Ultra-high strength steel sheet having excellent phosphatability and hole expandability and method for manufacturing same

Abstract

Provided is an ultra-high-strength steel sheet having high phosphatability and hole expandability, including, by wt %, carbon (C): 0.08% to 0.2%, silicon (Si): 0.05% to 1.3%, manganese (Mn): 2.0% to 3.0%, phosphorus (P): 0.001% to 0.10%, sulfur (S): 0.010% or less, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.3% to 1.2%, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.01% to 0.05%, nitrogen (N): 0.001% to 0.01%, and a balance of iron (Fe) and inevitable impurities, satisfying 3.4≤Ti / N≤10, 1.0≤Mn / (Si+Cr), and 0.7≤Mn* / (Si*+Cr*)≤Mn / (Si+Cr) where Ti, N, Mn, Si, and Cr refer to a weight percent (wt %), and Mn*, Si*, and Cr* refer to an average of values obtained by GDS component analysis from the surface to the 0.1 μm position in the thickness direction.

Description

TECHNICAL FIELD[0001]The present disclosure relates to an ultra-high-strength steel sheet for automobiles or the like, and more particularly, to an ultra-high-strength steel sheet having high phosphatability and hole expandability, and a method for manufacturing the ultra-high-strength steel sheet.BACKGROUND ART[0002]In recent years, the use of ultra-high-strength steel sheets for automobiles has increased to deal with fuel efficiency regulations for protecting the environment and to guarantee crash safety. When manufacturing such ultra-high-strength steels, it is not easy to ensure sufficient strength and ductility through using only general steel materials strengthened by the solid-solution strengthening or precipitation strengthening effect.[0003]Thus, transformation-strengthened steels having strength and ductility improved by using transformation phases have been developed, and examples thereof include dual phase (DP) steel, complex phase (CP) steel, and transformation induced ...

AI Technical Summary

Benefits of technology

The present invention is about a strong steel sheet that has a high tensile strength of 1 GPa or greater, a high yield ratio of 0.8 or greater, can prevent flange cracking during a forming process, and has a high crash energy absorbing ability. Additionally, the steel sheet has high phosphatability, meaning it can be easily coated with phosphate.

Problems solved by technology

When manufacturing such ultra-high-strength steels, it is not easy to ensure sufficient strength and ductility through using only general steel materials strengthened by the solid-solution strengthening or precipitation strengthening effect.

Although it has been attempted to guarantee elongation using various transformation phases as described above so as to deal with the accelerated use of high-strength steels for automobiles, it is still practically difficult to ensure sufficient elongation.

However, although such ultra-high-strength steels have high yield strength, the ultra-high-strength steels may easily fracture or may not smoothly absorb energy in a crash.

However, CP steel has ductility and bending workability markedly varying with phase fractions, and proper fractions of phases and manufacturing ranges for CP steel have not yet been studied.

However, the addition of silicon (Si) or chromium (Cr) leads to the formation of surface oxides during an annealing process, and the surface oxides may not be easily removed in a pickling process after the annealing process.

Therefore, final products have poor phosphatability.