High-Si Austenitic Stainless Steel Hot Rolling
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Solution Overview
Problem
High-Si austenitic stainless steel used in highly-concentrated nitric acid environments at high temperatures faces challenges such as trans-passive corrosion, intergranular corrosion, and surface defects like scabs during hot rolling due to the formation of Ni-Si intermetallic compounds, which affect ductility and workability.
Innovation Solution
A method for manufacturing austenitic stainless steel with a specific chemical composition (C: 0.04% or less, Cr: 7-20%, Ni: 10-22%, Si: 2.5-7%, Mn: 10% or less, and a sum of Nb, Ti, Ta, and Zr: 0.05-0.7%) is developed, where the heating temperature during hot rolling is controlled using the formula T h = 1135 - 90Si - 2.9Cr + 40Ni - ΔT ≥ 60°C, and subsequent heat treatment is performed within 1100-1160°C with a cooling rate of at least 100°C/min to prevent cracking and scab formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the heating temperature during hot working is increased to improve productivity, then the manufacturing efficiency is improved, but cracking occurs in the slab due to low solid solubility of Si in austenite phase and formation of brittle intermetallic compounds
Solution Approach 1:
The patent applies parameter changes by precisely controlling the heating temperature during hot working to be below the critical temperature where Ni-Si intermetallic compounds form. By adjusting the temperature parameter within a specific range, the patent avoids the formation of brittle phases while maintaining adequate ductility for hot working, thus resolving the contradiction between productivity and cracking resistance
Solution Approach 2:
The patent applies preliminary action by controlling the composition parameters (Si content at 3-7%, Ni content at 8-14%) before the hot working process. This preliminary compositional control ensures that the steel has appropriate ductility and delays the formation of brittle intermetallic compounds until after the hot working is complete, preventing cracking during the process
2Reliability
If the Si content is increased to improve corrosion resistance in concentrated nitric acid, then the corrosion resistance is improved, but the hot workability deteriorates due to formation of brittle phases at high temperature
Solution Approach 1:
The patent applies parameter changes by optimizing the Si content to be within 3-7% and controlling the heating temperature during hot working to remain below the formation temperature of Ni-Si intermetallic compounds. This parameter optimization allows the steel to exhibit both high corrosion resistance (achieved through adequate Si content) and good hot workability (maintained by avoiding brittle phase formation during processing)
Solution Approach 2:
The patent applies preliminary action by pre-controlling the compositional parameters (Si: 3-7%, Ni: 8-14%, and other alloying elements) before the hot working process. This preliminary compositional design ensures that the steel achieves the desired corrosion resistance while maintaining adequate ductility during hot working, as the composition is optimized to delay intermetallic compound formation until after processing
3Reliability
If the heating temperature is controlled below the formation temperature of Ni-Si intermetallic compounds to prevent cracking, then the cracking resistance is improved, but the manufacturing efficiency decreases
Solution Approach 1:
The patent applies preliminary action by optimizing the compositional parameters (Si: 3-7%, Ni: 8-14%, Cr: 16-18%, and other elements) before the hot working process. This preliminary compositional control ensures that the steel has adequate ductility and delays the formation of brittle Ni-Si intermetallic compounds until after the hot working is complete, allowing the use of higher heating temperatures that improve manufacturing efficiency without causing cracking
Solution Approach 2:
The patent applies parameter changes by precisely controlling both compositional parameters (alloying element contents) and process parameters (heating temperature during hot working) within optimized ranges. This dual parameter control allows the steel to maintain high ductility during hot working while achieving the desired corrosion resistance, thus resolving the contradiction between cracking resistance and manufacturing efficiency
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
This method ensures the production of high-Si austenitic stainless steel with improved corrosion resistance and ductility, preventing scab formation during hot rolling and enhancing its suitability for use in high-temperature, high-concentration nitric acid environments without the need for costly surface treatments.
Implementation Method 1
it is estimated that the Ni-Si based intermetallic compound having a low fusion point becomes partially melted at a temperature of more than 1100°C, and propagation of cracking along grain boundaries causes cracking
Implementation Method 2
subjects this steel to solution heat treatment at a temperature of at least 1000°C and at most 1200°C
Data Source
Figure 1~2
Figure 3
AI summary
It is possible to securely produce high Si-containing austenitic stainless steel having corrosion resistance usable in a highly-concentrated nitric acid environment at a high temperature without generating scabs by heating and hot-rolling a slab of stainless steel at a heating temperature during the hot rolling, and subsequently, carrying out heat treatment to heat the hot-rolled stainless steel at a temperature of 1100 to 1160°C, and thereafter, to cool this stainless steel at cooling rate of at least 100°C/min. , wherein the slab of stainless steel has a chemical composition containing: C: at most 0.04% ; Cr: 7 to 20%, Ni: 10 to 22%, Si: 2.5 to 7%, Mn: at most 10% , sol. Al: at most 0.03% , P: at most 0.03% , S: at most 0.03% ; N: at most 0.035% , a total amount of one or more types of elements selected from Nb, Ti, Ta, and Zr: 0.05 to 0.7%; and the remainder being Fe and impurities, and the heating temperature during the hot rolling is defined as Th in which ΔT of Formula (1): Th = 1135 - 90Si - 2.9Cr + 40Ni - ΔT is at least 30°C.