COLD-ROLLED STEEL SHEET AND PROCESS FOR MANUFACTURING SAME

摘要

A high-strength cold-rolled steel sheet includes a composition having controlled amounts of carbon, silicon, manganese, phosphorous, sulfur, titanium, niobium, sol. Aluminum, chromium, molybdenum, vanadium, boron, calcium, REM, and iron. A microstructure thereof has a main phase of ferrite of at least 40 area %, and a second phase of a low-temperature transformation phase consisting either or both of martensite and bainite, which comprises at least 10 area % in total and retained austenite (γ) at least comprising 3 area %. An average grain diameter of ferrite has a tilt angle of at least 15° is at most 5.0 mm, an average grain diameter of the low-temperature transformation-produced phase is at most 2.0 mm, an average grain diameter of lump-like retained γ having an aspect ratio of less than 5 is at most 1.5 mm, and an area fraction of the lump-like retained γ relative to the retained γ is at least 50%.

主权项

1. A cold-rolled steel sheet characterized by having:
a chemical composition comprising, in mass %, C: 0.06 to 0.3%, Si: 0.4 to 2.5%, Mn: 0.6 to 3.5%, P: at most 0.1%, S: at most 0.05%, Ti: 0 to 0.08%, Nb: 0 to 0.04%, a total content of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to 1%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM: 0 to 0.003% and the remainder of Fe and impurities, a microstructure having a main phase of ferrite which comprising at least 40 area %, and a second phase of a low-temperature transformation phase consisting either or both of martensite and bainite which comprising at least 10 area % in total and retained austenite at least which comprising 3 area %, and satisfying Equations (1) to (4):
dF≦5.0  (1);dM+B≦2.0  (2);dAd≦1.5  (3); andrAs≧50  (4), where dF is an average grain diameter (μm) of ferrite defined by high angle grain boundaries having a tilt angle of at least 15°; dM+B is an average grain diameter (μm) of the low-temperature transformation phase; dAs is an average grain diameter (μm) of retained austenite having an aspect ratio of less than 5; and rAs is an area fraction (%) of retained austenite having an aspect ratio of less than 5 relative to all the retained austenite.