JPWO2023053908A5 - - Google Patents

Claims (5)

in % by mass,
C: 0.06 to 0.24%,
Si: 0.4% or more and less than 1.60%,
Mn: 1.5-3.2%,
P: 0.02% or less,
S: 0.01% or less,
sol. Al: less than 1.0%,
N: contains less than 0.015%,
and satisfies the following formula (1),
Having a component composition in which the balance is Fe and unavoidable impurities,
Area ratio of polygonal ferrite: 20% or more and 85% or less,
Area ratio of upper bainite: 9% or more and 45% or less,
Volume fraction of retained austenite: 3% or more and 15% or less,
Area ratio of fresh martensite: 3% or more and 15% or less,
The total area ratio of tempered martensite and lower bainite: 50% or less (including 0%), and the area ratio of the remaining structure: 5% or less,
The ratio of the total number of fresh martensite particles and retained austenite particles having an equivalent circle diameter of less than 1.2 μm to the total number of fresh martensite particles and retained austenite particles is 50% or more,
Fresh martensite particles and residual austenite particles having an aspect ratio of 2.5 or more and an equivalent circle diameter of 1.2 μm or more and a total number of fresh martensite particles and retained austenite particles having an equivalent circle diameter of 1.2 μm or more and residual A steel sheet in which the ratio of austenite particles to the number of particles is 40% or more.
Si/Mn<0.50 Formula (1)
Here, in formula (1), Si and Mn represent Si content (% by mass) and Mn content (% by mass), respectively. The steel sheet according to claim 1, further comprising one or more selected from the following groups (A) and (B) as the component composition.
Group (A): % by mass, Nb: 0.2% or less, Ti: 0.2% or less, V: 0.2% or less, B: 0.01% or less, Cu: 0.2% or less, Ni : 0.2% or less, Cr: 0.4% or less, Mo: one or more selected from 0.15% or less,
Group (B): % by mass, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.10% or less, Sb: 0.10% or less, REM: 0.0050% or less 1 or 2 or more selected from A member using the steel plate according to claim 1 or 2 . A steel slab having the chemical composition according to claim 1 or 2 is subjected to hot rolling and pickling, and then the obtained hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. a cold rolling process;
An annealing step of obtaining a steel plate by subjecting the cold-rolled steel plate to an annealing treatment,
The cold rolling process is
Cumulative cold rolling rate: 30 to 85%,
By setting the reduction rate of the first pass to 5% or more and less than 25%, {111} <0-11> orientation, {111} <11-2> orientation, {211} <0-11> orientation, and { 100} <011> The cold rolling process for obtaining the cold-rolled steel sheet in which the total area ratio of the structure having the <011> orientation is 35% or more and 75% or less with respect to the entire structure of the bcc phase,
The annealing step includes
For the cold-rolled steel sheet, the temperature range of 500 ° C. or higher and Ac1 or lower is set to an average heating rate of 0.5 to 15 ° C./sec, and 840 ° C. or lower and 0.5 ≤ (T-Ac1) / (Ac3-Ac1 ) <1.0 by heating to the annealing temperature T,
After the heating, soaking and holding at the annealing temperature T in a furnace atmosphere with a dew point Td of −50° C. or more and −30° C. or less results in a needle-like austenite structure having a number density of 5/1000 μm ² or more. get a steel plate,
Then, the temperature range of 750 to 550 ° C. is set to an average cooling rate of 6.0 ° C./sec or more, and the first cooling is performed to cool to the first cooling stop temperature Tc1 of 550 ° C. or less and 400 ° C. or more,
After the first cooling, performing a first holding for 25 seconds or more at the first cooling stop temperature Tc1,
After the first holding, second cooling is performed to cool to a second cooling stop temperature Tc2 that is equal to or lower than the first cooling stop temperature Tc1 and is 450° C. or lower and 300° C. or higher,
Perform a second hold for 20 to 3000 seconds at the second cooling stop temperature Tc2,
A method for manufacturing a steel sheet, including the annealing treatment of performing a third cooling of cooling after the second holding. A method for manufacturing a member, comprising the step of subjecting the steel plate according to claim 1 or 2 to at least one of forming and joining to form a member.