<p>The automotive industry is under growing pressure to reduce its carbon footprint, with steel producers being one of the main contributors to lifecycle CO₂ emissions. Achieving carbon neutrality requires close collaboration with upstream industries, particularly the steel and energy sectors. This study develops a comprehensive process-level model to analyze the cradle-to-grave carbon footprint and costs of automotive steel products. The results show that lifecycle carbon emissions, considering different production routes, vehicle types, and energy sources, range from 1.02 tCO₂ to 10.71 tCO₂. Lifecycle carbon emissions decline substantially, with reductions of 50.46%-88.40% under independent and collaborative scenarios. Lifecycle cost trajectories show differentiated dynamics when time-varying electricity and hydrogen prices are considered. Initial cost reductions result primarily from early vehicle electrification and production decarbonization, while rising electricity prices in later stages partially offset these savings, especially in collaborative decarbonization scenarios relying heavily on low-carbon electricity. Despite higher absolute costs, the unit carbon footprint per cost remains lower under collaborative scenarios, highlighting superior carbon mitigation efficiency and the importance of coordinated action across the automotive, steel, and energy industries. CBAM fees for blast furnace-basic oxygen furnace (BF-BOF) steel rise steadily, while fees for low-carbon steel production remain initially low and increase gradually, highlighting rising cost pressure on high-emission steel and the need for proactive decarbonization. This study provides actionable insights into lifecycle decarbonization of automotive steel, supporting cross-industry strategies for achieving deep emission reductions and carbon neutrality.</p>