Ground Reinforcement Steel Pipe Composition for Strength and Formability
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Solution Overview
Problem
Existing hot-rolled steel sheets and steel pipes for ground reinforcement face challenges in achieving both high strength and formability, particularly due to issues with weld zone softening and shape defects during manufacturing.
Innovation Solution
A hot-rolled steel sheet with a specific alloy composition (0.05-0.1% C, 0.1% Si or less, 1.5-1.9% Mn, 0.05-0.15% Ti, 0.03-0.1% Nb, 0.03-0.1% Mo, 0.02% P or less, 0.02% S or less, 0.01% N or less) and microstructure (90% or more ferrite with grain size ≤15 μm) is developed, along with a manufacturing method involving reheating, finish hot rolling, and coiling within specific temperature ranges to produce a steel pipe with enhanced strength and formability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a low-temperature structure such as bainite or martensite is used to achieve high strength, then yield strength and tensile strength are improved, but the structure causes softening in the weld zone due to slow cooling rate after welding, deteriorating the physical properties of the weld zone
Solution Approach 1:
The patent changes the microstructural parameters by controlling the phase transformation during cooling. By adjusting the finish hot rolling temperature (800-950°C) and coiling temperature (550-700°C), the steel achieves a ferrite microstructure with specific grain size (15μm or less) instead of bainite or martensite. This parameter change in microstructure resolves the contradiction by providing high strength through fine-grained ferrite while avoiding weld zone softening.
Solution Approach 2:
The patent creates a composite microstructure consisting of fine-grained ferrite (90% or more by area) with controlled carbide precipitation. The specific alloy composition (C: 0.05-0.1%, Mn: 1.5-1.9%, Ti: 0.05-0.15%, Nb: 0.03-0.1%, Mo: 0.03-0.1%) creates a composite structure at the micro level, where the ferrite matrix provides ductility and the fine carbides provide strength, achieving both high strength and weldability.
2Ease of manufacture
If the steel pipe is subjected to extensive work hardening during piping to achieve small diameter, then formability is improved, but shape defects occur due to excessive hardening
Solution Approach 1:
The patent changes the material parameters by creating a fine-grained ferrite microstructure (average grain size 15μm or less) with specific alloy composition. This microstructural parameter change enables the steel to undergo extensive plastic deformation during piping operations without developing shape defects, as the fine grains distribute stress more uniformly and prevent localized hardening defects.
Solution Approach 2:
The patent performs preliminary microstructure control during hot rolling before the piping operation. By controlling the finish hot rolling temperature and coiling temperature to produce a uniform fine-grained ferrite structure with controlled carbide distribution, the steel is pre-conditioned to withstand subsequent piping operations without shape defects, enabling small diameter production with excellent formability.
3Strength
If the ferrite grain size is reduced to increase strength, then yield strength and tensile strength are improved, but formability deteriorates
Solution Approach 1:
The patent optimizes the ferrite grain size parameter to 15μm or less, which is a specific threshold that balances strength and formability. This parameter is achieved through controlled finish hot rolling temperature (800-950°C) and coiling temperature (550-700°C), creating a fine-grained ferrite structure that provides high strength while maintaining adequate formability for manufacturing operations.
Solution Approach 2:
The patent creates a composite microstructure of fine-grained ferrite (90% or more by area) with uniformly distributed fine carbides (average size 20nm or less). The ferrite matrix provides ductility and formability, while the fine carbide precipitates provide strength through precipitation hardening. This composite structure resolves the contradiction between grain size reduction for strength and maintenance of formability.
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 proposed solution achieves a yield strength of 700 MPa or more, tensile strength of 750 MPa or more, and elongation of 15% or more for the steel sheet, and a yield strength of 800 MPa or more, tensile strength of 800 MPa or more, and elongation of 10% or more for the steel pipe, while maintaining uniform hardness and preventing shape defects.
Implementation Method 1
the hot-rolled steel sheet includes by weight, 0.05% or more of a carbide containing Ti, Nb, and Mo, alone or in combination thereof, wherein the carbide has an average size of 20 nm or less
Implementation Method 2
finish hot rolling the reheated steel slab at a temperature within a range of 800 to 950° C. to obtain a hot-rolled steel sheet; wherein the hot-rolled steel sheet for ground reinforcement has a microstructure including by area, 90% or more of ferrite, wherein a grain of the ferrite has an average size of 15 μm or less
Data Source
AI summary
The present invention relates to: a hot rolled steel sheet for ground reinforcement and a steel pipe for ground reinforcement, which have excellent strength and formability; and manufacturing methods thereof. An aspect of the present disclosure is to provide a hot-rolled steel sheet for ground reinforcement and a steel pipe for ground reinforcement, having excellent strength and formability, and manufacturing methods thereof.