WO2017169836A1 - High-strength cold-rolled steel sheet, high-strength hot-dip-galvanized steel sheet, and production method for high-strength cold-rolled steel sheet and high-strength hot-dip-galvanized steel sheet - Google Patents

Abstract

This high-strength cold-rolled steel sheet: has a prescribed component composition; is configured such that a value X, which is the total number of measurement points at or below [0.40×(IQmax-IQmin)+IQmin] divided by the total number of measurement points and multiplied by 100, is 8 or lower, such that the difference Y-X between a value Y, which is the total number of measurement points at or below [0.75×(IQmax-IQmin)+IQmin] divided by the total number of measurement points and multiplied by 100, and X is 30.0 or higher but less than 45, and such that the difference Z-Y between a value Z, which is the total number of measurement points at or below [0.90×(IQmax-IQmin)+IQmin] divided by the total number of measurement points and multiplied by 100, and Y is 48.0 or higher; and has a volume fraction of retained austenite of 2% or lower with respect to the entire structure thereof. IQ is the sharpness of an electron backscatter diffraction pattern, IQmax is the maximum IQ value from among all measurement points, and IQmin is the minimum IQ value from among all measurement points.

Description

High-strength cold-rolled steel sheet, high-strength hot-dip galvanized steel sheet, and methods for producing them

The present invention relates to a high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet, and a method for producing them, and more specifically, a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more that is excellent in ductility and bendability and has a high yield ratio. The present invention relates to high-strength hot-dip galvanized steel sheets and methods for producing them. Hereinafter, these high-strength cold-rolled steel sheets and high-strength hot-dip galvanized steel sheets may be collectively referred to as high-strength steel sheets.

In recent years, workability such as ductility and bendability has been reduced with increasing strength of members such as automobile steel plates and steel plates for transport machinery. For this reason, it was difficult to process a steel plate for automobiles and a steel plate for transport machinery into a member having a complicated shape by press forming. Therefore, a high-strength steel sheet having excellent workability is demanded.

In addition, when an automotive steel plate is applied to a vehicle body structural member, the higher the yield ratio, the better the impact absorption energy at the same tensile strength. On the other hand, when the yield ratio is too high, the shape freezing property at the time of molding deteriorates, for example, the spring back becomes large. Therefore, a steel sheet having a yield ratio represented by YR (Yield Ratio) of, for example, 79% or more and less than 90% is demanded. The YR is a value obtained by dividing YS (Yield Strength), which is 0.2% proof stress, by TS (Tensile Strength), which is tensile strength, and multiplying by 100.

Among the above required characteristics, the following patent documents have been proposed as techniques for improving the workability of high-strength steel sheets. Patent Document 1 (Japanese Patent Application Laid-Open No. 2014-237871) describes a balance between strength and bendability by using martensite, bainite, or a microstructure in which they are combined, and by making the surface layer of a steel plate soft. It has been shown that it can be improved. However, in patent document 1, only high-strength and the said moldability are examined, and the yield ratio and ductility (elongation) are not considered.

Patent Document 2 (Japanese Patent Application Laid-Open No. 2013-147736) adds at least one element selected from Ti, Nb, and V, makes it essential to add B, has a structure mainly composed of bainite and martensite, and A high-strength steel sheet in which the average crystal grain size of bainite is controlled to 7 μm or less is shown. According to the high-strength steel sheet disclosed in Patent Document 2, a high yield ratio and excellent ductility (elongation) can be ensured. However, the steel sheet disclosed in the example of Patent Document 2 does not consider bendability.

In the above-mentioned patent document, a method for producing a high-strength steel sheet that satisfies all of the tensile strength, yield ratio, ductility, and bendability has not been studied.

Japanese Patent Application Laid-Open No. 2014-237887 JP 2013-147736 A

An object of the present invention is to provide a high-strength cold-rolled steel sheet and a high-strength hot-dip galvanized steel sheet that have a high yield ratio, excellent ductility and bendability in a high-strength region having a tensile strength of 980 MPa or more, and methods for producing the same. It is.

The high-strength cold-rolled steel sheet of the present invention that has achieved the above object is, in mass%, C: 0.12 to 0.19%, Si: more than 0%, 0.4% or less, Mn: 1.80 to 2. 45%, P: more than 0%, 0.020% or less, S: more than 0%, 0.0040% or less, Al: 0.015 to 0.06%, Ti: 0.010 to 0.035%, and B: 0.0025 to 0.0040% contained, the balance being iron and inevitable impurities, X defined by (1) below is 8 or less, Y defined by (2) below and X The difference value Y-X of 30.0 or more and less than 45, the difference Z-Y between Z and Y defined in (3) below is 48.0 or more, and the residual austenite for the entire structure The volume ratio is 2% or less, and the tensile strength is 980 MPa or more.
(1) X is a value obtained by dividing the total number of measurement points equal to or less than [0.40 × (IQmax−IQmin) + IQmin] by 100 and multiplying by 100.
(2) Y is a value obtained by dividing the total number of measurement points equal to or less than [0.75 × (IQmax−IQmin) + IQmin] by the total number of measurement points and multiplying by 100.
(3) Z is a value obtained by dividing the total number of measurement points equal to or less than [0.90 × (IQmax−IQmin) + IQmin] by the total number of measurement points and multiplying by 100, in the above (1) to (3) IQ is the sharpness of the electron beam backscatter diffraction pattern, IQmax is the maximum IQ value at all measurement points, and IQmin is the minimum IQ value at all measurement points.

According to the present invention, it is possible to provide a high-strength cold-rolled steel sheet in which the volume fraction of retained steel and retained austenite and IQ (Image Quality, image quality) are appropriately controlled. Therefore, it is possible to provide a cold-rolled steel sheet having a tensile strength of 980 MPa or more excellent in ductility and bendability and a high yield ratio, a manufacturing method thereof, and a hot-dip galvanized steel sheet.

FIG. 1 is a schematic diagram for explaining IQ requirements defined in the present invention. FIG. 2 is a graph schematically showing the configuration of the annealing process recommended for obtaining the high-strength steel sheet of the present invention.

In order to provide a high-strength steel plate having a tensile strength of 980 MPa or more, a high yield ratio, and excellent ductility and bendability (hereinafter, sometimes referred to as workability), The inventors have made extensive studies focusing on the retained austenite volume fraction and IQ. As a result, it was found that the steel components, retained austenite volume fraction, and IQ may be adjusted to the following ranges, respectively. In the present specification, high strength means that the tensile strength is 980 MPa or more.

First, the IQ that characterizes the present invention will be described in detail. IQ is the sharpness of an EBSD (Electron Back Scatter Diffraction, electron beam backscatter diffraction) pattern. IQ is known to be affected by the amount of strain in the crystal. Specifically, the smaller the IQ, the more strain tends to exist in the crystal. For example, martensite has a high dislocation density and includes disorder of the crystal structure, so that IQ tends to be small. Ferrite tends to have a high IQ due to its low dislocation density. Therefore, conventionally, a method for determining the metal structure using the absolute value of IQ as an index has been proposed. Conventionally, for example, a method of determining a structure having an IQ of 4000 or more as ferrite has been proposed. However, according to the examination results of the present inventors, it is found that the method based on the absolute value of IQ is easily affected by polishing conditions for structure observation, a detector, and the like, and the absolute value of IQ is likely to fluctuate. It was.

Therefore, the present inventors investigated the influence of the dispersion state of strain in the steel sheet, that is, the IQ distribution state, which is the sharpness of the EBSD pattern, on the yield ratio, ductility, and bendability. As a result, it has been found that it is important for IQ to satisfy the requirements described later in order to achieve any of good ductility, bendability, and high yield ratio. The details of the IQ measurement method will be described in the column of Examples described later.

In the present invention, X defined by the following (1) is 8 or less, the difference YX between Y and X defined by the following (2) is 30.0 or more and less than 45, The difference value Z−Y between Z and Y defined in (3) is 48.0 or more. IQ in (1) to (3) below is the sharpness of the electron backscatter diffraction pattern, IQmax is the maximum IQ value in all measurement points, and IQmin is the minimum IQ value in all measurement points. is there.
(1) X is a value obtained by dividing the total number of measurement points equal to or smaller than [0.40 × (IQmax−IQmin) + IQmin] by 100 and multiplying by 100.
(2) Y is a value obtained by dividing the total number of measurement points equal to or less than [0.75 × (IQmax−IQmin) + IQmin] by 100 and multiplying by 100.
(3) Z is a value obtained by dividing the total number of measurement points equal to or smaller than [0.90 × (IQmax−IQmin) + IQmin] by 100 and multiplying by 100.

The items specified by X, Y, and Z are schematically shown in FIG. In FIG. 1, the horizontal axis represents the IQ value, and the vertical axis represents the number ratio (%) of the measurement points indicating each IQ value. X is the number ratio (frequency in percentage) of the measurement points with an IQ value of [0.40 × (IQmax−IQmin) + IQmin] or less with respect to all measurement points. Y is the number ratio (frequency in percentage) of the measurement points whose IQ value is [0.75 × (IQmax−IQmin) + IQmin] or less to all measurement points. Z is the number ratio (frequency in percentage) of the measurement points whose IQ value is [0.90 × (IQmax−IQmin) + IQmin] or less to all measurement points. Therefore, X being 8 or less means that the ratio of the measurement points at which the IQ value is [0.40 × (IQmax−IQmin) + IQmin] or less to the total measurement points is 8% or less. Further, the value of Y−X is 30.0 or more and less than 45 means that the IQ value exceeds [0.40 × (IQmax−IQmin) + IQmin] and [0.75 × (IQmax−IQmin)]. + IQmin] means that the ratio of measurement points to all measurement points is 30.0% or more and less than 45%. In addition, the value of ZY is 48.0 or more means that the IQ value exceeds [0.75 × (IQmax−IQmin) + IQmin] and is equal to or less than [0.90 × (IQmax−IQmin) + IQmin]. This means that the ratio of the measurement points to the total measurement points is 48.0% or more.

When the value of X exceeds 8, the yield ratio is lowered and the bendability is also deteriorated. The reason for this is that as the value of X increases, the number of strained crystals increases. It is considered that the increase in the number of strained crystals increases the movable transition and lowers the yield ratio. It is considered that the bendability deterioration is caused by an increase in microcracks that are the starting points of fracture in the vicinity of a strained crystal. The value of X is preferably 6 or less, more preferably 5 or less. Although the minimum of the value of X is not specifically limited, For example, it is 0.5.

In the present invention, X is 8 or less, and the value of Y−X is 30.0 or more and less than 45. When the value of YX is less than 30.0, the yield ratio decreases. On the other hand, when the value of Y−X is 45 or more, the yield ratio becomes too high and the ductility is lowered. The reason for this is considered that the strain distribution in the steel sheet becomes uniform, the yield ratio increases, and the ductility decreases as the value of YX increases. The value of YX is preferably 33.0 or more and 44.0 or less, and more preferably 35.0 or more and 43.0 or less.

In the present invention, the value of YX is 30.0 or more and less than 45, and the value of ZY is 48.0 or more. By setting the value of ZY to 48.0 or more, ductility can be improved. The reason is considered that the yield ratio can be adjusted to a predetermined range by controlling the value of YX, and crystals with less strain in the steel sheet can be introduced as the value of ZY increases. The value of ZY is preferably 49.0 or more, more preferably 50.0 or more. The upper limit of the value of ZY is not particularly limited, but is 65, for example.

In the present invention, the volume ratio of retained austenite with respect to the entire structure is set to 2% or less. As the volume fraction of retained austenite increases, the yield ratio decreases. The volume fraction of retained austenite is preferably 1.5% or less, more preferably 1% or less, and most preferably 0%. The volume fraction of retained austenite was measured using ISIJ Int. Vol. 33. (1933), no. 7, p. 776 is a value measured by the method described in 776.

The microstructure of the high-strength steel sheet of the present invention is mainly a martensite structure and a bainite structure, and the total ratio of these structures to the entire structure is, for example, 95 area% or more.

In the present invention, in addition to controlling the volume ratio of IQ and retained austenite as described above, the chemical components in the steel sheet are controlled as follows. In the present specification, all chemical components mean mass%.

C: 0.12 to 0.19%
C is an element necessary for ensuring the strength of the steel sheet. If the amount of C is insufficient, the tensile strength decreases. Therefore, the lower limit of the C amount is 0.12% or more. The lower limit of the C amount is preferably 0.13% or more, more preferably 0.14% or more. However, if the amount of C is excessive, the X value calculated based on the IQ value is high, the YX value is low, and the yield ratio and bendability are lowered. Moreover, since a retained austenite is generated excessively, the yield ratio is lowered. Therefore, the upper limit of the C amount is set to 0.19% or less. The upper limit of the C amount is preferably 0.18% or less, and more preferably 0.17% or less.

Si: more than 0% and 0.4% or less When Si is added excessively, the X value calculated based on the IQ value is high, the YX value is low, and the yield ratio and ductility are lowered. Therefore, the upper limit of Si content is 0.4% or less. The upper limit of the Si amount is preferably 0.3% or less, and more preferably 0.2% or less. Although Si does not need to be contained, it is industrially difficult to reduce the amount to 0%. Si is known as a solid solution strengthening element. Si is an element that effectively acts to improve the tensile strength while suppressing a decrease in ductility. Further, Si is an element that improves bendability. In order to effectively exhibit such an effect, the Si content is preferably 0.01% or more, and more preferably 0.1% or more.

Mn: 1.80 to 2.45%
Mn is an element that contributes to increasing the strength of the steel sheet. In order to exhibit such an effect effectively, the lower limit of the amount of Mn is made 1.80% or more. The amount of Mn is preferably 1.9% or more, more preferably 2.0% or more. If Mn is too small, the YX value calculated based on the IQ value will be low, and the yield ratio will be reduced. When the amount of Mn is excessive, the X value calculated based on the IQ value is high, the YX value is low, and the yield ratio and bendability are lowered. Therefore, the upper limit of the Mn amount is 2.45% or less. The upper limit of the amount of Mn is preferably 2.35% or less, and more preferably 2.25% or less.

P: more than 0% and 0.020% or less P is an element inevitably contained. P is an element that segregates at grain boundaries and promotes grain boundary embrittlement, and degrades bendability. For this reason, it is recommended to reduce the amount of P as much as possible. Therefore, the upper limit of the P amount is 0.020% or less. The upper limit of the amount of P is preferably 0.015% or less, and more preferably 0.010% or less. Note that P is an impurity inevitably contained in the steel, and it is industrially impossible to reduce the amount to 0%.

S: more than 0% and 0.0040% or less S is an element inevitably contained in the same manner as P. Since S generates inclusions and degrades bendability, it is recommended that the amount of S be reduced as much as possible. Therefore, the upper limit of the S amount is set to 0.0040% or less. The upper limit of the amount of S is preferably 0.003% or less, more preferably 0.002% or less. In addition, S is an impurity inevitably contained in steel, and it is industrially impossible to reduce the amount to 0%.

Al: 0.015 to 0.06%
Al is an element that acts as a deoxidizer. In order to exhibit this effect effectively, the lower limit of the Al content is set to 0.015% or more. The lower limit of the Al content is preferably 0.025% or more, more preferably 0.030% or more. However, when the amount of Al becomes excessive, many inclusions such as alumina are generated in the steel sheet, and the bendability may be deteriorated. For this reason, the upper limit of the Al amount is set to 0.06% or less. The upper limit of the Al content is preferably 0.055% or less, more preferably 0.050% or less.

Ti: 0.010 to 0.035%
Ti is an element that improves the strength by forming carbides and nitrides. Ti is also an element for effectively utilizing the hardenability of B. Specifically, Ti forms a nitride to reduce N in the steel. Thereby, formation of B nitride is suppressed, and B is in a solid solution state, so that the hardenability of B can be effectively exhibited. Thus, since Ti improves hardenability, it contributes to the strengthening of a steel plate. In order to effectively exhibit such an effect, the lower limit of the Ti amount is set to 0.010% or more. The lower limit of the amount of Ti is preferably 0.013% or more, and more preferably 0.015% or more. However, when the amount of Ti is excessive, Ti carbide and Ti nitride are excessive. Accordingly, the X value calculated based on the IQ value is high and the YX value is low, so that the bendability is deteriorated and the yield ratio is also reduced. For this reason, the upper limit of the Ti amount is set to 0.035% or less. The upper limit of the Ti amount is preferably 0.030% or less. More preferably, it is 0.025% or less.

B: 0.0025 to 0.0040%
B is an element that contributes to increasing the strength of the steel sheet by improving the hardenability. In order to effectively exhibit such an effect, the lower limit of the B amount is set to 0.0025% or more. When B is too small, the YX value calculated based on the IQ value is lowered, and the yield ratio is lowered. The lower limit of the B amount is preferably 0.0027% or more, more preferably 0.0029% or more. However, when the amount of B becomes excessive, the effect is saturated and the cost increases. For this reason, the upper limit of the B amount is set to 0.0040% or less. The upper limit of the amount of B is preferably 0.0035% or less.

The basic components of the high-strength steel sheet of the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are contained in the steel. Inevitable impurities include, for example, N and O in addition to the above-described P and S, and these are preferably in the following ranges.

N: More than 0% and 0.01% or less N is inevitably present as an impurity element and deteriorates bendability. The upper limit of N is preferably 0.01% or less, more preferably 0.006% or less, and still more preferably 0.005% or less. The smaller the amount of N, the better. However, it is industrially difficult to make it 0%.

O: More than 0% and 0.002% or less O is unavoidably present as an impurity element and deteriorates bendability. The upper limit of O is preferably 0.002% or less, more preferably 0.0015% or less, and still more preferably 0.0010% or less. The smaller the amount of O, the better. However, it is industrially difficult to make it 0%.

Furthermore, in the present invention, it is preferable that (a) one or more of Cu, Ni, Cr, Mo, V and Nb or (b) Ca is contained within the following range as required.

The high-strength cold-rolled steel sheet of the present invention is, in mass%, Cu: more than 0%, 0.3% or less, Ni: more than 0%, 0.3% or less, Cr: more than 0%, 0.25% or less, Mo: more than 0%, 0.1% or less, V: more than 0%, 0.05% or less, Nb: more than 0%, 0.08% or less, and Ca: more than 0%, 0.005% or less It is preferable to contain 1 or more types chosen from these.

Cu, Ni, Cr, Mo, V, and Nb are all effective elements for improving the strength. These elements may be contained alone or in appropriate combination within the following ranges.

Cu: more than 0%, 0.3% or less Cu is an element that is further effective in improving the corrosion resistance of the steel sheet. In order to effectively exhibit such an effect, the lower limit of the Cu amount is preferably 0.03% or more, more preferably 0.05% or more. However, when the amount of Cu becomes excessive, the effect is saturated and the cost increases. Therefore, the upper limit of the amount of Cu is preferably 0.3% or less, more preferably 0.2% or less, and still more preferably 0.15% or less.

Ni: more than 0% and 0.3% or less Ni is an element that is further effective in improving the corrosion resistance of the steel sheet. In order to effectively exhibit such an effect, the lower limit of the Ni amount is preferably 0.03% or more, more preferably 0.05% or more. However, when the amount of Ni becomes excessive, the effect is saturated and the cost increases. Therefore, the upper limit of the Ni amount is preferably 0.3% or less, more preferably 0.2% or less, and still more preferably 0.15% or less.

Cr: more than 0% and 0.25% or less Cr is an element showing the effect of increasing the strength. In order to effectively increase the strength of Cr, the lower limit of the Cr amount is preferably 0.01% or more, more preferably 0.015% or more, still more preferably 0.03% or more, and particularly preferably 0.8%. 05% or more. However, when the amount of Cr becomes excessive, unplating occurs, so the upper limit of the amount of Cr is preferably 0.25% or less, more preferably 0.20% or less, and still more preferably 0.10% or less.

Mo: more than 0%, 0.1% or less Mo is an element showing the effect of increasing the strength. In order to effectively increase the strength of Mo, the lower limit of the amount of Mo is preferably 0.03% or more, more preferably 0.05% or more. However, when the amount of Mo becomes excessive, the effect is saturated and the cost increases. Therefore, the upper limit of the Mo amount is preferably 0.1% or less.

V: more than 0% and 0.05% or less V is an element showing the effect of increasing the strength. In order to effectively increase the strength of V, the lower limit of the amount of V is preferably 0.003% or more, and more preferably 0.005% or more. However, when the amount of V becomes excessive, the effect is saturated and the cost increases. Therefore, the upper limit of the V amount is preferably 0.05% or less, more preferably 0.03% or less, and still more preferably 0.02% or less.

Nb: more than 0% and 0.08% or less Nb is an element showing the effect of increasing the strength. In order to effectively increase the strength of Nb, the lower limit of the Nb amount is preferably 0.003% or more, more preferably 0.005% or more. However, if the amount of Nb is excessive, bendability is deteriorated. Therefore, the upper limit of the Nb amount is preferably 0.08% or less, more preferably 0.06% or less, and still more preferably 0.04% or less.

Ca: more than 0% and 0.005% or less Ca is an element effective for spheroidizing sulfides in steel and enhancing bendability. In order to effectively exhibit such an effect, the lower limit of the Ca content is preferably 0.0005% or more, more preferably 0.001% or more. However, when the amount of Ca becomes excessive, the effect is saturated and the cost increases. Therefore, the upper limit of the Ca content is preferably 0.005% or less, more preferably 0.003% or less, and still more preferably 0.0025% or less.

The high strength steel sheet of the present invention in which the chemical composition, the area ratio of retained austenite, and the values X, Y, and Z calculated from IQ values satisfy the above conditions, the tensile strength is 980 MPa or more, and yield Excellent in ratio, ductility and bendability. The yield ratio of the high strength steel sheet of the present invention is, for example, 79% or more and less than 90%, preferably 79.4% or more and less than 90%.

Next, a method for producing the high strength steel plate of the present invention will be described.

The high-strength steel sheet of the present invention that satisfies the above requirements includes processes of hot rolling, cold rolling, and annealing (soaking and cooling), and in particular, appropriately controls the annealing process after cold rolling. There is a feature. Hereinafter, the manufacturing process for obtaining the high-strength steel sheet of the present invention will be described in the order of hot rolling, cold rolling, and subsequent annealing.

Preferred conditions for hot rolling are as follows, for example.

If the heating temperature before hot rolling is low, the solid solution of carbides such as TiC in austenite may be reduced. For this reason, the minimum of the heating temperature before hot rolling becomes like this. Preferably it is 1200 degreeC or more, More preferably, it is 1250 degreeC or more. If the heating temperature before hot rolling is high, the cost increases. For this reason, the upper limit of the heating temperature before hot rolling is preferably 1350 ° C. or less, more preferably 1300 ° C. or less.

If the finish rolling temperature of hot rolling is low, rolling cannot be performed in the austenite single-phase region, deformation resistance during rolling is large, and operation may be difficult. For this reason, finish rolling temperature becomes like this. Preferably it is 850 degreeC or more, More preferably, it is 870 degreeC or more. If the finish rolling temperature is high, the crystal may be coarsened. For this reason, finish rolling temperature becomes like this. Preferably it is 980 degrees C or less, More preferably, it is 950 degrees C or less.

The average cooling rate from finish rolling to winding in hot rolling is preferably 10 ° C./second or more, more preferably 20 ° C./second or more in consideration of productivity. On the other hand, when the average cooling rate is high, the equipment cost becomes high. Therefore, it is preferably 100 ° C./second or less, and more preferably 50 ° C./second or less.

Next, preferable conditions for the process after hot rolling will be described.

Winding temperature after hot rolling: 550 ° C. or more When the winding temperature after hot rolling is less than 550 ° C., the strength of the hot rolled sheet increases and it is difficult to reduce the temperature by cold rolling. Therefore, the coiling temperature after hot rolling is 550 ° C. or higher, preferably 570 ° C. or higher, more preferably 600 ° C. or higher. On the other hand, if the coiling temperature after hot rolling becomes too high, the pickling property for removing the scale deteriorates. Therefore, the coiling temperature after hot rolling is preferably 800 ° C. or lower, more preferably 750 ° C. or lower.

Cold rolling rate: 20% or more, 60% or less The hot-rolled steel sheet is subjected to cold rolling after pickling to remove scale. When the cold rolling ratio of cold rolling is less than 20%, the plate thickness must be reduced in the hot rolling process in order to obtain a steel plate having a predetermined thickness. Become. This takes time for pickling and reduces productivity. Therefore, the lower limit of the cold rolling rate is preferably 20% or more, more preferably 25% or more. On the other hand, if the cold rolling rate exceeds 60%, a high capability of the cold rolling mill is required. Therefore, the upper limit of the cold rolling rate is preferably 60% or less, more preferably 55% or less, and still more preferably 50% or less.

In order to obtain the high-strength steel sheet of the present invention, the annealing step after cold rolling is (a) a soaking step for heating and holding, (b) a first cooling step performed following the soaking step, (c) A second cooling step performed subsequent to the first cooling step, (d) a third cooling step performed subsequent to the second cooling step, and (e) performed following the third cooling step. It is important to appropriately adjust each of the conditions (a) to (e). Specifically, after cold rolling, heating is carried out at an average heating rate of 1 to 20 ° C./second, and maintained at a range of Ac ₃ points to Ac ₃ points + 200 ° C. for 1 to 100 seconds; After the step, a first cooling step of cooling to a temperature range of 480 to 520 ° C. at an average cooling rate of 15 to 50 ° C./second, and subsequent to the first cooling step, 0.2 to 5.0 A second cooling step for cooling to a temperature range of 440 to 470 ° C. at an average cooling rate of ° C./second, and subsequent to the second cooling step, at an average cooling rate of 20 to 50 ° C./second, It is important to include a third cooling step for cooling to a temperature range of 310 ° C. and a fourth cooling step for cooling at an average cooling rate of 1 ° C./second or more subsequent to the third cooling step. .

FIG. 2 schematically shows the structures (a) to (e) of the annealing process of the present invention.

(A) Soaking step After the cold rolling, the steel is heated to a temperature of Ac ₃ point to Ac ₃ point + 200 ° C. (soaking temperature) and held for a predetermined time soaking (soaking step). When the soaking temperature is less than the Ac ₃ point, the value of X becomes high, and it becomes difficult to ensure the yield ratio. Therefore, the lower limit of the soaking temperature is preferably Ac ₃ point or higher, and more preferably Ac ₃ point + 25 ° C. or higher. On the other hand, if the soaking temperature exceeds the Ac ₃ point + 200 ° C., excessive energy is required for industrial production. For this reason, the upper limit is, Ac ₃ point + 200 ° C. or less, and more preferably not more than Ac ₃ point + 0.99 ° C..

The temperature at the Ac ₃ point is calculated based on the following formula (a). [% (Element name)] in the formula is the content (% by mass) of each element. This formula is described in “Leslie Steel Material Science” (published by Maruzen Co., Ltd., William C. Leslie, p. 273). In addition, the element which does not contain is calculated on the assumption that the content is 0%.
Ac ₃ = 910−203√ (% C) −15.2 (% Ni) +44.7 (% Si) +104 (% V) +31.5 (% Mo) +13.1 (% W) −30 (% Mn ) -11 (% Cr) -20 (% Cu) +700 (% P) +400 (% Al) +120 (% As) +400 (% Ti) (a)

The heating rate up to the soaking temperature is not particularly limited, but the average heating rate is preferably 1 ° C./second or more and 20 ° C./second or less. When the average heating rate after the cold rolling is less than 1 ° C./second, productivity is deteriorated. Therefore, the lower limit of the average heating rate is preferably 1 ° C./second or more, more preferably 3 ° C./second or more, and further preferably 5 ° C./second or more. On the other hand, when the average heating rate exceeds 20 ° C./second, the steel plate temperature becomes difficult to control, and the equipment cost increases. Therefore, the upper limit of the average heating rate is preferably 20 ° C./second or less, more preferably 18 ° C./second or less, and still more preferably 15 ° C./second or less.

It is preferable that the soaking temperature is soaked for 1 second to 100 seconds. When the soaking time is less than 1 second, the value of X becomes high and it becomes difficult to ensure the yield ratio. Therefore, the lower limit of the soaking time is preferably 1 second or longer, more preferably 10 seconds or longer. On the other hand, when the soaking time exceeds 100 seconds, productivity deteriorates. Therefore, the upper limit of the soaking time is preferably 100 seconds or less, more preferably 80 seconds or less.

(B) First Cooling Step After the soaking step, the average cooling rate CR1 from the soaking temperature to the cooling stop temperature T1 is preferably 15 ° C./second or more and 50 ° C./second or less (first Cooling process). When the average cooling rate CR1 in the first cooling step is less than 15 ° C./second, the productivity is deteriorated. Therefore, the lower limit of the average cooling rate CR1 is preferably 15 ° C./second or more, more preferably 20 ° C./second or more. On the other hand, when the average cooling rate CR1 exceeds 50 ° C./second, it becomes difficult to control the steel plate temperature, and the equipment cost increases. Therefore, the upper limit of the average cooling rate CR1 is preferably 50 ° C./second or less, more preferably 40 ° C./second or less, and further preferably 30 ° C./second or less.

It is preferable that the cooling stop temperature T1 in the first cooling step is 480 ° C. or more and 520 ° C. or less. When the cooling stop temperature T1 is less than 480 ° C., the value of ZY decreases and ductility decreases. Therefore, the lower limit of the cooling stop temperature T1 is preferably 480 ° C. or higher, more preferably 490 ° C. or higher. On the other hand, when the cooling stop temperature T1 exceeds 520 ° C., the value of X increases, the yield ratio decreases, and the bendability deteriorates. Therefore, the upper limit of the cooling stop temperature T1 is preferably 520 ° C. or less, more preferably 510 ° C. or less, and further preferably 500 ° C. or less.

(C) Second cooling step After the first cooling step, the average cooling rate CR2 from the cooling stop temperature T1 to the cooling stop temperature T2 is 0.2 ° C / second or more and 3.5 ° C / second or less. Preferably (second cooling step). When the average cooling rate CR2 in the second cooling step is less than 0.2 ° C./second, the productivity is deteriorated. Therefore, the lower limit of the average cooling rate CR2 is preferably 0.2 ° C./second or more, more preferably 1 ° C./second or more. On the other hand, when the average cooling rate CR2 exceeds 3.5 ° C./second, it becomes difficult to secure a time t _1-2 from the cooling stop temperature T1 described later to the cooling stop temperature T2, and the value of X Increases, yield ratio decreases, and bendability deteriorates. Therefore, the upper limit of the average cooling rate CR2 is preferably 3.5 ° C./second or less, more preferably 3 ° C./second or less, still more preferably 2.5 ° C./second or less.

The cooling stop temperature T2 in the second cooling step is preferably 440 ° C. or higher and 470 ° C. or lower. When the cooling stop temperature T2 is less than 440 ° C., the value of ZY decreases and ductility decreases. Therefore, the lower limit of the cooling stop temperature T2 is preferably 440 ° C. or higher, more preferably 450 ° C. or higher. On the other hand, even if the cooling stop temperature T2 exceeds 470 ° C., the ZY value decreases and ductility decreases. Therefore, the upper limit of the cooling stop temperature T2 is preferably 470 ° C. or less, more preferably 465 ° C. or less, and further preferably 460 ° C. or less.

In particular, when the cooling stop temperature T1 in the first cooling step and the cooling stop temperature T2 in the second cooling step are both less than 440 ° C., the yield ratio becomes too high because the value of Y−X increases. And ductility falls. Further, when the cooling stop temperature T1 in the first cooling step is less than 400 ° C. and the cooling stop temperature T2 in the second cooling step exceeds 450 ° C., the volume ratio of the retained austenite becomes high and the yield ratio decreases. To do.

Time t _1-2 from the cooling stop temperature T1 to the cooling stop temperature T2 is 20 seconds or more, it is preferably not more than 30 seconds. When the time _t1-2 is less than 20 seconds, the value of X is increased, the yield ratio is lowered, and the bendability is deteriorated. Therefore, the lower limit of the time t _1-2 is preferably 20 seconds or more, more preferably more than 22 seconds. On the other hand, if the time t _1-2 exceeds 30 seconds, the value of Y−X becomes low and the yield ratio decreases. Therefore, the upper limit of the time t _1-2 is preferably 30 seconds or less, and more preferably not more than 28 seconds.

(D) Third cooling step After the second cooling step, cooling to a cooling stop temperature T3 of 100 ° C. or higher and 310 ° C. or lower is performed at an average cooling rate CR3 of 20 ° C./second or higher and 50 ° C./second or lower. Is preferable (third cooling step). When the average cooling rate CR3 in the third cooling step is less than 20 ° C./second, the YX value is high and the ZY value is low. As a result, the yield ratio becomes too high and the ductility deteriorates. Therefore, the lower limit of the average cooling rate CR3 in the third cooling step is preferably 20 ° C./second or more, more preferably 25 ° C./second or more. On the other hand, when the average cooling rate CR3 exceeds 50 ° C./second, the equipment cost increases. Therefore, the upper limit of the average cooling rate CR3 is preferably 50 ° C./second or less, more preferably 40 ° C./second or less.

When the cooling stop temperature T3 in the third cooling step is less than 100 ° C., the equipment cost increases. Therefore, the lower limit of the cooling stop temperature T3 is preferably 100 ° C. or higher, more preferably 200 ° C. or higher. On the other hand, when the cooling stop temperature T3 exceeds 310 ° C., the YX value is high, the ZY value is low, the yield ratio is too high, and the ductility deteriorates. Therefore, the upper limit of the cooling stop temperature T3 is preferably 310 ° C. or lower, more preferably 300 ° C. or lower, and further preferably 290 ° C. or lower.

(E) Fourth Cooling Step After the third cooling step, it is preferable to cool at an average cooling rate CR4 of 1 ° C./second or more (fourth cooling step). The upper limit of the average cooling rate CR4 is not particularly limited, and is, for example, 10 ° C./second. Further, the cooling stop temperature T4 of the fourth cooling step is not particularly limited, and it may be normally cooled to room temperature.

The present invention includes a high-strength hot-dip galvanized steel sheet having a galvanized layer on the surface of a high-strength cold-rolled steel sheet.

The method for producing a high-strength hot-dip galvanized steel sheet according to the present invention includes a step of performing a galvanizing process after cooling to the cooling stop temperature T2 in the second cooling step. This galvanizing treatment is performed by immersing the cold-rolled steel sheet in a galvanizing bath at 440 ° C. or higher and 470 ° C. or lower for 1 second or more and 5 seconds or less after the second cooling step. By immersing the steel sheet in the galvanizing bath, a galvanized layer can be formed on the surface of the steel sheet. The galvanizing treatment is preferably performed before the third cooling step. The temperature of the galvanizing bath is preferably 455 ° C. or higher and 465 ° C. or lower.

Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples, and can be implemented with modifications within a range that can be adapted to the above and the gist. They are all included in the technical scope of the present invention.

An experimental slab was produced that satisfied the component composition shown in Table 1 below, with the balance being iron and inevitable impurities other than P, S, N, and O. Table 1 below shows temperatures of Ac ₃ point and Ac ₃ point + 200 ° C. calculated based on the above formula (a).

The obtained slab was heated to 1250 ° C. and hot-rolled to a thickness of 2.8 mm. The finish rolling temperature was 900 ° C., the average cooling rate from finish rolling to winding in hot rolling was 20 ° C./second, and the winding temperature was 600 ° C. Subsequently, the obtained hot-rolled steel sheet was pickled and then cold-rolled to a thickness of 1.4 mm. Thereafter, heat treatment (annealing) was performed under the conditions shown in FIG. In any of the heat treatments shown in Table 2, (a) the average heating rate until the soaking step was 8 ° C./sec, and (b) the average cooling rate CR1 in the first cooling step was 20 ° C./sec. . When the average cooling rate CR2 shown in Table 2 is negative, it means heating. Furthermore, temper rolling with an elongation of 0.1% was performed. In Table 1, “-” means no addition. In Table 1, P, S, N, and O are inevitable impurities as described above, and the values shown in the columns of P, S, N, and O mean the amounts inevitably included.

In addition, among the heat histories shown in Table 2, the example in which (c) the cooling stop temperature T2 is set to 460 ° C. in the second cooling step is the heat history when the hot dip galvanization is performed on the cold-rolled steel sheet. Mock up.

For each cold-rolled steel sheet thus obtained, IQ (image quality), volume fraction of retained austenite, and various properties were measured as follows.

[Volume ratio of retained austenite]
Residual austenite was cut out from a 1.4 mm × 20 mm × 20 mm test piece from the annealed cold rolled steel sheet. After grinding to 1/4 part of the thickness of the test piece, the amount of retained austenite was measured by X-ray diffractometry after chemical polishing (ISIJ Int. Vol. 33. (1933), No. 7, P.A. 776).

[IQ (Image Quality)]
Moreover, IQ (image quality) which is the definition of an EBSD pattern was measured as follows. First, a sample was prepared by mechanically polishing a cross section parallel to the rolling direction. Next, this sample was set in an OIM system manufactured by Texemra Laboratories Inc. and tilted by 70 °, and an area of 100 μm × 100 μm was taken as a measurement visual field, and acceleration voltage: 20 kV, 1 step: 185 μm at 0.25 μm EBSD measurement was performed. By this measurement, IQ of a body-centered cubic lattice (BCC) crystal including a body-centered tetragonal lattice (BCT) was measured. Here, the body-centered tetragonal lattice is one in which the C atoms are dissolved in a specific interstitial position in the body-centered cubic lattice so that the lattice extends in one direction. Since the body-centered tetragonal lattice has the same structure as the body-centered cubic lattice, the measurement of the body-centered cubic lattice includes the body-centered square lattice in this embodiment. In addition, a measurement location is W / 4 part when the length in the direction perpendicular to the rolling direction in a plane parallel to rolling is W, and t / 4 part when the plate thickness is t. One field of view was carried out.

The maximum value (IQmax) and minimum value (IQmin) of IQ at all measurement points were extracted, and the values of X, Y, and Z were calculated. Tables 3-1 and 3-2 below show X values, YX values, and ZY values.

[Tensile properties]
Regarding the tensile strength (TS), 0.2% yield strength (YS), and ductility (El), the direction perpendicular to the rolling direction in the plane parallel to the rolling surface of the cold rolling is the length of the test piece. , JIS No. 13B test piece (mark distance: 50 mm, parallel part width: 12.5 mm) was collected and tested according to JIS Z2241. The measurement results are shown in Table 3-1 and Table 3-2 below. The yield ratio (YR) was calculated based on the tensile strength (TS) and the 0.2% yield strength (YS). The results are shown in Tables 3-1 and 3-2 below.

[Bendability]
The bendability (R / t) was obtained by taking a 1.4 mm × 30 mm × 20 mm test piece from the cold-rolled steel sheet so that the direction perpendicular to the rolling direction on the rolling surface is the length of the test piece, and JIS Z2248. The test was conducted according to the V-block method. And the minimum bending radius R which a crack and a crack do not generate | occur | produce in a test piece was measured. The bending direction is the longitudinal direction of the test piece. The bending angle of the V block was 90 °. A value obtained by dividing R found by a bending test by a nominal plate thickness of 1.4 mm was defined as R / t. The measurement results are shown in Table 3-1 and Table 3-2 below.

The characteristics were evaluated according to the following criteria according to the tensile strength of the obtained steel sheet. These results are shown in Tables 3-1 and 3-2.
(I) Steel sheet having a tensile strength of 980 MPa or more and less than 1180 MPa The elongation (El) was 10.0% or more and the bendability (R / t) was 3 or less. The higher the El, the better (the upper limit is not particularly limited, but usually about 15%), and the smaller the R / t, the better (the lower limit is not particularly limited, but usually 0.5).
(Ii) Steel sheet having a tensile strength of 1180 MPa or more Elongation (El) was 9% or more and bendability (R / t) was 4 or less. The higher the El, the better (the upper limit is not particularly limited, but usually about 13%), and the smaller the R / t, the better (the lower limit is not particularly limited, but usually 1.0).

From Table 3-1 and Table 3-2 below, it can be considered as follows.

No. in Table 3-1 and Table 3-2 3, 4, 25, 37 to 43, 47, and 48 are steel types No. 1 in Table 1 that satisfy the composition of the present invention. 1 to 3, 11 to 17, 19 and 20 are examples of the present invention produced under the preferred heat treatment conditions of the present invention (heat treatment No. 3 in Table 2). Since these satisfy the requirements of the present invention, the tensile strength is 980 MPa or more, the yield ratio is 79% or more and less than 90%, and the ductility (El) and bendability (R / t) are excellent. What you are getting. In Table 2, heat treatment No. 3 has undergone a thermal history simulating hot dip galvanizing.

No. 3, 4, 25, 37 to 43, 47, and 48 confirm that the total area ratio of bainite and martensite is 95% or more with respect to the entire structure.

On the other hand, it was confirmed that the following characteristics that do not satisfy any of the requirements of the present invention do not provide the desired characteristics.

No. in Table 3-2 Nos. 28 to 34 and 45 are steel types No. 1 in Table 1 that do not satisfy the composition of the present invention. 4 to 10, 18 and heat treatment No. 1 in Table 2. 3 is an example manufactured under the heat treatment conditions of No. 3.

No. No. 28 has a small amount of C and does not satisfy the tensile strength (TS). Further, the value of Y−X did not satisfy the requirements of the present invention, and the yield ratio (YR) was lowered.

No. No. 29 had a large amount of C, a high volume fraction of retained austenite, a high X value, and a low YX value. As a result, in addition to the low yield ratio (YR), the bendability (R / t) is not satisfied.

No. No. 30 has a small amount of Mn and does not satisfy the tensile strength (TS). Moreover, the value of YX did not satisfy the requirements of the present invention, and the yield ratio (YR) was low.

No. No. 31 had a large amount of Mn, a high X value, and a low YX value. As a result, the yield ratio (YR) is low and the bendability is not satisfied.

No. No. 32 has a small amount of Ti and does not satisfy the tensile strength (TS). Moreover, the value of YX did not satisfy the requirements of the present invention, and the yield ratio (YR) was low.

No. No. 33 had a large amount of Ti, a high X value, and a low YX value. As a result, the yield ratio (YR) is low and the bendability (R / t) is not satisfied.

No. No. 34 has a small amount of B and does not satisfy the tensile strength (TS). Further, the value of Y−X did not satisfy the requirements of the present invention, and the yield ratio (YR) was lowered.

No. No. 45 had a large amount of Si, a high X value, and a low YX value. As a result, the yield ratio (YR) was low and the ductility (El) was also low.

No. in Table 3-1 and Table 3-2 1, 2, 5 to 24, 26, 27, 35, 36, 44, and 46 are steel types No. 1 in Table 1 that satisfy the composition of the present invention. 1 to 3 and 11 to 17 were used, and heat treatment Nos. This is an example manufactured under the heat treatment conditions of 1, 2, 4 to 21.

No. 1, 2, 7, 10 to 19, 35 and 36, the cooling stop temperature T1 and the cooling stop temperature T2 are low, the values of YX and ZY do not satisfy the requirements of the present invention, and The ratio (YR) was high and the ductility (El) was low.

No. _No. 5 had a long time t1-2, the _YX value did not satisfy the requirements of the present invention, the tensile strength (TS) was low, and the yield ratio (YR) was high.

No. 6, the cooling stop temperature T1 and the cooling stop temperature T2 are high, the values of X and ZY do not satisfy the requirements of the present invention, the yield ratio (YR) is low, the ductility (El) is low, The bendability (R / t) deteriorated.

No. In Nos. 8 and 9, the cooling stop temperature T1 was low, the ZY value did not satisfy the requirements of the present invention, and the ductility (El) deteriorated.

No. No. 20 had a low soaking temperature, the values of X and ZY did not satisfy the requirements of the present invention, the tensile strength (TS) was low, and the yield ratio (YR) was low.

No. No. 21, the cooling stop temperature T1 was high, the average cooling rate CR2 was large, the value of X did not satisfy the requirements of the present invention, and the yield ratio (YR) was low.

No. In No. 22, the cooling stop temperature T2 was low, the ZY value did not satisfy the requirements of the present invention, and the ductility (El) deteriorated.

No. No. 23 had a high cooling stop temperature T2, the ZY value did not satisfy the requirements of the present invention, and the ductility (El) deteriorated.

No. 24 has a large average cooling rate CR2, less time t _1-2, the value of X values and Z-Y does not meet the requirements of the present invention, the yield ratio (YR) is low and bendability ( R / t) deteriorated.

No. 26 has a long time t _1-2, the value of Y-X does not meet the requirements of the present invention, the yield ratio (YR) becomes lower.

No. In No. 27, the cooling stop temperature T3 was high, the values of YX and ZY did not satisfy the requirements of the present invention, the yield ratio (YR) was high, and the ductility (El) was low.

No. No. 44 had a low cooling stop temperature T1, a high volume ratio of retained austenite, and a low ZY value. As a result, the yield ratio (YR) was lowered.

No. 46 is an example that was not retained in the soaking step, the values of X and ZY did not satisfy the requirements of the present invention, the tensile strength (TS) was low, and the yield ratio (YR) was It became low.

This application is based on Japanese Patent Application No. 2016-068969 filed on Mar. 30, 2016 and Japanese Patent Application No. 2016-223879 filed on Nov. 17, 2016. The content of which is included in the present application.

Claims (5)

% By mass
C: 0.12 to 0.19%,
Si: more than 0%, 0.4% or less,
Mn: 1.80 to 2.45%,
P: more than 0%, 0.020% or less,
S: more than 0%, 0.0040% or less,
Al: 0.015 to 0.06%,
Ti: 0.010-0.035%, and B: 0.0025-0.0040%, the balance is iron and inevitable impurities,
X defined in (1) below is 8 or less,
The value Y−X of the difference between Y and X defined in (2) below is 30.0 or more and less than 45,
The difference Z-Y between Z and Y defined in (3) below is 48.0 or more,
The volume ratio of retained austenite with respect to the whole structure is 2% or less,
A high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more.
(1) X is a value obtained by dividing the total number of measurement points equal to or less than [0.40 × (IQmax−IQmin) + IQmin] by 100 and multiplying by 100.
(2) Y is a value obtained by dividing the total number of measurement points equal to or less than [0.75 × (IQmax−IQmin) + IQmin] by the total number of measurement points and multiplying by 100.
(3) Z is a value obtained by dividing the total number of measurement points equal to or less than [0.90 × (IQmax−IQmin) + IQmin] by the total number of measurement points and multiplying by 100.
IQ in (1) to (3) above is the sharpness of the electron beam backscatter diffraction pattern, IQmax is the maximum IQ value in all measurement points, and IQmin is the minimum IQ value in all measurement points. is there. Furthermore, in mass%,
Cu: more than 0%, 0.3% or less,
Ni: more than 0%, 0.3% or less,
Cr: more than 0%, 0.25% or less,
Mo: more than 0%, 0.1% or less,
V: more than 0%, 0.05% or less,
The high-strength cold-rolled steel sheet according to claim 1, comprising at least one selected from the group consisting of Nb: more than 0%, 0.08% or less and Ca: more than 0%, 0.005% or less. A high-strength hot-dip galvanized steel sheet having a galvanized layer on the surface of the high-strength cold-rolled steel sheet according to claim 1 or 2. A method for producing a high-strength cold-rolled steel sheet according to claim 1 or 2,
A cold-rolled steel sheet satisfying the content of each component according to claim 1 or 2 is heated at an average heating rate of 1 to 20 ° C./second, and in a range of Ac ₃ points to Ac ₃ points + 200 ° C. A soaking step for 1 to 100 seconds;
After the soaking step, a first cooling step of cooling the steel sheet to a temperature range of 480 to 520 ° C. at an average cooling rate of 15 to 50 ° C./second;
After the first cooling step, a second cooling step of cooling the steel sheet to a temperature range of 440 to 470 ° C. at an average cooling rate of 0.2 to 5.0 ° C./second;
After the second cooling step, a third cooling step of cooling the steel sheet to a temperature range of 100 to 310 ° C. at an average cooling rate of 20 to 50 ° C./second;
After the third cooling step, a fourth cooling step of cooling the steel sheet to room temperature at an average cooling rate of 1 ° C./second or more,
Temperature of the Ac ₃ The method for manufacturing a high-strength cold-rolled steel sheet is calculated based on the following formula (a).
Ac ₃ = 910−203√ (% C) −15.2 (% Ni) +44.7 (% Si) +104 (% V) +31.5 (% Mo) +13.1 (% W) −30 (% Mn ) -11 (% Cr) -20 (% Cu) +700 (% P) +400 (% Al) +120 (% As) +400 (% Ti) (a)
([% (Element name)] in the formula (a) is the content (% by mass) of each element.) A method for producing a high-strength hot-dip galvanized steel sheet according to claim 3,
A cold-rolled steel sheet satisfying the content of each component according to claim 1 or 2 is heated at an average heating rate of 1 to 20 ° C./second, and in a range of Ac ₃ points to Ac ₃ points + 200 ° C. A soaking step for 1 to 100 seconds;
After the soaking step, a first cooling step of cooling the steel sheet to a temperature range of 480 to 520 ° C. at an average cooling rate of 15 to 50 ° C./second;
After the first cooling step, a second cooling step of cooling the steel sheet to a temperature range of 440 to 470 ° C. at an average cooling rate of 0.2 to 5.0 ° C./second;
After the second cooling step, a step of forming a galvanized layer on the surface of the steel sheet by immersing the steel sheet in a galvanizing bath at 440 ° C. or higher and 470 ° C. or lower for 1 second or more and 5 seconds or less;
After the second cooling step, a third cooling step of cooling the steel sheet to a temperature range of 100 to 310 ° C. at an average cooling rate of 20 to 50 ° C./second;
After the third cooling step, a fourth cooling step of cooling the steel sheet to room temperature at an average cooling rate of 1 ° C./second or more,
Temperature of the Ac ₃ The method for manufacturing a high strength galvanized steel sheet is calculated based on the following formula (a).
Ac ₃ = 910−203√ (% C) −15.2 (% Ni) +44.7 (% Si) +104 (% V) +31.5 (% Mo) +13.1 (% W) −30 (% Mn ) -11 (% Cr) -20 (% Cu) +700 (% P) +400 (% Al) +120 (% As) +400 (% Ti) (a)

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