Effect of cooling mode on microstructure and mechanical properties in an extremely low carbon Cu bearing steel

• Different cooling modes were adopted in an extremely low carbon Cu-bearing steel.
• The microstructure-mechanical property relationship was determined.
• The strengthening and toughening mechanism were discussed in detail.
• A steel with good comprehensive mechanical properties was produced.

The effect of cooling mode on microstructure-mechanical property relationship in an extremely low carbon Cu-bearing steel has been comparatively studied. It has been demonstrated that the cooling mode has a decisive effect on phase transformation mechanism. The morphology, size and crystal structure of copper precipitates are greatly affected by cooling modes. Coarser copper precipitates showing fcc structure can be observed in furnace + air cooling process, while finer copper precipitates with twinned 9R structure can be found in air cooling process. Quantitative analyses have been applied to evaluate the strengthening mechanism, revealing that cooling mode has a significant effect on grain-boundary strengthening and copper precipitation strengthening, which are the main factors that contribute to the yield strength difference. The low temperature toughness was influenced by the ferrite grain size and copper precipitation behavior. In general, a steel with excellent comprehensive mechanical properties have been developed through air cooling process.