Hot dip galvanized steel sheet and method for producing same

1. A hot dip galvanized steel sheet comprising a base steel sheet and a hot dip galvanized layer on at least one surface of the base steel sheet, wherein the base steel sheet has a chemical composition comprising, by mass %,

C: 0.050% to 0.350%,

Si: 0.10% to 2.50%,

Mn: 1.00% to 3.50%,

P: 0.050% or less,

S: 0.0100% or less,

Al: 0.001% to 1.500%,

N: 0.0100% or less,

O: 0.0100% or less,

Ti: 0% to 0.200%,

B: 0% to 0.0100%,

V: 0% to 1.00%,

Nb: 0% to 0.100%,

Cr: 0% to 2.00%,

Ni: 0% to 1.00%,

Cu: 0% to 1.00%,

Co: 0% to 1.00%,

Mo: 0% to 1.00%,

W: 0% to 1.00%,

Sn: 0% to 1.00%,

Sb: 0% to 1.00%,

Ca: 0% to 0.0100%,

Mg: 0% to 0.0100%,

Ce: 0% to 0.0100%,

Zr: 0% to 0.0100%,

La: 0% to 0.0100%,

Hf: 0% to 0.0100%,

Bi: 0% to 0.0100%,

REM other than Ce and La: 0% to 0.0100% and

a balance of Fe and impurities,

a steel microstructure at a range of ⅛ thickness to ⅜ thickness centered about a position of ¼ thickness from a surface of the base steel sheet contains, by area %,

ferrite: 0% to 50%,

retained austenite: 0% to 30%,

tempered martensite: 5% or more,

fresh martensite: 0% to 10%, and

pearlite and cementite in total: 0% to 5%,

when there are remaining structures, the remaining structures consist of bainite,

when defining a region having a hardness of 90% or less of the hardness at a position of ¼ thickness to the base steel sheet side from an interface of the base steel sheet and the hot dip galvanized layer as a “soft layer”, there is a soft layer having a thickness of 10 μm or more at the base steel sheet side from the interface,

the soft layer contains tempered martensite, and

an increase rate in a thickness direction of an area % of tempered martensite from the interface to the inside of the base steel sheet inside the soft layer is 5.0%/μm or less.

2. The hot dip galvanized steel sheet according to claim 1 , wherein the steel microstructure further contains, by area %, retained austenite: 6% to 30%.

3. A method for producing the hot dip galvanized steel sheet according to claim 1 , comprising:

a hot rolling step of hot rolling a slab having the chemical composition of the base steel sheet according to claim 1 to obtain a hot rolled steel sheet;

a cold rolling step of cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet; and

a hot dip galvanizing step of hot dip galvanizing the cold rolled steel sheet to obtain the hot dip galvanized steel sheet;

wherein:

(A) the cold rolling step satisfies the conditions of the following (A1) and (A2):

(A1) a rolling line load satisfies the following formula (1) and cold rolling with a rolling reduction of 6% or more is performed one time or more:

13≤A/B≤35  formula (1)

 wherein in formula (1), A is the rolling line load in kgf/mm and B is the tensile strength of the hot rolled steel sheet in kgf/mm 2 ;

 (A2) a total cold rolling reduction is 30 to 80%; and

(B) the hot dip galvanizing step comprises: subjecting the cold rolled steel sheet to a first soaking treatment followed by a first cooling to obtain a first cooled steel sheet; subjecting the first cooled steel sheet to a second soaking treatment to obtain a second soaked steel sheet; dipping the second soaked steel sheet in a hot dip galvanizing bath to obtain a coated steel sheet; a second cooling of the coated steel sheet to obtain a second cooled steel sheet; and subjecting the second cooled steel sheet to a third soaking treatment; and wherein the hot dip galvanizing step further satisfies the following conditions (B1) to (B6):

(B1) in the first soaking treatment, the cold rolled steel sheet is heated to a maximum heating temperature in an atmosphere satisfying the following formulas (2) and (3), wherein the maximum heating temperature is at least Ac1+30° C. or more and 950° C. or less, and an average heating rate from 650° C. to the maximum heating temperature is 0.5° C./s to 10.0° C./s;

(B2) in the first soaking treatment, the cold rolled steel sheet which has been heated to the maximum heating temperature is held at the maximum heating temperature for 1 second to 1000 seconds;

(B3) in the first cooling, an average cooling rate in a temperature range of 700° C. to 600° C. is 10° C./s to 100° C./s;

(B4) in the second soaking treatment, the first cooled steel sheet is held in a range of 300° C. to 600° C. for 80 seconds to 500 seconds in an atmosphere satisfying the following formulas (4) and (5);

(B5) in the second cooling, cooling the coated steel sheet to a temperature of Ms-50° C. or less wherein Ms represents martensite transformation start temperature of the base steel sheet; and

(B6) in the third soaking treatment, the second cooled steel sheet is heated to a temperature region of 200° C. to 420° C., then held in the temperature region for 5 seconds to 500 seconds;

−1.10≤log(PH 2 O/PH 2 )≤−0.07  formula (2)

0.010≤PH 2 ≤0.150  formula (3)

log(PH 2 O/PH 2 )<−1.10  formula (4)

0.0010≤PH 2 ≤0.1500  formula (5)

wherein PH 2 O and PH 2 represent partial pressures of water vapor and hydrogen respectively of the respective atmosphere of the conditions (B1) and (B4).