0% and 30% or less of a thickness of the internal oxide layer from an interface between the internal oxide layer and base iron toward a direction of the surface layer oxide scale, one or more branches of the internal oxide exist in a cross section of 1 μm×1 μm square, and in any crystal grain boundary having a length of 1 μm, one or more of the internal oxides in the crystal grain are connected to an internal oxide of the crystal grain boundary to form a net-like structure.