High-strength cold-rolled steel sheet, hot-dipped galvanized steel sheet, alloyed hot-dipped galvanized steel sheet, and methods for producing of these

A hot-dip galvannealed steel sheet comprising:

• a high-strength cold-rolled steel sheet;
• and
• a hot-dip galvannealed layer provided on a surface of the high-strength cold-rolled steel sheet, wherein
• the high-strength cold-rolled steel sheet comprises, in % by mass:
• C:
• 0.070 to 0.140%;
• Si:
• 0.80 to 1.80%;
• and
• Mn:
• 1.80 to 2.80%,
• in area ratio in a structure observed with a scanning electron microscope,
• bainite, tempered martensite, and hard phases composed of at least one of cementite and MA made up of quenched martensite and retained austenite combined together account for 85% or more in total,
• the hard phase that has a breadth of 0.4 μm or less and a length of 1.2 μm or more accounts for 0.5% or more,
• the hard phase that has a breadth of 1.2 μm or more accounts for 5.0% or less, and
• structures other than the bainite, the tempered martensite, and the hard phases account for 15% or less;
• and
• in volume ratio measured by X-ray diffractometry, retained austenite accounts for 3.0% or more and 7.0% or less.

A method for producing the high-strength cold-rolled steel sheet, the method comprising:

• a rolling step of subjecting a steel slab satisfying a composition comprising, in % by mass:
• C:
• 0.070 to 0.140%;
• Si:
• 0.80 to 1.80%;
• and Mn:
• 1.80 to 2.80%, sequentially to hot rolling and cold rolling;
• a heating step of heating the steel sheet obtained by cold rolling the steel slab to a temperature region of (Ac3 point+200° C.) or lower while adjusting a heating rate at 700° C. or more to 1.5° C./sec or more and 30° C./sec or less;
• a soaking step of holding the steel sheet subjected to the heating step for 10 seconds or more and 100 seconds or less;
• a first cooling step of cooling the steel sheet subjected to the soaking step to a first cooling temperature of 100° C. or higher and 410° C. or lower at a cooling rate of 10° C./sec or more and 50° C./sec or less;
• a holding step of holding the steel sheet cooled to the first cooling temperature at a holding temperature of 100° C. or higher and 410° C. or lower for 10 seconds or more and 80 seconds or less;
• and
• a second cooling step of cooling the steel sheet subjected to the holding step to room temperature at a cooling rate of 15° C./sec or more, wherein
• the high-strength cold-rolled steel sheet comprises
• in area ratio in a structure observed with a scanning electron microscope,
• bainite, tempered martensite, and hard phases composed of at least one of cementite and MA made up of quenched martensite and retained austenite combined together account for 85% or more in total,
• the hard phase that has a breadth of 0.4 μm or less and a length of 1.2 μm or more accounts for 0.5% or more,
• the hard phase that has a breadth of 1.2 μm or more accounts for 5.0% or less, and
• structures other than the bainite, the tempered martensite, and the hard phases account for 15% or less;
• and
• in volume ratio measured by X-ray diffractometry, retained austenite accounts for 3.0% or more and 7.0% or less.