JP2013144830A - Hot-dip galvanized steel sheet and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-dip galvanized steel sheet having yield strength of at least 420 MPa and tensile strength of at least 780 MPa, and also excellent in elongation and hole expandability.SOLUTION: The steel sheet has: chemical composition containing, in mass%, 0.03-0.30% of C, 0.005-2.5% of Si, 1.9-3.5% of Mn, 0.1% or less of P, 0.01% or less of S, 0.001-1.5% of sol.Al, and 0.02% or less of N; and steel structure containing, in vol%, at least 3% of tempered martensite and at least 1% of retained austenite. The steel sheet is heated to at least 720°C, cooled to 450-600°C at a rate of 2-200°C/s, cooled to 200°C or less from the temperature of alloying treatment after hot dip galvanizing at a rate of 5°C/s or more, and further subjected to tempering treatment in a temperature range of 200-600°C for 1 s-10 min.

Description

  The present invention relates to an galvannealed steel sheet and a method for producing the same. The present invention particularly relates to a high-strength galvannealed steel sheet having both elongation and hole expansibility, which is suitable for use in press forming such as a car body of an automobile, and a method for producing the same. The alloyed hot-dip galvanized steel sheet of the present invention comprises an alloyed hot-dip galvanized hot-rolled steel sheet that has been hot-dip galvanized by heating the hot-rolled steel sheet, and a cold-rolled steel sheet obtained by cold rolling the hot-rolled steel sheet. Includes both alloyed hot-dip galvanized cold-rolled steel sheets that are hot-dip galvanized after annealing.

  In recent years, there has been a demand for improving the fuel efficiency of automobiles in order to protect the global environment, and there is an increasing need for high-strength steel sheets to reduce the weight of the vehicle body and ensure the safety of passengers. A steel plate used for a member for automobiles is not sufficient if it has only high strength, and high corrosion resistance and good press formability are required.

Here, a steel sheet using the TRIP effect of retained austenite is known as a hot-dip galvanized steel sheet having good elongation.
For example, JP-A-11-279691 discloses a high-tensile hot-dip galvanized steel sheet having an excellent strength-ductility balance. However, when hard martensite is contained for increasing the strength, there is a problem that the hole expandability deteriorates.

  As a technique for improving hole expansibility, methods utilizing tempered martensite are disclosed in JP-A-6-93340, JP-A-6-108152, JP-A-2005-256089, and JP-A-2009-19258. It is disclosed.

  In the method described in JP-A-6-93340, martensite is generated by rapid cooling before hot dipping, and then hot dipping is performed. Therefore, it contains a lot of hard martensite that has not been tempered, which is newly generated by cooling after plating, and has an adverse effect on hole expansibility, and residual austenite that contributes to elongation during alloying treatment decomposes. It has been difficult to achieve both high strength and stable expansion and hole expansion.

  JP-A-6-108152, JP-A-2005-256089, and JP-A-2009-19258 disclose methods of tempering after plating. However, since the end point temperature of cooling before tempering is not sufficiently low, hard martensite is newly generated in the cooling process after tempering, and the hole expandability is deteriorated. Moreover, since the retained austenite does not remain sufficiently, it is difficult to ensure sufficient elongation stably.

Journal of the Japan Welding Society 50 (1981), No. 1, p37-46

Japanese Patent Application Laid-Open No. 11-296991 Japanese Patent Laid-Open No. 6-93340 JP-A-6-108152 JP 2005-256089 A JP 2009-19258 A

  An object of this invention is to provide the galvannealed steel plate which can make elongation and hole expansibility both compatible stably, and its manufacturing method.

  As a result of diligent experiments on alloyed hot-dip galvanized steel sheets having sufficient elongation, the present inventors have determined that the form of martensite and retained austenite is MA (Martensite-Austenite constituent, also known as island martensite). It was found that high elongation characteristics can be obtained. Here, as described in Non-Patent Document 1, M-A is a concentration of C to untransformed austenite when steel is transformed into ferrite or bainite, and martensite is transformed during subsequent cooling. This is a region of a composite of martensite and retained austenite, and is scattered in islands in the matrix.

  However, excessively hard martensite degrades hole expansibility. Accordingly, the present inventors have further experimented to improve the hole expandability, and tempering MA at a relatively low temperature so that residual austenite remains, thereby improving the hole expandability while maintaining good elongation. I also found that it can be realized.

  The present invention has been completed based on the above knowledge, and provides an alloyed hot-dip galvanized steel sheet excellent in workability and a method for producing the same. In the present invention, “steel plate” means “steel strip”.

Here, the present invention is as follows.
An alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of the steel sheet,
The steel plate
In mass%, C: 0.03% to 0.30%, Si: 0.005% to 2.5%, Mn: 1.9% to 3.5%, P: 0.1% or less Chemical composition containing: S: 0.01% or less, sol.Al: 0.001% or more and 1.5% or less, and N: 0.02% or less,
A steel structure containing, by volume, 3% or more of tempered martensite and 1% or more of retained austenite,
The alloyed hot-dip galvanized steel sheet has a mechanical property that yield strength is 420 MPa or more and tensile strength is 780 MPa or more in a tensile test in a direction perpendicular to rolling.
An alloyed hot-dip galvanized steel sheet.

The chemical composition may further contain at least one element selected from the following (all are% by mass):
(A) One selected from the group consisting of Ti: 0.001% to 0.30%, Nb: 0.001% to 0.30% and V: 0.001% to 0.30% Or two or more;
(B) One or two selected from the group consisting of Cr: 0.001% to 2.0% and Mo: 0.001% to 2.0%;
(C) Cu: 0.001% or more and 2.0% or less and Ni: 0.001% or more and 2.0% or less selected from the group consisting of 2.0% or less;
(D) B: 0.0001% to 0.02%;
(E) Ca: 0.0001% or more and 0.01% or less and REM: 0.0001% or more and 0.1% or less selected from the group consisting of 0.1% or less; and (G) Bi: 0.0001 % To 0.05%.

  From another aspect, the present invention heats a hot-rolled steel plate or a cold-rolled steel plate having the chemical composition described above to 720 ° C. or higher, and reaches a temperature range of 450-600 ° C. at a rate of 2 to 200 ° C./second. Cool, apply hot dip galvanizing, then perform alloying treatment, cool from the alloying treatment temperature to 200 ° C. or less at an average cooling rate of 5 ° C./second or more, and further in a temperature range of 200 to 600 ° C. for 1 second. It is a manufacturing method of the galvannealed steel sheet characterized by performing the tempering process for 10 minutes or less above.

  The alloyed hot-dip galvanized steel sheet according to the present invention has both excellent extensibility and hole expansibility, and is excellent in formability, and is therefore optimal for structural parts of automobiles such as pillars.

  The alloyed hot-dip galvanized steel sheet and the manufacturing method thereof according to the present invention are as follows: chemical composition of base steel sheet, metal structure, alloy composition of alloyed hot-dip galvanized layer, mechanical characteristics of alloyed hot-dip galvanized steel sheet and production The method will be specifically described in order. In the following description, all the percentages relating to the chemical composition of the steel sheet are mass%.

(A) Chemical composition of steel sheet [C: 0.03% to 0.30%]
C is an effective component for obtaining a high tension. If the C content is less than 0.03%, the required high tension cannot be obtained. On the other hand, when C is contained exceeding 0.30%, the toughness and weldability of the steel sheet are lowered. Therefore, the C content was determined as described above. A preferable C content is 0.05% or more and 0.22% or less.

[Si: Si: 0.005% to 2.5%]
Si is an element that increases the strength of a steel sheet, and is an effective component for strengthening ferrite, homogenizing the structure, and improving workability. Moreover, it has the effect | action which suppresses precipitation of cementite and promotes the austenite residue. In order to obtain such an effect, it is necessary to contain 0.005% or more of Si. On the other hand, if more than 2.5% of Si is contained, the occurrence of non-plating in hot dipping becomes a problem. At the same time, the toughness and weldability of the steel sheet decrease. Therefore, the Si content was determined as described above. The Si content is preferably 0.05% or more and 2.0% or less, more preferably 1.5% or less.

[Mn: 1.9% to 3.5%]
Mn is an element essential for generating MA and obtaining strength and elongation. In order to obtain a desired effect, it is necessary to contain 1.9% or more of Mn. On the other hand, when Mn is contained exceeding 3.5%, the toughness and weldability of the steel sheet are lowered. Therefore, the Mn content is determined as described above. 2.2 to 3.5% is a preferable range. A more preferable Mn content is 2.2% or more and 3.1% or less.

[P: 0.1% or less]
P is an undesirable element that is contained as an impurity and deteriorates toughness. Therefore, the P content is set to 0.1% or less. P: 0.02% or less is a preferable range.

[S: S: 0.01% or less]
S is contained as an impurity, forms MnS in the steel, and deteriorates the hole expandability. Therefore, the S content is determined to be 0.01% or less. The S content is preferably 0.005% or less, more preferably 0.0012% or less.

[Sol.Al: 0.001% to 1.5%]
Al is added for deoxidation. Further, like Si, it is also effective for suppressing the precipitation of cementite and increasing the amount of retained austenite. Therefore, the lower limit of Al is set to 0.001%. On the other hand, when Al is contained exceeding 1.5%, inclusions increase and workability deteriorates. Therefore, the sol.Al content was determined as described above. A preferable range is 0.005% or more and 1.0% or less.

[N: 0.02% or less]
N is contained as an impurity, and nitrides are formed during continuous casting to cause cracks in the slab. Therefore, the content is preferably low. Therefore, the N content is determined to be 0.02% or less. Preferably it is 0.01% or less.

The following elements are optional elements that may optionally be included.
[Ti: 0.001% or more and 0.30% or less, Nb: 0.001% or more and 0.30% or less, and V: one or more selected from 0.001% or more and 0.30% or less]
Since Ti, Nb, and V have the effect of becoming precipitates and refining crystal grains, they may be contained in the base material steel sheet for the purpose of improving strength and toughness. However, if each content is less than 0.001%, the effect is not sufficient, and if each content exceeds 0.30%, the effect is saturated, which is disadvantageous in terms of cost. Therefore, each element has a content of 0.001% or more and 0.30% or less as described above. Ti and Nb have a remarkable austenite refining effect, and then promote C concentration to austenite due to the formation of ferrite, making it easier to produce MA. Accordingly, at least one of Ti and Nb is preferably contained in an amount of 0.01% or more, and more preferably 0.03% or more.

[Cr: One or two selected from 0.001% to 2.0% and Mo: 0.001% to 2.0%]
Cr and Mo, like Mn, have the function of promoting transformation strengthening by stabilizing austenite and are effective in increasing the strength of the steel sheet, so may be contained. However, when the content is less than 0.001%, the effect is not sufficient, and when the content exceeds 2.0%, the characteristic variation increases. Therefore, the Cr content and the Mo content are both 0.001% to 2.0%. A preferable Cr content is 0.1% or more and 1.0% or less, and a preferable Mo content is 0.05% or more and 0.5% or less.

[Cu: 0.001% or more and 2.0% or less and Ni: 0.001% or more and 2.0% or less selected from one or two]
Cu and Ni have a corrosion-inhibiting effect and have a function of concentrating on the surface to suppress the entry of hydrogen and to suppress delayed fracture. Therefore, Cu and Ni may be contained. However, if the content is less than 0.001%, the effect is not sufficient, and if the content exceeds 2.0%, the effect is saturated and disadvantageous in terms of cost. Therefore, the Cu content and the Ni content are both 0.001% or more and 2.0% or less. Preferably, both are 0.01% or more and 0.8% or less.

[B: 0.0001% to 0.02%]
B is an element that suppresses the nucleation from the grain boundary, enhances the hardenability, and contributes to high strength. Moreover, it can produce | generate M-A effectively and contributes to the improvement of elongation. Therefore, you may make it contain. However, if the content of B is less than 0.0001%, the effect is not sufficient, and even if the content exceeds 0.02%, the effect is saturated and disadvantageous in terms of cost. Therefore, the B content is determined to be 0.0001 to 0.02%.

[Ca: One or two selected from 0.0001% to 0.01% and REM: 0.0001% to 0.1%]
Ca and REM may be contained because they have the effect of improving the local ductility by spheroidizing the sulfide. However, when Ca is contained in an amount of less than 0.0001%, the effect is not sufficient, and even if Ca is contained in an amount exceeding 0.01%, the effect is saturated, which is disadvantageous in terms of cost. Therefore, the Ca content is as described above. Moreover, the effect of REM is not sufficient if the content is less than 0.0001%, and if the content exceeds 0.1%, the effect is saturated and disadvantageous in terms of cost. Therefore, the REM content is as described above.

  Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid. In the case of lanthanoid, it is added industrially in the form of misch metal. In the present invention, the content of REM refers to the total content of these elements.

[Bi: 0.0001% to 0.05%]
When Mn and the like are segregated microscopically, a band structure with non-uniform hardness develops and the workability decreases. Bi has the function of concentrating on the solidification interface to narrow the dendrite interval and reduce the solidification segregation, so it may be contained. However, if the content is less than 0.0001%, the effect is insufficient, and if more than 0.05% is contained, surface quality is deteriorated. Therefore, the content is determined as described above. It was. A preferable range of Bi content is 0.0003 to 0.01%, and a more preferable range is 0.0003 to 0.0050%.

(B) Metal structure of steel sheet As described above, in order to obtain an alloyed hot-dip galvanized steel sheet having good workability and strength of 780 MPa or more, MA is baked at a relatively low temperature so that residual austenite remains. It is effective to use the returned organization. Thereby, the hole expandability is improved while maintaining the good elongation provided by MA.

  Therefore, the metal structure of the galvannealed steel sheet according to the present invention needs to contain 3% or more of tempered martensite and 1% or more of retained austenite by volume ratio. In order to obtain higher strength, the tempered martensite is preferably 5% or more. The retained austenite is preferably 2% or more. The volume ratio of each phase related to the metal structure is the area ratio of each phase measured at the position of the plate thickness ¼.

  The balance of the metal structure is preferably ferrite and / or bainite. Moreover, it is preferable not to contain 5 [mu] m or more of cementite inside the grains for the purpose of promoting the formation of MA. As described above, both elongation and hole expansibility can be achieved by tempering MA at a relatively low temperature so that residual austenite remains. From the viewpoint of hole expandability, it is preferable that all martensite contained is tempered.

(C) Alloyed hot-dip galvanized layer When the Fe concentration of the alloyed hot-dip galvanized layer is less than 7% by mass, weldability and slidability tend to be insufficient. Therefore, the Fe concentration of the alloyed hot-dip galvanized layer is preferably 7% by mass or more. The upper limit of the Fe concentration of the alloyed hot-dip galvanized layer is preferably 20% or less, more preferably 15% or less from the viewpoint of powdering resistance. The Fe content of the plating layer is adjusted by the conditions of heat treatment (alloying treatment) after hot dipping.

(D) Mechanical properties The alloyed hot-dip galvanized steel sheet according to the present invention has mechanical properties such that the yield strength (YS) is 420 MPa or more and the tensile strength (TS) is 780 MPa or more in a tensile test in the direction perpendicular to the rolling direction. In this tensile test, if the yield strength is less than 420 MPa and / or the tensile strength is less than 780 MPa, it is difficult to ensure sufficient impact absorption in the case of an automobile part. The yield strength is preferably 500 MPa or more, more preferably 600 MPa or more, and the tensile strength is preferably 800 MPa or more, more preferably 900 MPa or more. In consideration of application to automobile parts that require formability, the total elongation is preferably 12% or more and the hole expansion ratio is preferably 35% or more.

(C) Production method In order to produce MA, it is necessary that a C concentration gradient exists in the retained austenite during the production process. A portion with a high C concentration remains as retained austenite, and a portion with a low C concentration is transformed into martensite, resulting in M-A. Since M-A contains retained austenite and martensite is hard, strain concentrates on a relatively soft matrix, and high strength and good elongation can be obtained. However, since excessively hard martensite is disadvantageous for hole expansibility, alloyed hot dip galvanizing excellent in elongation and hole expansibility can be produced by appropriately tempering so that residual austenite remains. In addition, by performing tempering after plating, it is possible to suppress problems of surface oxidation and remaining untempered martensite.

  Specifically, a hot-rolled steel plate or a cold-rolled steel plate containing 1.9% or more of Mn is used as a plating base material, and after heating the steel plate to a temperature of 720 ° C. or higher, 2 to 200 ° C./second. Is cooled to a temperature range of 450 to 600 ° C. at a rate of, subjected to hot dip galvanization, and then subjected to alloying treatment, and is cooled from the alloying treatment temperature to 200 ° C. or less at an average cooling rate of 5 ° C./second or more The alloyed hot-dip galvanized steel sheet containing MA thus manufactured is further tempered at a temperature range of 200 to 600 ° C. for 1 second to 10 minutes.

  1.9% or more of Mn is necessary for suppressing precipitation of cementite and obtaining MA. As described above, the base steel plate may be either a hot-rolled steel plate or a cold-rolled steel plate, and there are no particular restrictions on the conditions for hot rolling or cold rolling, but preferred hot rolling conditions include a rolling finish temperature of 800 ° C. or higher. The winding temperature is in the range of 350 ° C. or higher and 750 ° C. or lower in the range of 1100 ° C. or lower.

  The annealing temperature at the time of hot dipping is 720 ° C. or higher in order to generate austenite during heating. In order to obtain a uniform structure advantageous for hole expansibility, it is preferable to heat to an austenite single phase region. The upper limit of the heating temperature is not particularly specified, but is preferably set to 1000 ° C. or less from the viewpoint of suppressing the coarsening of the particle size and ensuring good toughness.

  When the cooling rate after heating is less than 2 ° C./second, precipitation of cementite occurs, and when the cooling rate exceeds 200 ° C./second, the concentration gradient of C to austenite is insufficient and MA does not occur. This cooling rate is preferably 5 to 50 ° C./second, more preferably 8 to 30 ° C./second.

  The reason why the cooling stop temperature of the cooling is set to a temperature range of 450 to 600 ° C. is that it is necessary to provide a gradient so that MA generates the C concentration in austenite. This cooling stop temperature is preferably 480 to 570 ° C.

  Thereafter, isothermal holding and cooling were performed as necessary, hot dip galvanizing was performed, and further, alloying treatment was performed by heating to a temperature necessary for alloying the plating. Cool the galvannealed steel sheet.

The bath temperature and bath composition of hot dip plating may be general, and there is no particular limitation. The plating adhesion amount is not particularly limited, and may be within a normal range. For example, the amount of adhesion per side is in the range of ²⁰ to 80 g / m2. The alloying treatment is preferably performed under conditions such that the Fe concentration in the plating layer is 7% by mass or more. Necessary conditions vary depending on the plating adhesion amount, but are performed, for example, by heating at a temperature of 490 to 560 ° C. for 5 to 60 seconds.

  In cooling from the alloying treatment temperature after the alloying treatment, it is important to cool to 200 ° C. or less at 5 ° C./second or more. This cooling promotes the production of MA and suppresses the formation of martensite after subsequent tempering. A preferable cooling end temperature is 100 ° C. or lower. A preferable cooling rate is 10 ° C./second or more. The upper limit of the cooling rate is not particularly defined, but is preferably 500 ° C./second or less from the viewpoint of economy.

  Tempering is performed in a temperature range of 200 to 600 ° C. for 1 second to 10 minutes in order to properly temper the MA martensite. If the temperature is low or the time is short, the martensite is hard and inferior in hole expansibility. On the other hand, when the temperature is high or the time is long, the retained austenite decomposes or the martensite becomes too soft, and the elongation deteriorates. In the tempering, it is necessary to keep the maximum temperature within a range of 200 to 600 ° C. for 1 second to 5 minutes. However, it is preferable to keep the temperature isothermal at the highest temperature from the viewpoint of stabilizing the characteristics. A preferred maximum plate temperature is in the range of 250 to 500 ° C. The MA martensite is preferably tempered.

  After this tempering, there is no problem even if it is processed with a skin pass or leveler for flatness correction, and a film having oiling or lubricating action may be applied.

  Steel having the chemical composition shown in Table 1 was melted in an experimental furnace to produce a slab having a thickness of 40 mm. The slab was hot-rolled to the indicated thickness at the finishing temperature shown in Table 2. After hot rolling, water spray cooling of about 30 ° C./second was performed, and a hot-rolled steel sheet was manufactured at the indicated winding temperature. Winding was simulated by charging in a furnace after water spray cooling to the winding temperature, holding at the winding temperature for 60 minutes, and then cooling the furnace to 100 ° C. or less at a cooling rate of 20 ° C./hour. The obtained hot-rolled steel sheet was scale-removed by pickling and partly cold-rolled as shown in Table 2.

  From the hot-rolled steel sheet or the cold-rolled steel sheet thus obtained, a heat treatment test material was collected and subjected to hot-dip galvanization and alloying treatment according to the heat treatment conditions in the alloyed hot-dip galvanizing treatment shown in Table 3. That is, first, the test material was heated to the indicated annealing temperature and held for 45 seconds for annealing, and then cooled to the primary cooling stop temperature at the indicated primary cooling rate. Thereafter, for some materials, isothermal holding is performed under the indicated conditions, followed by cooling to 460 ° C., which is a hot dipping bath temperature, at 5 ° C./s, followed by hot dip galvanizing, and then at 510 ° C. The alloying heat treatment was performed for 20 seconds, and cooling after the alloying treatment was started at a cooling rate of 15 ° C./second. The Fe concentration in the galvannealed layer thus obtained was 8 to 12% by mass. This cooling was performed up to the indicated temperature in Test Nos. 23 to 25, and to room temperature otherwise. Moreover, as a result of collecting thermal expansion curves with the same heat treatment pattern as in Table 3 and investigating the transformation behavior, the martensite formation temperature was 170 to 270 ° C. for Test Nos. 23 to 25, and the others were martensite at room temperature. The site transformation was complete.

  The alloyed hot-dip galvanized steel sheet thus cooled was subjected to tempering that maintained the temperature under the tempering conditions shown in Table 3 (temperature is the highest temperature) immediately after cooling, except for test No. 28. The rate of temperature increase to the tempering temperature was 20 ° C./second. By this tempering, all martensite generated before tempering is tempered. Martensite was tempered by confirming the presence of carbides in martensite in SEM observation after nital corrosion.

The following measurements were performed on the tempered galvannealed steel sheet obtained. These measurement results are summarized in Table 4.
Metal structure: FS Lepera: Journal of Metals 32, No. 3, (1980) 38-39, corrodes the cross section in the rolling direction to reveal martensite and austenite and It observed with the optical microscope of 1000 times magnification in the 1/4 position, and the total of the area ratio of a martensite and an austenite was measured by image processing from the structure | tissue photograph. The value obtained by subtracting the residual austenite area ratio measured by X-ray at the 1/4 position of the plate thickness was taken as the martensite volume ratio.

  Tensile test: JIS No. 5 tensile test specimens were taken from various heat-treated materials so that the direction perpendicular to the rolling direction was the tensile direction, and yield strength (YS), tensile strength (TS) and total elongation (El) were measured. It was measured.

  Hole expansion test: In the hole expansion test conducted in accordance with Japan Iron and Steel Federation Standard JFST1001, the hole expansion ratio (%) is the ratio of the amount of increase in the hole diameter until the crack that penetrates the plate thickness occurs around the hole and the initial hole diameter. )

  The alloyed hot-dip galvanized steel sheets of Test Nos. 1 to 16, 18 to 24, and 30 to 33, which are invention examples according to the present invention, have a high strength of 780 MPa or more in tensile strength, good elongation, and a hole expansion rate of 40%. The above good hole expansibility was shown.

  On the other hand, Test No. 17 having a low Mn content had low strength, and both elongation and hole expansibility were low. In Test No. 25, in which the temperature before tempering was as high as 300 ° C., harder martensite was generated after tempering, so the hole expandability was low. In test No. 26 where the annealing temperature was low, an unrecrystallized structure remained, no martensite was formed, and elongation and hole expansibility were low. Test No. 27, which had a slow primary cooling rate, had low elongation because martensite was not sufficiently generated. Test No. 28, which had a low tempering temperature, had insufficient martensite softening and low hole expansibility, and Test Nos. 29 and 34, which had a high tempering temperature, had too much martensite to become soft. Low, low elongation, and insufficient hole expansibility.

Claims (8)

An alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of the steel sheet,
The steel plate
In mass%, C: 0.03% to 0.30%, Si: 0.005% to 2.5%, Mn: 1.9% to 3.5%, P: 0.1% or less Chemical composition containing: S: 0.01% or less, sol.Al: 0.001% or more and 1.5% or less, and N: 0.02% or less;
Containing 3% or more of tempered martensite and 1% or more of retained austenite in volume%,
The alloyed hot-dip galvanized steel sheet has a mechanical property that yield strength is 420 MPa or more and tensile strength is 780 MPa or more in a tensile test in a direction perpendicular to rolling.
An alloyed hot-dip galvanized steel sheet.   The chemical composition is a group consisting of Ti: 0.001% to 0.30%, Nb: 0.001% to 0.30% and V: 0.001% to 0.30% by mass%. The alloyed hot-dip galvanized steel sheet according to claim 1, further comprising one or more selected from the group consisting of:   The chemical composition further contains, in mass%, one or two selected from the group consisting of Cr: 0.001% to 2.0% and Mo: 0.001% to 2.0%. The alloyed hot-dip galvanized steel sheet according to claim 1 or 2.   The chemical composition contains one or two selected from the group consisting of Cu: 0.001% to 2.0% and Ni: 0.001% to 2.0% by mass%. The alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 3.   The alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 4, wherein the chemical composition further contains B: 0.0001% to 0.02% by mass.   The chemical composition further contains one or two selected from the group consisting of Ca: 0.0001% to 0.01% and REM: 0.0001% to 0.1% by mass%. The alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 5.   The alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 6, wherein the chemical composition further contains, by mass%, Bi: 0.0001% to 0.05%.   A hot-rolled steel plate or a cold-rolled steel plate having the chemical composition according to any one of claims 1 to 7 is heated to 720 ° C or higher and cooled to a temperature range of 450 to 600 ° C at a rate of 2 to 200 ° C / second. Then, hot dip galvanizing is performed, followed by alloying treatment, cooling from the alloying treatment temperature to 200 ° C. or less at an average cooling rate of 5 ° C./second or more, and further for 1 second or more in a temperature range of 200 to 600 ° C. A method for producing an galvannealed steel sheet, characterized by performing a tempering treatment for 10 minutes or less.