Steel purity is an important factor in its quality. Therefore, the need for cleaner steels is growing every year.. The so-called pure steels are usually steels, in which the content of impurities, such as, phosphorus, sulfur, oxygen, nitrogen, hydrogen - sometimes carbon, as well as non-metallic inclusions very low.
Steel purity – theory and practice
Improving the purity of steel is therefore becoming an increasingly important goal for metallurgical scientists., and an important challenge for iron and steel producers. The need for steels with improved mechanical properties is forcing steelmakers to improve the purity of their final products.. To achieve satisfactory steel purity, it is necessary to control and improve the entire range of technological operations throughout the entire steel production process., such as deoxidation, alloying, secondary metallurgical treatments, casting.
What is "pure steel"
Since the term "pure steel" is rather vague, some authors introduce more precise formulations of such steel:
- "High-purity steel" - steel with a low level of soluble impurities;
- "Low impurity steel" – impurities, that occur when remelting steel scrap;
- "Pure steel" – low defect steel, which are associated with the presence of oxides.
Benefits of clean steel
Well known, that individual and joint exposure to carbon, phosphorus, sulfur, nitrogen, hydrogen and total oxygen content in steel can have a noticeable effect on the properties of the steel, such as:
- tensile strength;
- formability - significant plastic deformation without cracking;
- cracking resistance;
- corrosion resistance;
- fatigue strength and so on.
Besides, pure steel requires control over non-metallic oxide inclusions - their size, distribution, morphology and chemical composition.
Control of impurity elements, which were listed above, may be different for different types and grades of steel. The thing is, that the effect of ordinary impurities in steel on their mechanical properties in some cases can be very significant and harmful, and in others - insignificant and even useful.
Influence of sulfur and oxygen
Oxygen and sulfur form oxide and sulfide inclusions.
These inclusions adversely affect:
- plastic properties (elongation, narrowing and bending ability);
- cold forging and drawing ability;
- low temperature viscosity level;
- fatigue strength.
Effect of carbon and nitrogen
Carbon and nitrogen:
- increase the hardenability of steel, but reduce its plastic and viscous properties;
- promote the formation of pearlite and cementite with a decrease in plasticity and toughness;
- promote embrittlement of steel through precipitation of carbides and nitrides along grain boundaries.
Effect of phosphorus
Phosphorus forms a solid solution with iron:
- increases hardenability;
- promotes temper brittleness;
- increases the tendency to embrittlement.
Impurities and inclusions in various steels
As mentioned, the purity of the steel depends on the quantity, morphology and size distribution of non-metallic inclusions. Inclusions generate most defects in steels. Therefore, for many products, the maximum size of inclusions is limited., however, the total amount of inclusions and their size distribution are also important factors in the cleanliness of the steel..
for instance, maximum carbon content in automotive sheet steel and deep upset steels should not exceed 30 ppm and nitrogen - 50 ppm, the size of non-metallic inclusions should not exceed 100 μm.
IN sheet steel for stamping cans carbon content should not be more than 30 ppm, nitrogen - 40 ppm, and the total oxygen content is 20 ppm.
IN alloy steels for the manufacture of pressure vessels, the phosphorus content does not exceed 70 ppm.
Pipe steels have a sulfur limit - 30 ppm, nitrogen - 50 ppm, oxygen - 30 ppm, as well as the maximum size of non-metallic inclusions – 100 μm.
IN welded steel plates the hydrogen content should not exceed 1,5 ppm.
Bearing steel contains oxygen no more 10 ppm, and non-metallic inclusions – not more 10 μm.
Cord steel for car tires may contain no more than: 2 ppm hydrogen, 40 ppm nitrogen, 15 ppm oxygen, as well as non-metallic inclusions no more 10 μm.
Steel for the production of thick plates: hydrogen - no more 2 ppm, nitrogen - no more 30-40 ppm, total oxygen - no more 20 ppm, as well as single non-metallic inclusions - no more 13 μm, clusters of inclusions - no more 200 μm.
Steel wire: nitrogen - no more 60 ppm, total oxygen content - no more 30 ppm, non-metallic inclusions - no more 20 μm.
Technological control of steel purity
Steel purity is controlled during all technological operations of steel production. This control includes:
- time and place for steel deoxidation;
- time and place for alloying steel;
- duration and sequence of furnace and out-of-furnace steel treatments;
- stirring steel;
- transfer of liquid steel from unit to unit;
- steel ladle design and handling;
- features of various metallurgical fluxes and their application;
- methods of casting steel.