1. Question: What are the mechanical properties of 10# steel pipe? Why is its plasticity and toughness better than Q255 and Q275 steel pipe?
Answer: The mechanical properties of 10# steel pipe are "low strength, high plasticity, and high toughness." Specific indicators (hot-rolled state): yield strength ≥205MPa, tensile strength 335-475MPa, elongation ≥31%, impact toughness (αk) ≥49J/cm². Its plasticity and toughness are superior to Q255 and Q275 steel pipes, primarily due to its lower carbon content (10# steel contains 0.07%-0.14% carbon, far lower than the 0.18%-0.38% of Q255 and Q275), and lower impurity content-the lower the carbon content, the fewer lattice defects within the steel, the stronger its plastic deformation capacity, the better its toughness, and the less prone it is to brittle fracture.
2. Question: What is the mechanical property level of 20# steel pipe? How does it balance strength and ductility, and in which scenarios is it suitable for applications requiring a balanced performance?
Answer: The mechanical properties of 20# steel pipe are at a balanced level of "medium strength and good ductility." Specific indicators (hot-rolled state): yield strength ≥245MPa, tensile strength ≥410MPa, elongation ≥25%, impact toughness (αk) ≥39J/cm². The core reason for its ability to balance strength and ductility is that the carbon content is controlled between 0.17% and 0.24%, avoiding both the insufficient strength caused by the low carbon content of 10# steel and the decreased ductility caused by the high carbon content of Q275 steel. Therefore, it is suitable for scenarios requiring a balance between strength and ductility, such as shafts and sleeves in machining, fluid transport pipelines (bearing certain pressure and requiring a certain level of toughness), and structural components requiring welding and bending.
3. Question: What is the hardness ranking of Q255, Q275, 10#, and 20# steel pipes? What is the relationship between hardness and carbon content? Answer: The hardness ranking of the four types of steel pipes (from highest to lowest) is: Q275 > Q255 > 20# > 10#. There is a positive correlation between hardness and carbon content: when other chemical compositions are similar, the higher the carbon content, the higher the hardness of the steel. Q275 has the highest carbon content (0.28%-0.38%), therefore the highest hardness; Q255 has the second highest carbon content (0.18%-0.28%), and the second highest hardness; 20# steel has a slightly higher carbon content (0.17%-0.24%) than 10# steel (0.07%-0.14%), therefore its hardness is higher than 10# steel, while 10# steel has the lowest carbon content and the lowest hardness.
4. Question: What effect does temperature change have on the mechanical properties of Q255 steel pipes? What precautions should be taken when using Q255 steel pipes in low-temperature environments? Answer: Temperature changes significantly affect the mechanical properties of Q255 steel pipes: at high temperatures (above 300℃), its yield strength and tensile strength decrease significantly, while its plasticity increases, making it prone to creep deformation; at low temperatures (below -20℃), its plasticity and toughness decrease sharply, making it prone to brittle fracture, i.e., the "cold brittleness" phenomenon. When using Q255 steel pipes in low-temperature environments, the following precautions should be taken: avoid impact loads, minimize the stress intensity, and if necessary, perform low-temperature aging treatment to improve its low-temperature toughness. Also, avoid bending, stamping, and other cold working processes in low-temperature environments to prevent pipe breakage.
5. Question: Compared to Q255 steel pipes, which has better impact toughness, 20# steel pipes? How is this difference manifested in practical applications?
Answer: The impact toughness (αk≥39J/cm²) of 20# steel pipes is superior to that of Q255 steel pipes (αk≥34J/cm²). This difference manifests in practical applications as follows: 20# steel pipes are more suitable for scenarios that may withstand slight impact loads, such as connecting pipes in mechanical transmissions and support structures of vehicle chassis. When subjected to sudden impacts, they can absorb energy through plastic deformation and avoid breakage. On the other hand, Q255 steel pipes are not suitable for scenarios with impact loads due to their poor impact toughness. Otherwise, they are prone to brittle fracture, leading to equipment failure or safety accidents.
