As the basic material of modern industry, the performance of steel is directly regulated by the chemical composition. Among them, carbon (C), manganese (Mn), silicon (Si), sulfur (S), phosphorus (P) five elements by changing the metallurgical organization, crystal structure and distribution of impurities, significantly affecting the strength, toughness, processability and corrosion resistance of steel.
First, carbon (C) elements: strength and plasticity of the core regulator
Carbon is the most important alloying element in steel, and its content has a decisive role in steel performance. In the sub-eutectic steel (carbon content of 0.02% -0.77%) range, with the increase in carbon content, the number of carburized particles in the ferrite matrix, tensile strength and hardness was linearly increased, but the elongation and impact toughness decreased significantly. When the carbon content exceeds the eutectic point (0.77%) to form a peritectic steel, the narrowing of the spacing between the pearlite lamellae leads to a continued increase in strength, but the carbide bias at the grain boundaries triggers the risk of brittleness.
Typical cases show that the carbon content of 0.45% of medium carbon steel after tempering treatment, tensile strength of up to 800MPa, elongation maintained at 15%; and carbon content of 1.2% of high carbon steel although the hardness of HRC62, but the impact toughness is less than 10J/cm². Welding performance, the carbon content of each increase of 0.1%, weld crack sensitivity index increased by 20%, need to use low-hydrogen electrodes and preheat to 150 ℃ or more.
Second, manganese (Mn) element: hardenability and hot workability of the double regulator
Manganese as a weak carbide-forming elements, through solid solution strengthening and organization control dual mechanism to enhance the performance of steel. In ferrite, manganese atoms replace iron atoms to trigger lattice distortion, yield strength increased by about 30MPa/%; in austenite, manganese expansion of the γ-phase region so that the critical temperature of Ac3 increased by 50-80 ℃, significantly improving the hardenability. Experimental data show that 45 steel containing 1.2% manganese can reach HRC45 hardness after water quenching, which is 3 Rockwell hardness levels higher than that of manganese-free steel.
In terms of hot working performance, manganese and sulfur form high melting point MnS (melting point 1610℃), which replaces low melting point FeS (melting point 988℃) to eliminate thermal embrittlement. However, excess manganese (>1.5%) leads to grain coarsening during tempering and a 40% increase in the temper brittleness index, and residual austenite needs to be eliminated by holding at 700 °C. In typical applications, 20MnSi steel with 0.8%-1.2% manganese is widely used for construction rebar, and its yield strength is increased by 25% compared with Q235 steel.
Third, Silicon (Si) element: synergistic enhancer of solid solution strengthening and corrosion resistance
As a strong ferrite-forming element, silicon enhances steel properties through the dual mechanism of solid solution strengthening and surface oxide film. In ferrite, the radius of silicon atoms is 11% larger than that of iron atoms, which triggers lattice distortion to increase the yield strength by about 50MPa/%. Surface oxidation experiments show that the silicon content of 1.5% of the steel oxidized at 800 ℃ for 24 hours, the thickness of the oxide film is 60% less than ordinary steel, thanks to the formation of SiO₂ dense protective layer.
In terms of machinability, a silicon content of more than 0.8% increases the cold deformation resistance by 20%, requiring a multi-pass process with small deformation volumes. Typical applications, silicon content of 0.2% -0.5% of 40SiMn steel used in the manufacture of automotive connecting rods, its fatigue life than ordinary carbon steel to enhance 1.5 times; silicon content of 15% -20% of high-silicon cast iron in sulfuric acid medium corrosion rate <0.1mm / a, become the preferred material for corrosion-resistant parts of chemical equipment.
Fourth, sulfur (S) elements: hot working performance of the invisible destroyer
Sulfur in the form of FeS inclusions in the steel grain boundaries, its harm is mainly reflected in the thermal processing and welding two scenes. FeS and Fe formed by the co-crystal melting point of only 988 ℃, when the steel is heated to 1150 ℃, the grain boundaries at the liquid FeS lead to a decline in the local strength, prone to thermal cracking. Experimental data show that the sulfur content of 0.05% of the steel in the continuous casting process, the incidence of thermal cracking rate is 5 times higher than the sulfur content of 0.01%.
In terms of welding performance, SO₂ gas generated by the reaction between sulfur and oxygen forms pores in the weld, reducing the effective cross-sectional area of the weld metal by 30%. Typical cases show that the sulfur content of 0.08% of Q235 steel in the manual arc welding, weld metal impact toughness is less than 8J/cm ², only 1 / 3 of the base material. modern steelmaking process by adding rare earth elements to form a high melting point of sulfide, the sulfur hazard index reduced by 70%.
Five, phosphorus (P) elements: low-temperature toughness of the fatal killer
Phosphorus in ferrite solid solubility of 0.9%, its atomic radius is 14% larger than the iron atom, triggering serious lattice distortion. Experimental data show that the phosphorus content of 0.1% of the steel at -20 ℃ when the impact toughness of 65% lower than the normal temperature, which stems from the phosphorus atoms in the {100} crystal plane bias formation of Kirchner gas clusters on the dislocation movement of the pinning effect. Low-temperature embrittlement experiments show that steel with 0.15% phosphorus content undergoes deconvoluted fracture at -40°C, with a fracture characterized by typical icosahedral features.
In terms of cutting machinability, the synergistic effect of phosphorus and sulfur resulted in a 20% reduction in cutting forces and a 1.5-fold increase in tool life. In typical applications, the free-cutting steel 1215 with a phosphorus content of 0.08%-0.15% is widely used for precision parts machining, with a surface roughness of up to Ra0.8 μm. It should be noted, however, that with a phosphorus content of more than 0.12%, the rate of corrosion of the steel in the marine environment is increased by a factor of 3, which needs to be inhibited by adding copper elements to form a protective film.
