1.How does temperature affect toughness?
Body-centered cubic steel has a unique characteristic: it exhibits a ductile-brittle transition temperature. When the ambient temperature falls below this critical point, the material's fracture mode abruptly changes from ductile fracture (microscopically manifested as dislocation slip) that requires the absorption of a large amount of energy to brittle fracture (microscopically manifested as "cleavage fracture," i.e., separation along specific crystal planes) that absorbs almost no energy. The "brittleness" you mentioned refers to this change in fracture mode.
2.What effect does cold rolling have on toughness?
Decreased toughness: Studies have found that as the cold rolling reduction rate increases, the impact absorption energy and fracture toughness of the material at extremely low temperatures (such as 4K) decrease significantly.
Changes in fracture mode: When cold-rolled materials fracture at low temperatures, their fracture surfaces exhibit characteristics such as "flat brittle fracture" and "intergranular fracture," indicating that their resistance to crack propagation is very weak.
Simply put: if you compare ordinary steel to resilient "playdough," the combined effects of cold rolling and low temperatures are like freezing this "playdough" hard, then repeatedly pounding and compacting it, ultimately turning it into a hard biscuit that's easily broken.
3.How do the specific characteristics of steel grade and composition affect toughness?
High-manganese steel: For example, the 32Mn-7Cr steel mentioned in the study, even at ultra-low temperatures of -269℃ (4K), retains considerable strength despite a decrease in toughness after cold rolling.
IF steel (interstitial atomless steel): This type of steel is commonly used in deep-drawn automotive parts, but under certain conditions (such as phosphorus content and improper annealing), it may exhibit "secondary processing brittleness," meaning brittle fracture at low temperatures during post-forming use. This illustrates the crucial role of composition and process design in low-temperature performance.
4.What causes cold-rolled steel to become brittle?
The fundamental reason lies in the fact that the body-centered cubic crystal structure of steel determines its ductile-brittle transition characteristics, and the microstructural changes brought about by cold rolling (such as dislocations and grain deformation) further exacerbate this tendency for low-temperature embrittlement.
5.How to select materials for long-term use in environments below 0℃ or even colder?
Special low-temperature steels (such as steels for low-temperature pressure vessels) should be selected.
Complete annealing treatment after cold rolling is necessary to eliminate the effects of work hardening.
Design optimizations should be implemented to avoid stress concentration at low temperatures.