Hot-rolled steel sheet, cold-rolled steel sheet, galvanized steel sheet, and methods of manufacturing the same

A method of manufacturing the hot-rolled steel sheet, the method comprising,

• first hot rolling carried out at least once at a rolling reduction ratio of 20% or more in a temperature range of 1000° C. to 1200° C., and an austenite grain diameter is set to 200 μM or less, wherein
• an ingot or slab containing, by mass %:
• C:
• 0.0001% to 0.40%,
• Si:
• 0.001% to 2.5%,
• Mn:
• 0.001% to 4.0%,
• P:
• 0.001% to 0.15%,
• S:
• 0.0005% to 0.03%,
• Al:
• 0.001% to 2.0%,
• N:
• 0.0005% to 0.01%,
• O:
• 0.0005% to 0.01%,
• and further comprising one or two or more of:
• Ti:
• 0.001% to 0.20%,
• Nb:
• 0.001% to 0.20%,
• V:
• 0.001% to 1.0%,
• W:
• 0.001% to 1.0%,
• B:
• 0.0001% to 0.0050%,
• Mo:
• 0.001% to 1.0%,
• Cr:
• 0.001% to 2.0%;
• Cu:
• 0.001% to 2.0%,
• Ni:
• 0.001% to 2.0%,
• Co:
• 0.0001% to 1.0%,
• Sn:
• 0.0001% to 0.2%,
• Zr:
• 0.0001% to 0.2%,
• As:
• 0.0001% to 0.50%,
• Mg:
• 0.0001% to 0.010%,
• Ca:
• 0.0001% to 0.010%, and
• REM:
• 0.0001% to 0.1%
• and balance composed of iron and inevitable impurities;
• second hot rolling in which a total of rolling reduction ratios is 50% or more is carried out in a temperature range of T1+30° C. to T1+200° C.;
• third hot rolling in which a total of rolling reduction ratios is less than 30% is carried out in a temperature range of T1° C. to lower than T1+30° C.;
• and
• hot rolling ends at an Ar3 transformation temperature or higher,
• where, T1 is a temperature determined by steel sheet components, and expressed by the following formula 1,
• T1(° C.)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V (Formula 1).