Fire Resistant Steel Composition and Production Process
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Solution Overview
Problem
Conventional fire-resistant steel materials for thick-gauge steel applications face challenges in maintaining toughness, reheating embrittlement resistance, and high temperature strength, particularly in H-beams, due to issues with precipitation of nitrides and martensite, and the economic concern of high Mo prices.
Innovation Solution
A fire-resistant steel material is developed with a composition of C: 0.001% to 0.030%, Si: 0.05% to 0.50%, Mn: 0.4% to 2.0%, Nb: 0.03% to 0.50%, Ti: 0.005% to less than 0.040%, N: 0.0001% to less than 0.0050%, and Al: 0.005% to 0.030%, optimized to inhibit carbide and nitride precipitation, using Nb as a solid solution element for enhanced high temperature strength and toughness without adding B, and controlling the balance of elements to prevent reheating embrittlement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Mo is added to cause fine Mo carbides precipitation for improving high temperature strength, then high temperature strength is improved, but production cost increases due to high Mo prices
Solution Approach 1:
The patent replaces expensive Mo carbide precipitation strengthening with a cheaper alternative using B to inhibit carbide precipitation and maintain C in solid solution form. This substitution uses a low-cost element (B) to achieve the same strengthening effect without the high material cost of Mo, directly addressing the technical contradiction between high temperature strength and production cost.
Solution Approach 2:
The patent changes the strengthening mechanism from precipitation strengthening (Mo carbides) to solid solution strengthening (C in ferrite). By controlling the absence of carbide precipitation through B addition and optimizing C content (0.05-0.20%), the patent achieves high temperature strength through a different physical-chemical state, eliminating dependence on expensive Mo while maintaining performance.
2Strength
If Ti and Nb are added for strengthening, then high temperature strength is improved, but nitride precipitation occurs which reduces toughness and causes reheating embrittlement
Solution Approach 1:
The patent introduces B as an intermediary element that mediates between Ti/Nb strengthening and carbide/nitride precipitation. B preferentially combines with C to form BC carbides, preventing C from forming harmful nitrides with Ti and Nb. This intermediary action allows Ti and Nb to provide strengthening while B protects against the harmful precipitation that would otherwise reduce toughness and cause reheating embrittlement.
Solution Approach 2:
The patent converts the potential harm of C (which can form harmful nitrides with Ti/Nb) into a benefit by using B to control C's behavior. The same C that could cause reheating embrittlement through nitride formation is instead directed to form beneficial solid solution strengthening and BC carbides, transforming a harmful element into a useful strengthening mechanism.
3Strength
If B is added to inhibit carbide precipitation and maintain solid solution C, then high temperature strength is improved, but reheating embrittlement resistance deteriorates
Solution Approach 1:
The patent optimizes the B content parameter to a specific range (0.0005-0.0050%) to achieve the desired balance. By precisely controlling B concentration, the patent maintains enough B to inhibit harmful carbide precipitation and preserve solid solution C for high temperature strength, while limiting B to levels that prevent excessive grain boundary segregation and maintain reheating embrittlement resistance.
Solution Approach 2:
The patent creates local quality differences by controlling B distribution and interaction with other elements. B is present in sufficient quantity to inhibit carbide precipitation in the bulk material, providing high temperature strength, but its concentration is limited to prevent harmful effects at grain boundaries during reheating. This spatial and concentration differentiation resolves the contradiction between the two opposing requirements.
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 material achieves superior high temperature strength, toughness, and reheating embrittlement resistance, reducing costs and improving the reliability and safety of large-sized building structures by eliminating the need for cold working and thermal refining treatments.
Implementation Method 1
using Nb as a solid solution element for enhanced high temperature strength and toughness
Implementation Method 2
optimized to inhibit carbide and nitride precipitation
Implementation Method 3
controlling the balance of elements to prevent reheating embrittlement
Data Source
AI summary
The present invention provides a fire resistant steel material excellent in high temperature strength, toughness, and reheating embrittlement resistance containing, by mass %, C: 0.001% to 0.030%, Si: 0.05% to 0.50%, Mn: 0.4% to 2.0%, Nb: 0.03% to 0.50%, Ti: 0.005% to less than 0.040%, N: 0.0001% to less than 0.0050%, and Al: 0.005% to 0.030%, limiting P: 0.03% or, less and S: 0.02% or less, satisfying C—Nb/7.74≦0.005 and 2≦Ti/N≦12, and having a balance of Fe and unavoidable impurities and, further, a process for production of a fire resistant material comprising heating a steel slab comprised of this chemical composition to 1100 to 1350° C. and hot rolling it by a cumulative reduction rate at 1000° C. or less of 30% or more.