[0001]The present invention relates to a galvannealed steel sheet and a manufacturing method thereof.
Background technique
[0002]
　Recently, concern for the global environment, for the purpose of improving fuel economy of automobiles, weight reduction of the vehicle body is required. In addition, it is necessary to ensure the safety of the passengers, there is an increasing need for high-strength steel sheet. Recently, also desired steel sheet having a tensile strength of at least 1470 MPa. However, the steel sheet subjected to automobile members is insufficient only a high strength, good press formability is required.
[0003]
　In order to improve the press formability of the steel sheet, it is necessary to improve the ductility of the steel sheet. However, a conflicting elements between tensile strength and ductility, to improve the tensile strength and fine ductility both at the same time is usually difficult. Furthermore, the automobile member, together with corrosion resistance, continuous dotting property of the spot weld is required. Therefore, by using the galvannealed steel sheet, it is necessary to achieve a high strength and good press formability.
[0004]
　Yet high strength, as a steel sheet having a good uniform elongation, a steel sheet utilizing TRIP effect of the retained austenite is known. For example, Patent Document 1, a manufacturing method thereof and good workability high strength galvannealed steel sheet is disclosed.
[0005]
　Further, Patent Document 2, with excellent galvannealed layer on the plating wettability and coating adhesion, galvannealed steel sheets of high strength having the above tensile strength 590 MPa, and manufacturing method thereof It has been disclosed.
[0006]
　Further, Patent Document 3, excellent processability and high strength galvannealed steel sheet manufacturing method thereof having corrosion resistance is disclosed. Then, Patent Document 4, which has an ultra-high strength by heat treatment, for molded part steel, molded member and a manufacturing method thereof are disclosed having excellent ductility.
CITATION
Patent Literature
[0007]
Patent Document 1: JP-A-11-279691 Patent Publication
Patent Document 2: WO 2014/073520
Patent Document 3: JP 2011-168816 Patent Publication
Patent Document 4: JP-T 2014-508854 Patent Publication
Summary of the Invention
Problems that the Invention is to Solve
[0008]
　Patent Document 1, C, Si, by optimizing the ratio of Mn, reheat even were conducted, galvannealed steel sheet having excellent press workability as well as a high strength for the alloying treatment it is described that obtained by.
[0009]
　Incidentally, the TRIP steel sheet, in order to remain the austenite, it is necessary to include Si. However, the Si plating resistance, in particular, to increase inhibits alloying of galvanized. Since assuming the intensity levels in the technique described in Patent Document 1 is less than 880 MPa, C content is less and less 0.15 mass%. When C content is high, since the alloying is further suppressed, while promoting the alloying, it is difficult to achieve both the balance between strength and elongation.
[0010]
　In TRIP steel sheet, as a technique to promote alloying, as described in Patent Document 2, a method of controlling the atmosphere during annealing before plating are known. This technique, oxides of Si, since it is formed on its internal surface of the steel sheet, the alloying of zinc and iron is likely to proceed.
[0011]
　Here, the technique of Patent Document 2, although the C content is related to relatively low steel of less than 0.3 wt%, in order to achieve further higher strength is 0.3 mass content of C there needs to be greater than or equal to%. However, progress when the C content more than 0.3 mass%, since the alloying is less likely to proceed, be employed the technique described in Patent Document 2, as well as securing retained austenite, sufficient alloying it is difficult to.
[0012]
　On the other hand, Patent Document 3, in TRIP steel sheet during reheating due to alloying treatment, a method of suppressing decomposition of residual austenite is described. According to the method described in Patent Document 3, by lowering the C content in austenite prior to alloying treatment, lowering the driving force carbide from the austenite is generated, it is possible to secure the austenite.
[0013]
　However, in the case of the method of Patent Document 3, C content in the residual austenite is lowered, since the austenite is unstable, tend to local elongation is lowered. In particular, in the above high-strength steel 1470 MPa, because it is preferable to increase the local elongation little addition to uniform elongation, it is impossible to employ a method described in Patent Document 3.
[0014]
　In the method described in Patent Document 4, after heating, subjected to press the steel, then heat treated, mainly a tempered martensite to produce a TRIP steel containing residual austenite, to achieve high strength and high ductility ing.
[0015]
　However, in the method of Patent Document 4, since zinc during heating to melt, can not be carried out in a continuous annealing line, it is impossible to mass production. Further, in the method of Patent Document 4, since the tempering temperature is lower than 480 ° C., alloying of the plating is estimated that almost occur.
[0016]
　As described above, in the conventional techniques described above, while progress thoroughly alloyed, about getting galvannealed steel sheet having a high strength and ductility, room for improvement is left.
[0017]
　The present invention has been made to solve the above problems, to provide a uniform deformation property (uniform elongation) and local deformability excellent galvannealed steel sheet (local elongation) and its manufacturing method and an object thereof.
Means for Solving the Problems
[0018]
　The present invention has been made to solve the above problems, and the gist of the galvannealed steel sheet and a manufacturing method thereof below.
[0019]
　(1) A galvannealed steel sheet comprising a galvannealed layer on a surface of the steel sheet,
　the chemical composition of the steel sheet contains, by
　mass%, C: 0.25 ~
　0.70%, Si: 0
　~
　2.50% .25, Mn: 1.00 ~ 5.00%, Al: 0.005 ~
　3.50%, P: 0.15% or
　less, S: 0.03% or
　less, N: 0. 02% or
　less, O: 0.01% or
　less,
　Ti:
　0 ～ 0.50 Pasento, Nb: 0 ～ 0.50
　Pasento, V: 0 ～ 0.50 Pasento, Cr: 0 ～ 1.50
　Pasento, Mo:
　~
　1.50%
　0,
　0 ~ 5.00%, B: 0 ~ 0.003%,
　Ca: 0 ~ 0.05%, REM: 0 ~ 0.05
　%,
　Mg:
　0 ~ 0.05%,
　W: 0 ~ 0.50%, Zr: 0 ~
　0.05%, Sb: 0 ~ 0.50%, Sn: 0 ~
　0.50%, As: 0 - 0.0
　%,
　Te:
　0 ~ 0.05%, Y: 0 ~ 0.20%,
　Hf: 0 ~ 0.20%, Co: 0 ~ 1.00%,
　the balance is Fe and impurities,
　the plate of said steel plate metal structure in the thickness 1/4 position, by volume%,
　retained austenite: 10.0 to 60.0%,
　high-temperature tempered martensite: 5.0% or more,
　low-temperature tempered martensite: 5.0% or more,
　fresh martensite site: 10.0% or less,
　ferrite: 0 to 15.0%,
　perlite 0 to 10.0%,
　the balance: a bainite
　total volume ratio of the high-temperature tempered martensite and the low temperature tempered martensite and bainite 30 and at 2.0% or more,
　a tensile strength of more than 1470 MPa,
　the tensile strength and the product of the uniform elongation is 13000 mPa% or more,
　the product of the tensile strength and local elongation is not less than 5000 MPa%,
　alloyed hot Lead-plated steel sheet.
[0020]
　(2) the chemical composition, in
　mass%, Si + Al: 0.80% or more,
　galvannealed steel sheet according to (1).
[0021]
　(3) the chemical composition, by
　mass%,
　Ti: 0.005
　~ 0.50%, Nb: 0.005 ~ 0.50%, V: 0.005
　~ 0.50%, Cr: 0.01
　1.50
　Pasento ～,
　Mo: 0.01 ～ 1.50 Pasento, Cu: 0.01 ～ 5.00
　Pasento, Ni: 0.01 ～ 5.00 Pasento, B: 0.0001 ～ 0.003 Pasento, 　Ca:
　　0.0001 ~ 0.05%, REM: 0.0005 ~ 　0.05%, Mg: 0.0001 ~ 0.05%, W: 0.005 　~ 0.50%, Zr: 0.005 ~ 　% 　0.05, 　Sb: 0.005 ~ 0.50%, Sn: 0.005 ~ 　0.50%, As: 0.005 ~ 0.05%, Te: 0.001 ~ 　0.05%, Y : 　0.001 ~ 0.20%, Hf: 0.001 ~ 0.20%, 　and, Co: 0.001 ~ 1.00%, 　is selected from 1 Containing more, 　the (1) or galvannealed steel sheet according to (2).

[0022]
　(4) the amount of C contained in the residual austenite is greater than or equal to 0.85 wt%,
　galvannealed steel sheet according to any one of (1) to (3).
[0023]
　(5) Fe content in the galvannealed layer is 3.0 to 20.0 mass%,
　galvannealed steel sheet according to any one of (1) to (4) .
[0024]
　(6) to the steel sheet surface layer comprises a surface layer soft layer having 0.9 times or less of the hardness of the average hardness in the region from the sheet thickness 1/4 position to a thickness of 1/2 position,
　the surface soft layer the thickness from the interface between the steel sheet and the galvannealed layer is 10μm greater,
　galvannealed steel sheet according to any one of (1) to (5).
[0025]
　(7) to the steel sheet surface layer comprises a surface layer soft layer having 0.9 times or less of the hardness of the average hardness in the region from the sheet thickness 1/4 position to a thickness of 1/2 position,
　the surface soft layer the thickness from the interface between the steel sheet and the galvannealed layer has a 10μm or less,
　the ratio of fatigue limit for the tensile strength of the steel sheet is 0.30 or more,
　from the (1) ( galvannealed steel sheet according to any one of up to 5).
[0026]
　(8) A method for manufacturing a galvannealed steel sheet according to any one of (1) to (5),
　according to any one of the up to (a) above (1) to (3) melting step of melting a steel ingot or slab having a chemical composition,
　(b) heating the steel ingot or slab subjected to hot rolling, hot rolling step of hot-rolled steel sheet,
　(c) said hot-rolled steel sheet a first cooling step of cooling,
　the winding step of winding (d) is the hot-rolled steel sheet,
　rewind the (e) the hot-rolled steel sheet, after pickling, subjected to cold rolling, the cold-rolled steel sheet cold rolling
　step, (f) the cold-rolled steel sheet to Ac 1 annealing step of holding 5s or more in a temperature range of point-920 °
　C., at (g) the cold-rolled steel sheet of 1 ° C. / s or more average cooling rate from 100 to second cooling step of cooling to a temperature range of 600 ° C.,
　or cooling (h) the cold-rolled steel sheet to hot-dip galvanizing bath temperature Pretreatment step of heating is
　by immersion in molten zinc plating bath: (i) the cold-rolled steel sheet, galvanized steel sheet, plating process to galvanized steel sheet,
　the (j) the hot-dip galvanized steel sheet 480 is heated to ~ 600 ° C., galvanizing alloyed, the alloying step of the galvannealed steel sheet,
　in (k) the galvannealed steel sheet 1 ° C. / s or more average cooling rate, 80 the third step of cooling to a temperature range of ~ 300 ° C.,
　and, tempering step, which holds 1s or 48h or less in a temperature range of 100 ~ 450 ° C. the (l) the galvannealed steel sheet
　wherein the above ( the steps from a) to (l) carried out sequentially, the production method of the galvannealed steel sheet.
[0027]
　(9) above a method of manufacturing a galvannealed steel sheet according to (6),
　(a) a steel ingot or slab having a chemical composition according to any one of (1) to (3) melting step of melting a,
　(b) subjecting the steel ingot or slab is heated to hot rolling, hot rolling step of hot-rolled steel sheet,
　a first cooling step of cooling the (c) said hot-rolled steel sheet,
　(d) winding step of winding the hot-rolled steel sheet,
　rewind the (e) the hot-rolled steel sheet, after pickling, subjected to cold rolling, cold rolling step of cold-rolled steel sheet,
　(f) the the cold-rolled steel sheet, in an atmosphere of higher dew point -25 ° C., Ac 1 annealing step of holding 5s or more in a temperature range of point ~ 920 ° C.,
　at (g) the cold-rolled steel sheet of 1 ° C. / s or more average cooling rate the second step of cooling to a temperature range of ~ 600 ° C. 100,
　(h) the cold-rolled steel sheet to hot-dip galvanizing bath Process prior to cooling or heating up time,
　(i) the cold-rolled steel sheet is immersed in molten zinc plating bath, galvanized steel sheet, plating process to galvanized steel sheet,
　(j) the molten zinc the coated steel sheet was heated to 480 ~ 600 ° C., galvanizing alloyed, the alloying step of the galvannealed steel sheet,
　(k) an average cooling rate of 1 ° C. / s or higher the galvannealed steel sheet in a third cooling step of cooling to a temperature range of 80 ~ 300 ° C.,
　and, tempering step, which holds 1s or 48h or less in a temperature range of 100 ~ 450 ° C. the (l) the galvannealed steel sheet
　comprises , a step from the (a) to (l) in this order, the manufacturing method of the galvannealed steel sheet.
[0028]
　(10) A method of manufacturing a galvannealed steel sheet according to the above (7),
　(a) a steel ingot or slab having a chemical composition according to any one of (1) to (3) melting step of melting a,
　(b) subjecting the steel ingot or slab is heated to hot rolling, hot rolling step of hot-rolled steel sheet,
　a first cooling step of cooling the (c) said hot-rolled steel sheet,
　(d) winding step of winding the hot-rolled steel sheet,
　rewind the (e) the hot-rolled steel sheet, after pickling, subjected to cold rolling, cold rolling step of cold-rolled steel sheet,
　(f) the the cold-rolled steel sheet, in an atmosphere of a dew point of -15 ° C. or less, Ac 1 annealing step of holding 5s or more in a temperature range of point ~ 920 ° C.,
　at (g) the cold-rolled steel sheet of 1 ° C. / s or more average cooling rate the second step of cooling to a temperature range of 100 ~ 600 ° C.,
　molten zinc plating (h) the cold-rolled steel sheet Pretreatment step of cooling or heating to a temperature,
　(i) the cold-rolled steel sheet is immersed in molten zinc plating bath, galvanized steel sheet, plating process to galvanized steel sheet,
　(j) the molten zinc the coated steel sheet was heated to 480 ~ 600 ° C., galvanizing alloyed, the alloying step of the galvannealed steel sheet,
　(k) an average cooling rate of 1 ° C. / s or higher the galvannealed steel sheet in a third cooling step of cooling to a temperature range of 80 ~ 300 ° C.,
　and, tempering step, which holds 1s or 48h or less in a temperature range of 100 ~ 450 ° C. the (l) the galvannealed steel sheet
　comprises , a step from the (a) to (l) in this order, the manufacturing method of the galvannealed steel sheet.
Effect of the invention
[0029]
　According to the present invention, while having a high tensile strength of over 1470 MPa, it is possible to obtain a uniform deformation property (uniform elongation) and local deformability excellent galvannealed steel sheet (local elongation).
DESCRIPTION OF THE INVENTION
[0030]
　The present inventors have, while sufficiently advanced alloying, extensive studies were carried out results on how to obtain excellent galvannealed steel sheet ductility has high strength, has led to obtain the following findings.
[0031]
　As described above, since the progress of securing sufficient alloying of the residual austenite is to contradictory, it is difficult to manufacture a galvannealed steel having both a high strength and ductility. The present inventors have found that the method of obtaining a galvannealed steel sheet having a sufficient elongation and 1470MPa or more tensile strength was performed fundamentally reviewed.
[0032]
　As a result, the C content in the steel sheet 0.3% by mass or more, the residual austenite is 10 vol% or more, a tensile strength of more than 1470 MPa, the product of the tensile strength and the uniform elongation is 13000 mPa% or more, tensile strength and product of local elongation is successful in obtaining a high strength and high ductility galvannealed steel sheet becomes more than 5000 MPa%.
[0033]
　For the method will be explained. By convention, the plating is performed after yelling build a metal structure of the steel plate. Therefore, the case of producing a TRIP steel, prior to plating, thereby thickening the C in the austenite. However, after performing plating, increasing the temperature for alloying, carbides are precipitated at 500 ° C. or higher.
[0034]
　Accordingly, the present inventors have, at the stage of the alloying treatment, it was decided not to enrich C in austenite. Furthermore, it was decided to add after alloying, heat treatment for promoting the transformation. By this heat treatment, to enrich C in austenite, it found that it is possible to obtain a TRIP steel containing a stable retained austenite.
[0035]
　Furthermore, in order to obtain a high ductility while having a high tensile strength of at least 1470MPa have found that each of the high-temperature tempered martensite and a low temperature tempered martensite described below it is effective to present at a predetermined volume ratio. Then, the present inventors have found that in order to present each hot tempered martensite and a low temperature tempered martensite at a predetermined volume ratio, the effect is to perform an annealing process prior to alloying treatment at a predetermined heating condition and cooling condition It was found to be specific.
[0036]
　The present invention has been made based on the above findings. It will be described in detail below each requirement of the present invention.
[0037]
　(A) Chemical composition
　reasons for limiting each element are as follows. Incidentally, "%" for the content in the following description means "mass%".
[0038]
　C: 0.25 ~ 0.70%
　C is an element effective for obtaining high strength. Further, C is also a element to stabilize the contributing residual austenite to the improvement of ductility of the steel sheet. The C content is less than 0.25%, the above effect is not sufficiently expressed, it is difficult to obtain the necessary tensile strength (more than 1470 MPa). On the other hand, when the C content exceeds 0.70%, the too hard, cold rolling becomes difficult. Therefore, C content is made 0.25 to 0.70%.
[0039]
　Strength of the steel sheet is responsible tempered martensite and bainite to be described later, the low C content, these tissues become soft, there are cases where the required tensile strength can not be obtained. Therefore, it is preferable C content is 0.28% or more, more preferably 0.30% or more. Further, it is preferable C content is 0.60%, and more preferably 0.50% or less.
[0040]
　Si: 0.25 ~ 2.50%
　Si, in addition to being an element of increasing the strength of the steel sheet by strengthening the ferrite, by homogenizing the tissue is the element effective for improving the workability . Further, Si suppresses the precipitation of cementite, there is also an element an activity of accelerating the residual austenite.
[0041]
　The Si content is less than 0.25%, the above effects are not sufficiently exhibited. On the other hand, when the Si content exceeds 2.50%, toughness is greatly decreased, production becomes difficult. Therefore, Si content is from 0.25 to 2.50%. Si content is preferably 0.30% or more, and more preferably 0.60% or more. Further, it is preferable Si content is less 2.30%, and more preferably not more than 2.00%.
[0042]
　Mn: 1.00 ~
　5.00% Mn is to generate M-A (Martensite-Austenite Constituent ), is an essential element to achieve both strength and elongation. The Mn content is less than 1.00%, the above effects are not sufficiently exhibited. On the other hand, when the Mn content exceeds 5.00%, the progress of the bainite transformation is slow, C is no longer concentrated in the austenite. Then, as a result, austenite is not stabilized, the volume ratio of the finally fresh martensite becomes excessive. Therefore, Mn content is made 1.00 to 5.00%. Mn content is preferably 1.20% or more, more preferably 1.50% or more. Further, it is preferable Mn content is less 4.5%, and more preferably not more than 4.00%.
[0043]
　Al: 0.005 ~
　3.50% Al, along with a deoxidizing element, as with Si, suppresses the precipitation of cementite, is an element effective to increase the residual austenite. The Al content is less than 0.005%, the above effects are not sufficiently exhibited. On the other hand, when the Al content exceeds 3.50% inclusions is increased, processability is deteriorated. Therefore, Al content is set to 0.005 to 3.50%. Al content is preferably 0.010% or more, and more preferably 0.020% or more. Further, it is preferable Al content is less 3.30%, and more preferably not more than 3.00%.
[0044]
　Si + Al: 0.80% or more
　is also the content of each of Si and Al is not meet the above range, the Si + Al is less than 0.80%, the synergistic effect of Si and Al is not sufficient, when the bainite transformation , cementite is precipitated, the residual austenite may not be stabilized. Therefore, in order to further stabilize the residual austenite, the total content of Si and Al is preferably to 0.80% or more, more preferably to 0.90% or more, to 1.00% or more but more preferable.
[0045]
　P: 0.15% or less
　P is an impurity element, an element degrading the toughness segregated. When the P content exceeds 0.15%, toughness is significantly degraded. Therefore, P content is at most 0.15%. P content is preferably not more than 0.12%, and more preferably not more than 0.10%. In order to reduce the P content to less than 0.003%, the production cost is greatly increased. Therefore, the lower limit of the substantial P content 0.003%.
[0046]
　S: 0.03% or less
　S is an impurity element, an element that inhibits growth to form MnS. When S content exceeds 0.03%, the elongation is remarkably reduced. Therefore, S content is at most 0.03%. It is preferably the S content is 0.02% or less, and more preferably not more than 0.01%. In order to reduce the S content to less than 0.0002%, the manufacturing cost is greatly increased. Therefore, the lower limit of the substantial S content 0.0002%.
[0047]
　N: 0.02% or less
　N is an impurity element, in the continuous casting, an element that forms nitrides which cause cracking of the slab. Exceeds 0.02%, cracking of the slab becomes considerably. Therefore, N content is 0.02% or less. N content is preferably 0.01% or less. In order to reduce the N content to less than 0.0007%, the manufacturing cost is greatly increased. Therefore, 0.0007% is the lower limit of the substantial N.
[0048]
　O: 0.01% or less
　O is an inclusions formed, is an element that inhibits local ductility and toughness. When O content exceeds 0.01%, the local ductility and toughness is significantly lowered. Therefore, O content is 0.01% or less. O content is preferably 0.008% or less, and more preferably 0.006% or less. In order to reduce the O content to less than 0.0001%, the manufacturing cost is greatly increased. Therefore, it is the lower limit of the substantial O 0.0001%.
[0049]
　The galvannealed steel sheet of the present invention, in addition to the above elements, Ti in an amount shown below, Nb, V, Cr, Mo, Cu, Ni, B, Ca, REM, Mg, W, Zr , Sb, Sn, as, Te, Y, may contain one or more elements selected from Hf and Co.
[0050]
　Ti:
　0 ~
　0.50% Nb: 0 ~ 0.50% V: 0 ~
　0.50% Ti, Nb and V, the precipitate formed by refining crystal grains, contribute to the improvement of strength and toughness for an element that may contain as necessary. However, when the content exceeds even 0.50% each element, the above effect saturates, manufacturing cost is increased. Therefore, Ti, both the content of Nb and V to 0.50% or less. The content of these elements is preferably not more than 0.35% both.
[0051]
　To obtain the above effect, Ti, one or more selected from Nb and V, it is preferable to contain 0.005% or more. Ti and Nb is to fine austenite, to stabilize the austenite, one or two of Ti and Nb is more preferably contained more than 0.010%, be contained more than 0.030% or A further preferred.
[0052]
　Cr:
　0 ~ 1.50% Mo: 0 ~
　1.50% Cr and Mo, as well as the Mn, stabilizes the austenite, promoting transformation strengthening is an effective element for increasing the strength of the steel sheet. Moreover, Cr and Mo, when alloying treatment is also an element forming an action to suppress the decomposition of austenite. Therefore, it may be contained as necessary these elements. However, when any of the elements may be contained in excess of 1.50%, the progress of the bainite transformation is slow, C is no longer concentrated in the austenite. Then, as a result, austenite is not stabilized, the volume ratio of the finally fresh martensite becomes excessive. Therefore, both the content of Cr and Mo to 1.50% or less. The content of these elements is preferably not more than 1.30% both. Further, more preferably the Cr content is less 1.20%, Mo content is more preferably not more than 1.00%.
[0053]
　To obtain the above effect, at least one selected from Cr and Mo, it is preferable to contain 0.01% or more. Further, Cr is more preferably contained more than 0.10%, Mo is more preferably contained 0.05% or more.
[0054]
　Cu:
　0 ~ 5.00% Ni: 0 ~
　5.00% Cu and Ni are elements having the effect of suppressing corrosion. Further, Cu and Ni, and concentrated on the surface of the steel sheet to suppress hydrogen from entering the steel sheet, which has an action to suppress the delayed fracture, is also an element contributing to the stabilization of austenite. Therefore, it may be contained as necessary these elements. However, when the content exceeds even 5.00% each element, the above effect saturates, manufacturing cost is increased. Therefore, both the content of Cu and Ni and 5.00% or less. The content of these elements is preferably either equal to or less than 4.00%.
[0055]
　To obtain the above effect, at least one selected from Cu and Ni, preferably contained 0.01% or more, it is preferable to contain 0.02% or more.
[0056]
　B: 0 ~ 0.003% B
　is suppressed nucleation originating grain boundaries, to enhance the hardenability, since an element which contributes to the high strength, may be contained as needed. However, when the B content exceeds 0.003%, the above effect is saturated and the production cost is increased. Therefore, B content is at most 0.003%. B content is preferably 0.002% or less. To obtain the effect, B content is preferably although 0.0001% or more, more preferably 0.0002% or more.
[0057]
　Ca:
　0 ~
　0.05% REM: 0 ~ 0.05% Mg: 0
　~ 0.05% Ca, REM and Mg, and spheroidizing sulfides, are elements contributing to the improvement of local elongation of the steel sheet Therefore, it may be contained if necessary. However, when the content exceeds even 0.05% each element, the above effect saturates, manufacturing cost is increased. Therefore, Ca, both the content of REM and Mg is 0.05% or less. The content of these elements is preferably 0.04% or less both.
[0058]
　To obtain the above effect, Ca, one or more selected from REM and Mg, 0.0001% or more for Ca and Mg, preferably contained 0.0005% for REM.
[0059]
　In the present invention, REM refers to a total of 16 elements of Sc and lanthanoids, the content of the REM means the total content of these elements. It should be noted that the lanthanide is industrially, it is added in the form of misch metal.
[0060]
　W: 0 ~ 0.50% W
　increases the hardenability, since an element which contributes to the improvement of the strength of the steel sheet, may be contained as necessary. However, when the W content exceeds 0.50%, the above effect is saturated and the production cost is increased. Therefore, W content is 0.50% or less. W content is preferably not more than 0.35%. To obtain the above effect, it is preferable W content is 0.005% or more, more preferably 0.010% or more.
[0061]
　Zr: 0 ~ 0.05% Zr
　increases the hardenability, since an element which contributes to the improvement of the strength of the steel sheet, may be contained as necessary. However, when the Zr content exceeds 0.05%, the above effect is saturated and the production cost is increased. Therefore, Zr content is 0.05% or less. Zr content is preferably 0.03% or less. To obtain the above effect, it is preferable Zr content is 0.005% or more, more preferably 0.07% or more.
[0062]
　Sb:
　0 ~ 0.50% Sn: 0 ~
　0.50% Sb and Sn, together with contributing to the improvement of the coating adhesion and the plating wettability, since an element which forms an effect of preventing the decarburization of steel, it may be contained as needed. However, when the content exceeds even 0.50% each element, the steel plate heat embrittlement occurs, cracking during hot working occurs, there is a possibility that surface defects occur on steel sheets, also, shear cutting there is a possibility that cracking even at the time of cold working, such as occur. Therefore, both the content of Sb and Sn to 0.50% or less. The content of these elements is preferably not more than 0.35% both.
[0063]
　To obtain the above effect, at least one selected from Sb and Sn, preferably contained more than 0.005%, it is preferable to contain 0.010% or more.
[0064]
　As:
　0 ~ 0.05% Te: 0 ~
　0.05% As and Te are the elements contributing to the improvement of the mechanical strength of the steel sheet, may be contained as necessary. However, when any of the elements may be contained by more than 0.05%, there is a possibility that the local deformability is reduced. Therefore, both the content of As and Te 0.05% or less. The content of these elements is preferably 0.03% or less both.
[0065]
　To obtain the above effect, at least one selected from As and Te, As 0.005% or more is preferably to contain Te least 0.001%, As 0.010% or more, the Te more preferably contained more than 0.007% or.
[0066]
　Y:
　0 ~ 0.20% Hf: 0 ~ 0.20%
　Y and Hf are the effective element in improving the corrosion resistance of the steel sheet, may be contained as necessary. However, when any of the elements may be contained in excess of 0.20%, there is a possibility that local elongation of the steel sheet deteriorates significantly. Therefore, both the content of Y and Hf and 0.20% or less. The content of these elements is preferably 0.15% or less both.
[0067]
　To obtain the above effect, at least one selected from Y and Hf, preferably contained more than 0.001%, it is preferable to contain 0.005% or more.
[0068]
　Co: 0 ~
　1.00% Co is an element having an effect of promoting bainite transformation. To facilitate TRIP effect is cause bainite transformation, it is necessary to enrich C to the austenite, Co is an element useful in the promotion of TRIP effect. Therefore, it may be contained as needed Co. However, when the Co content exceeds 1.00%, there is a possibility that the weldability and local elongation of the steel sheet deteriorates significantly. Therefore, Co content is at most 1.00%. Co content is preferably at 0.80%. To obtain the above effect, it is preferred Co content is 0.001% or more, more preferably 0.008% or more.
[0069]
　In the chemical composition of the steel sheet of the present invention, the balance being Fe and impurities.
[0070]
　Here, the "impurities", in manufacturing the steel sheet industrially, ores, raw material scraps, a component mixed by various factors of the manufacturing process, is allowed to the extent that the present invention does not adversely affect means shall.
[0071]
　(B) metal structure of the steel sheet
　metal structure in the thickness 1/4 position of the steel sheet of the present invention will be described. Incidentally, "%" in the following description means "% by volume".
[0072]
　Retained austenite: 10.0 to 60.0%
　steel sheet containing retained austenite, during processing, austenite transformation induced plasticity resulting in transformed into martensite: the (Transformation Induced Plasticity TRIP), excellent elongation, specifically has excellent uniform elongation.
[0073]
　When the volume fraction of retained austenite is less than 10.0%, the insufficient uniform elongation of the steel sheet. On the other hand, if the volume fraction of retained austenite exceeds 60.0% local elongation of the steel sheet may be deteriorated. Therefore, the volume fraction of retained austenite is set to 10.0 to 60.0%. A high ductility, because the car bodies of collision safety is further improved, if you want to ensure a higher ductility, the volume fraction of retained austenite is preferably 13.0% or more. The volume fraction of retained austenite is preferably not more than 50.0%, more preferably at most 40.0%, more preferably not more than 30.0%.
[0074]
　C content in the retained austenite (Cγ): 0.85 mass% or more
　in order to present the retained austenite stably, it is preferable that the C in the retained austenite is enriched. When Cγ is less than 0.85 mass%, residual austenite becomes unstable, it tends to disappear. As a result, it becomes difficult to ensure the retained austenite 10.0% or more, it may not be secured and the required uniform elongation and local elongation. Therefore, C gamma is preferably not less than 0.85 mass%, more preferably from 0.90% by mass or more, further more preferably not less than 0.95 mass%.
[0075]
　The upper limit of the Cγ is not specified in particular, if it exceeds 1.50 wt%, the austenite is not transformed to martensite during deformation, it can not be obtained TRIP effect, there is a possibility that the ductility deteriorates. Therefore, C gamma is preferably less 1.50 mass%.
[0076]
　Hot tempered martensite: 5.0% or more
　hot tempered martensite is tempered martensite at a temperature of 480 ~ 600 ° C.. Hot tempered martensite is a hard compared with ferrite, and is soft compared to the low-temperature tempered martensite described later, is estimated to be effective in improving the ductility. To obtain the above effect, it is necessary that the volume fraction of the high-temperature tempered martensite 5.0% or more. The volume ratio of the high-temperature tempered martensite is preferably 10.0% or more. However, if it is excessive volume fraction of the high-temperature tempered martensite, cold tempered martensite, it becomes impossible to secure the volume fraction of retained austenite and bainite, it is preferable to 70.0% or less.
[0077]
　Cold tempered martensite: 5.0% or more
　cold tempered martensite is tempered martensite at a temperature of 100 ~ 450 ° C.. Cold tempered martensite are the rigid than the aforementioned high-temperature tempered martensite, it is an organization necessary for ensuring tensile strength of at least 1470 MPa. Therefore, it is necessary that the volume fraction of the low-temperature tempered martensite 5.0% or more. The volume ratio of the low-temperature tempered martensite is preferably 10.0% or more, more preferably 15.0% or more. However, when an excessive volume percentage of the low-temperature tempered martensite, high temperature tempered martensite, it becomes impossible to secure the volume fraction of retained austenite and bainite, it is preferable to 70.0% or less.
[0078]
　Fresh Martensite: 10.0% or less
　when manufacturing a high strength steel sheet, usually to increase the fresh martensite (martensite is not tempered), when the plated steel sheet of the present invention, the volume of fresh martensite When the rate is more than 10.0% is not preferable because the local ductility and yield ratio of the steel sheet is reduced. Therefore, the volume rate of fresh martensite is less 10.0%. The volume ratio of fresh martensite is preferably 7.0% or less.
[0079]
　Ferrite: 0 to 15.0%
　ferrite because it is a soft tissue, can not be a volume ratio to obtain a tensile strength of at least 1470MPa exceeds 15.0%. Therefore, the volume ratio of ferrite is not more than 15.0%.
[0080]
　Perlite: 0 to 10.0%
　during the alloying process the pearlite occurs, which may reduce the volume fraction of retained austenite. Further, since the soft tissues than tempered martensite, the strength is lowered. Therefore, the volume ratio of pearlite is not more than 10.0%. Volume fraction of pearlite is as low as possible is preferable, but preferably not more than 5.0%, more preferably 0%.
[0081]
　In the metal structure in the thickness 1/4 position of the steel sheet of the present invention, the balance being bainite.
[0082]
　Hot tempered martensite and the low temperature tempered martensite and the total volume ratio of bainite: 30.0% or more
　in the description of the tempered martensite (hereinafter, collectively "hot tempered martensite" and "cold tempered martensite" "tempered martensite also referred to as a site. ") and in the case where the total volume fraction of bainite is less than 30.0%, when you try to secure the tensile strength 1470MPa, it is necessary to increase the volume fraction of fresh martensite. However, increasing the volume fraction of fresh martensite, local ductility decreases. Therefore, from the viewpoint of ensuring the required local ductility while maintaining higher tensile strength 1470 MPa, the total volume rate of the to 30.0% or more.
[0083]
　In order to make the volume fraction of retained austenite than 10.0%, upon tempering of the bainite transformation or during martensite, it is necessary to enrich C to retained austenite. In order to obtain this effect, the total volume fraction of the tempered martensite and bainite to 30.0% or more. From the viewpoint of the local deformability and strength improvement of the total volume fraction of the tempered martensite and bainite it is preferably set to 40.0% or more.
[0084]
　Incidentally, tempered martensite and bainite is not only improve the tensile strength, in order to contribute to the improvement of yield strength by the total volume fraction of the predetermined value or more, the yield ratio 0.58 or more it becomes possible. Therefore, the present invention coated steel sheet is suitable as an automobile member.
[0085]
　In the present invention, it will be described below how to determine the volume fraction of each tissue as described above.
[0086]
　The volume fraction of retained austenite (V.gamma) may be from the data obtained using Mo-K [alpha line, is calculated by the following equation.
　= V? (2/3) {100 / (0.7 × alpha (111) / gamma
　　　　(200) +1)} + (1/3) {100 / (0.78 × alpha (211) / gamma (311) +1)}
　However, α (211), γ ( 200), α (211), and gamma (311) represents the surface strength.
[0087]
　Further, C of the residual austenite (C gamma) is the X-ray analysis by Cu-K [alpha line, (200) plane of the austenite, (220) plane, and (311) plane lattice constant from the angle of reflection (in Angstroms) the calculated can be calculated according to the following equation.
　Cγ = (lattice constant -3.572) /0.033
[0088]
　Then, FSLepera: Journal of Metals 32, No. 3, by the method described in (1980) 38-39, and corrode the cross-section in the rolling direction, thereby revealing the fresh martensite and retained austenite. Thereafter, the sheet thickness 1/4 position, and observed at a magnification 1000 times using an optical microscope, the structure photographs with image processing, to measure the total area ratio of the fresh martensite and retained austenite (%), total it and volume fraction.
[0089]
　Then, from the value of the total volume fraction of fresh martensite and residual austenite, by subtracting the volume fraction of retained austenite was measured by the method described above to determine the volume fraction of fresh martensite.
[0090]
　Moreover, cut out a cross section perpendicular to the rolling direction, after mirror polishing, the samples electrolytic polishing, measured at 0.1step intervals 100 [mu] m × 100 [mu] m or more regions in the SEM-EBSD. Then, using the analysis software Inc. TSL Solutions, the average value of Image Quality in the grains in each of the crystal grains: calculating the (Grain Average Image Quality GAIQ value).
[0091]
　Then, the fraction of grain GAIQ value of 5,000 or less and the total volume ratio of the low-temperature tempered martensite and fresh martensite. From this value, by subtracting the volume fraction of fresh martensite, determine the volume fraction of the low-temperature tempered martensite.
[0092]
　Also, it cuts out a cross section perpendicular to the rolling direction, after mirror polishing, perform corrosion at nital. Perform SEM observation for the sample, at a lath-shaped tissue, and obtains the fraction of those containing cementite as total area ratio of the high-temperature tempered martensite and the low temperature tempered martensite, make it a total volume ratio. SEM observation was performed at a magnification of 5000 times, measuring region, and the region of 25 [mu] m × 20 [mu] m 4 or more visual fields. From this value, by subtracting the volume fraction of the low-temperature tempered martensite, determine the volume fraction of the high-temperature tempered martensite.
[0093]
　The sum of the bainite and tempered martensite is also determined by SEM observation. Those blocks of bainite or martensite is observed, the bainite or tempered martensite. Then, by measuring the total area ratio of bainite and tempered martensite, make it a total volume ratio.
[0094]
　Similarly, the ferrite and pearlite, after nital corrosion, was observed by SEM, no infrastructure, and a region in which hollowed ferrite, and pearlite what lamellar structure is visible. Then, a respective area ratios of ferrite and pearlite and it the volume ratio.
[0095]
　(C) galvannealed layer
　Fe amount included in the galvannealed layer: 3.0 to 20.0 wt%
　galvannealed layer may be a conventional galvannealed layer. However, when the Fe content in the plating layer is less than 3.0 mass%, the weldability and sliding of the galvannealed steel sheet becomes insufficient. Therefore, Fe content of the plating layer is preferably 3.0 mass% or more. On the other hand, from the viewpoint of ensuring the powdering resistance, Fe of the plating layer is preferably not more than 20.0 wt%.
[0096]
　Fe content of the plating layer is more preferably from 5.0 mass% or more, more preferably at least 7.0 mass%. Further, Fe of the plating layer is more preferably at most 15.0 mass%. Incidentally, Fe of the plating layer can be adjusted by the conditions of heat treatment after hot-dip galvanizing (alloying treatment).
[0097]
　(D) the mechanical properties
　galvannealed steel sheet according to the present invention, from the viewpoint of securing sufficient impact absorption is assumed to have a tensile strength of at least 1470 MPa. The upper limit of the tensile strength is not particularly limited. Tensile strength, depending on the application, may suitably be selected within the range of 1470 ~ 2200 MPa.
[0098]
　In consideration of application to automobile parts formability it is required, pulling the product of the strength and the uniform elongation and 13000 mPa% or more, the product of the tensile strength and local elongation and 5000 MPa% or more. Automotive parts, since uniform deformation characteristics and local deformation characteristics are required, it is necessary to satisfy the two.
[0099]
　Yield ratio affects the strength of the mechanical part obtained by molding a steel plate. For example, increasing the collision safety of automobile parts (increasing collision energy) for the high yield ratio is obtained. Therefore, the yield ratio of the plated steel sheet according to the present invention is preferably at 0.58 or more, more preferably 0.70 or more, still more preferably 0.80 or more. From the same viewpoint, the plated steel sheet according to the present invention preferably has a yield strength of at least 850 MPa.
[0100]
　In the present invention, as the tensile strength and yield strength, employing the value determined in the tensile test perpendicular to the rolling direction. The direction perpendicular to the rolling direction, refers to a direction perpendicular to the rolling direction and the thickness direction of the steel sheet, i.e., to mean the width direction.
[0101]
　Further, the galvannealed steel sheet according to the present invention, when used as a steel sheet for automobile parts subjected to repeated loads, in addition to the excellent uniform deformation characteristics and local deformation characteristics, determined excellent fatigue properties It is. If you want to secure predetermined fatigue characteristics, it is preferable to the ratio of the fatigue limit for the tensile strength of 0.30 or more, more preferably 0.35 or more. Incidentally, the fatigue limit of the steel sheet, the stress ratio -1, and the repetition frequency and 25 Hz, the maximum number of repeated × 10 2 6 measured by the times.
[0102]
　(E) surface layer structure of the steel sheet
　galvannealed steel sheet according to the present invention comprises a surface layer soft layer. The surface soft layer, present on the steel sheet surface layer means a layer having a 0.9 times or less of the hardness of the average hardness in the region from the sheet thickness 1/4 position to a thickness of 1/2 position.
[0103]
　In the present invention, by controlling the metallographic structure in the chemical composition and thickness 1/4 position, it is possible to obtain a high strength and excellent formability, any special restriction on the thickness of the surface layer soft layer Absent. However, by adjusting the thickness of the appropriate surface soft layer depending on the application, it is possible to obtain additional properties.
[0104]
　For example, when welding the automobile steel sheet galvanized, the weld may be liquid metal embrittlement cracks may occur. The liquid metal embrittlement cracking, tends to occur as the strength of the steel sheet is high strength. However, in addition to the provisions of the above-mentioned chemical composition and metal structure, by a 10μm than the thickness of the surface layer the soft layer, it is possible to ensure excellent liquid resistant metal embrittlement cracking resistance.
[0105]
　On the other hand, as described above, the plated steel sheet, when used as a steel sheet for automobile parts subjected to repeated loading is required to excellent fatigue characteristics. In addition to the provisions of the above-mentioned chemical composition and metal structure, by setting the thickness of the surface layer the soft layer and 10μm or less, the fatigue characteristics is improved, the ratio of fatigue limit for the tensile strength can be made 0.30 or more to become.
[0106]
　The thickness of the surface layer the soft layer shall be determined by the following procedure. First, cut out a cross section perpendicular to the rolling direction, and mirror polishing. Then, from 10μm position from the interface between the plated layer and the steel sheet of the sample to a thickness of the center (sheet thickness 1/2 position), the micro-Vickers hardness measured sequentially at 10μm pitch. Test force may be appropriately selected depending on the hardness of the tissue, for example, it is 2 ~ 25 gf. Also, if the indentation overlap is measured by shifting in a direction perpendicular to the plate thickness.
[0107]
　From the above measurement results, an average hardness in the region from the sheet thickness 1/4 position to a thickness of 1/2 position, identifies a 0.9-fold and a position of its hardness. Then, the distance from the interface between the plated layer and the steel sheet to a position of 0.9 times the average hardness of above is defined as the thickness of the surface layer the soft layer.
[0108]
　However, when the hardness at 10μm position from the interface between the plated layer and the steel sheet, which exceeds 0.9 times the average hardness in the region to a thickness of 1/2 position from the sheet thickness 1/4 position, it becomes difficult to measure the thickness of the surface layer the soft layer by the method described above. In that case, to investigate the change in the structure fraction by SEM observation, and obtaining the thickness of the surface layer the soft layer.
[0109]
　Specifically, the surface layer of the tissue was measured at 500 to 1000-fold magnification to observe the metal structure over a range of 100 ~ 200 [mu] m in a direction perpendicular to the thickness direction. Then, a fraction of the hard tissue at the position of 2,4,6,8,10μm from the interface between the plated layer and the steel sheet, respectively. Alternatively, it is acceptable to obtain the average fraction of hard structures in the region from the sheet thickness 1/4 position to a thickness of 1/2 position to identify 0.9 times a position of the average fraction, plating and its position It defines the distance between the interface layer and the steel sheet as the thickness of the surface layer the soft layer. Here, the fraction of hard structures refers to the total area of ​​the tissue other than the ferrite and pearlite.
[0110]
　(F) the production method
　is not particularly limited production conditions of the galvannealed steel sheet according to the present invention, by using the manufacturing method described below, can be produced. In the following manufacturing method, the step of the following (a) to (l) in this order. It will be described in detail each of the steps.
[0111]
　(A) melting process
　to melting the steel ingot or slab having a chemical composition described above. No particular limitation is imposed on the conditions in the melting process, it may be used conventional methods.
[0112]
　(B) hot rolling step
　heating the ingot or slab subjected to hot rolling, and hot-rolled steel sheet. Although not provided any special restriction on conditions in the hot rolling step, for example, the heating temperature before hot rolling the 1000 ~ 1300 ° C., the finishing temperature of hot rolling preferably in the 800 ~ 1000 ° C..
[0113]
　The heating temperature is less than 1000 ° C., the temperature during the transport until the hot rolling is lowered, it may be impossible to end the finish rolling at a required temperature. On the other hand, if the heating temperature exceeds 1300 ° C., there is a risk of melt reaches the melting point of the steel having the chemical composition described above.
[0114]
　Moreover, since hard steel having the chemical composition defined in the present invention, when the finishing temperature is lower than 800 ° C., cause excessive load on the rolling mill, there is a possibility that the hot rolling becomes difficult. On the other hand, if the finishing temperature exceeds 1000 ° C., the steel sheet crystal becomes coarse in after rolling, various characteristics of the finally obtained alloyed hot dip plated steel sheet may be deteriorated.
[0115]
　(C) the first cooling step
　to cool the hot-rolled steel sheet of finish rolling after the end. Although not provided particularly limited cooling conditions in the first cooling step, cooling at an average cooling rate of more than 10 ° C. / s, it is preferable to stop cooling at a temperature range of 300 ~ 700 ° C..
[0116]
　Once the tissue of the hot-rolled steel sheet is fine, easy to obtain the effect of Mn concentrated, but if the average cooling rate is less than 10 ° C. / s, phase transformation occurring at a high temperature, tissue tends to be coarsened . The upper limit of the average cooling rate is not particularly limited, industrially, the average cooling rate exceeds 200 ° C. / s, it becomes difficult to control the cooling stop temperature, variation in the material occurs. Therefore, the average cooling rate is preferably not more than 200 ° C. / s, more preferably to 100 ° C. / s or less, further preferably set to 60 ° C. / s or less.
[0117]
　Further, when the cooling stop temperature is lower than 300 ° C., organization of the steel sheet becomes martensite mainly, the winding may become difficult. On the other hand, if the cooling stop temperature is higher than 700 ° C., the scale produced on the surface of the steel sheet reaches the inside steel pickling may become difficult. Incidentally, a problem in manufacturing are the strength and ease of pickling hot-rolled steel sheet, in consideration of these, the cooling stop temperature may be appropriately set.
[0118]
　(D) winding process
　to wind the hot-rolled steel sheet after cooling is stopped. Although not limited particularly also the coiling temperature preferably set to 700 ° C. or less. Similar to the cooling stop temperature in the first cooling step also the coiling temperature may be appropriately set in consideration of the strength and pickling of the ease of hot-rolled steel sheet.
[0119]
　(E) cold rolling process
　again unwinding the wound taken hot rolled steel sheet, after pickling, subjected to cold rolling, the cold-rolled steel sheet. Not provided any special restriction on conditions in the cold rolling process. However, since the steel having the chemical composition defined in the present invention is hard, the rolling reduction exceeds 90%, it is difficult to terminate in a short time cold rolling. Therefore, the rolling reduction in the cold rolling step is preferably 90% or less. The rolling reduction in the range of 90% or less, may be appropriately set in consideration of the ability of the desired thickness and rolling mill.
[0120]
　(F) annealing process
　with respect to cold-rolled steel sheet after cold rolling, Ac 1 subjected to annealing for holding 5s or more in a temperature range of point ~ 920 ° C.. Annealing temperature Ac 1 is less than points, cementite does not become austenite, as a final tissue, bainite, it is not possible to obtain a residual austenite and tempered martensite, Ac 1 is required to be points or more. Meanwhile, the scale to produce enough steel sheet surface annealing temperature is a high temperature becomes thick, is deteriorated wettability during plating. Further, in view to ensure good toughness by suppressing the coarsening of grain size, and in view of reducing the energy cost, the annealing temperature is set to 920 ° C. or less. Annealing temperature is preferably set to 900 ° C. or less.
[0121]
　The holding time for holding the above annealing temperature is less than 5s, the temperature unevenness caused by the location of the steel sheet can not be sufficiently uniform tissue, it is difficult to obtain a sufficient local elongation. Therefore, the retention time 5s or more. Retention time is preferably more than 10s.
[0122]
　For the annealing atmosphere is not provided particularly limited. However, in order to adjust the thickness of the surface layer the soft layer of the steel sheet is appropriately according to the annealing temperature, it is desirable to dew point control an annealing atmosphere. As described above, the thickness of the surface layer the soft layer is the case of 10μm or less, the higher the ratio of fatigue limit for the tensile strength of the steel sheet, thereby improving the fatigue characteristics. When the thickness of the surface layer the soft layer is 10μm greater, it improves liquid resistant metal embrittlement cracking resistance of the steel sheet.
[0123]
　Specifically, the thickness of the surface layer the soft layer to a 10μm greater, it is preferred to the dew point of the annealing atmosphere and -25 ° C. or higher, more preferably to -15 ° C. greater, -10 ° C. greater more preferably in the. Meanwhile, in order to make the thickness of the surface layer the soft layer and 10μm or less, preferably to -15 ° C. or less the dew point of the annealing atmosphere, and more preferably be lower than -20 ℃, it is -25 ° C. or less but more preferable.
[0124]
　(G) a second cooling step
　at an average cooling rate of cold-rolled steel sheet of more than 1 ° C. / s after annealing is cooled to a temperature range of 100 ~ 350 ° C.. If the average cooling rate in the second cooling step is less than 1 ° C. / s, resulting possibly cementite is precipitated in the steel sheet. Preferably has an average cooling rate is 5 ° C. / s or more, more preferably 8 ° C. / s or higher.
[0125]
　However, when the average cooling rate exceeds 100 ° C. / s, the cooling rate is too high, it becomes difficult to induce steel sheet to a temperature range in which the residual austenite is generated (100 ~ 350 ℃). Therefore, the average cooling rate is preferably set to the desired control easily speed cooling stop temperature may preferably be 100 ° C. / s or less, and more preferably not more than 50 ° C. / s.
[0126]
　Further, when the cooling stop temperature is lower than 100 ° C., most austenite transformed into martensite, residual austenite in the final structure may become impossible to secure more than 10% by volume. On the other hand, if the cooling stop temperature is higher than 350 ° C., less martensitic transformation amount, there may not be possible to ensure subsequent resulting hot tempered martensite 5 vol% or more. Therefore, the cooling stop temperature is set to 100 ~ 350 ° C..
[0127]
　The lower limit of the cooling stop temperature may be appropriately set according to the type of steel or heat treatment conditions, may preferably be 130 ° C. or higher, more preferably in a 0.99 ° C. or more, more preferably in the 175 ° C. or higher, 200 ℃ to or more is particularly preferable. Also preferred for the cooling stop temperature 300 ° C. or less.
[0128]
　(H) pre-treatment step
　prior to application of the galvanized cold-rolled steel sheet, subjected to a pretreatment for cooling or heating a cold rolled steel sheet to hot-dip galvanizing bath temperature. While the temperature of the steel sheet is deviated greatly from the plating temperature and immersing the steel sheet in a plating bath, which can lead to poor appearance. It is not necessary to match exactly the temperature and plating bath temperature of cold-rolled steel sheet, is acceptable as long as until the difference of about 50 ° C..
[0129]
　(I) plating step
　after the pre-processing has been completed, by immersing the cold-rolled steel sheet in a molten zinc plating bath, galvanized steel sheet, a galvanized steel sheet. Bath composition of the molten zinc plating bath, there is no particular limitation on the bath temperature, and coating weight may be suitably set depending on the composition and thickness of the desired galvanized layer. The coating weight, for example, the coating weight per one side 20 ~ 80 g / m 2 may be set within the range of.
[0130]
　(J) alloying step
　by heating the galvanized steel sheet to 480 ~ 600 ° C., and galvannealed steel sheet by alloying molten zinc plating. The conditions of the alloying treatment may be appropriately set so that the amount of Fe galvannealed layer can be secured more than a predetermined amount. For example, coating weight per one side 20 ~ 80 g / m 2 if in the range of, by heating the galvanized steel sheet to 490 ~ 560 ° C., preferably kept 5 ~ 60s.
[0131]
　(K) the third cooling step
　after the alloying treatment, the galvannealed steel sheet at 1 ° C. / s or more average cooling rate, cooling to a temperature range of 80 ~ 300 ° C.. Cooling start temperature in the third cooling step is a steel sheet temperature at the end of the alloying step.
[0132]
　By cooling of the third cooling step, thereby facilitating the generation part of martensite, to promote the C distribution to austenite bainite transformation and martensite, to stabilize the residual austenite. In the third cooling step after the tempering process, in order to ensure the retained austenite more than 10% by volume, at the end of the third cooling step, it is necessary to austenite in the steel sheet is present more than 10% by volume.
[0133]
　If the average cooling rate is less than 1 ° C. / s, the above effects may not be obtained. The average cooling rate is preferably set to 5 ° C. / s or higher. The upper limit of the average cooling rate is not particularly limited, from the viewpoint of economy, preferably not more than 500 ° C. / s.
[0134]
　Also, if the cooling stop temperature is lower than 80 ° C., or exceeds 300 ° C., similarly, the above effects may not be obtained. Cooling stop temperature is preferably set to 110 ° C. or higher. Since the cooling stop temperature is lower strength is increased, the cooling end temperature is preferably between 250 ° C. or less.
[0135]
　(L) tempering step
　against galvannealed steel sheet after stopping the cooling, subjected to a tempering process for holding in a temperature range of 100 ~ 450 ° C. 1s or 48h or less. Tempering the effect of promoting effect of tempering the martensite, bainite transformation, and carried out to obtain the effect of thickening the C martensite and bainite to residual austenite.
[0136]
　When tempering temperature is lower than 100 ° C., there is a possibility that the above effects can not be obtained. On the other hand, when the tempering temperature exceeds 450 ° C., heated to a high temperature tempered martensite, the strength is greatly deteriorated. Further, decompose austenite C is concentrated within pearlite. For this reason, the tempering temperature is set to 100 ~ 450 ℃. Tempering temperature is preferably in the 120 ° C. or higher, and more preferably, 140 ° C. or higher. Further, tempering temperature is preferably set to 430 ° C. or less.
[0137]
　Further, tempering time (retention time) is less than 1s, no tempering effect. On the other hand, when the tempering time exceeds 48h, even 100 ~ 450 ° C. The tempering temperature, carbides are precipitated, there is a possibility that the residual austenite is greatly reduced. Therefore, tempering time should be not more than 48h more than 1s. It is preferable to be tempering time is 10s or higher, and more preferably, more than 30s. Further, tempering time is preferably at most 45h, and more preferably, less 40h.
[0138]
　(M) other
　after the tempering step, in order to improve the flatness of the galvannealed steel sheet may be subjected to skin pass rolling or leveler processing the steel sheet. Further, the galvannealed steel sheet may be formed a film with a coating oil or lubrication.
[0139]
　As described above, by using the above manufacturing method, even if the steel sheet contain C of more than 0.25 mass%, to produce a galvannealed steel sheet in and high ductility 1470MPa or higher grade can.
[0140]
　The following examples illustrate the present invention more specifically, the present invention is not limited to these examples.
Example 1
[0141]
　Steels having the chemical compositions shown in Table 1 were melted to prepare a thick 40mm slab. The slab was hot rolled under the conditions shown in Table 2-4 were produced hot-rolled steel sheet.
[0142]
　Next, the hot rolled steel sheet was cooled with water sprays to a coiling temperature at a rate (first cooling rate) shown in Tables 2-4. Then charged into the furnace, and 60min maintained at a coiling temperature, simulating the winding by furnace cooling until the average cooling rate at the 100 ° C. or less temperature of 20 ° C. / h. The obtained hot rolled steel sheet, after removing scale by pickling, and cold rolling under the conditions shown in Table 2-4.
[0143]
[Table 1]

[0144]
[Table 2]

[0145]
[table 3]

[0146]
[Table 4]

[0147]
　From the resulting cold-rolled steel sheets were collected test material. The test material performs annealing for holding heated to a predetermined temperature and then cooled at a predetermined rate (second cooling rate). Maximum annealing temperature in the annealing step, the annealing time and the dew point, and a second cooling rate and the second cooling stop temperature in the second cooling step are shown in Table 2-4.
[0148]
　Then, for some of the test material, after performing isothermal holding, 5 ° C. / s at heated or cooled to 460 ° C. a molten plating bath temperature, was galvanized. Thereafter, the alloying treatment with GA conditions shown in Table 2-4, then at a rate of 10 ° C. / s (third cooling rate), and cooled to third cooling stop temperature. After cooling, heating at a rate of 10 ° C. / s, after the tempering treatment was carried out under the conditions shown in Table 2-4, and cooled to room temperature at a cooling rate of 10 ° C. / s.
[0149]
　For comparison, in conventional manufacturing conditions shown in Table 5 were prepared galvannealed steel sheet. Until cold process is the same process as described above. Annealing after the cold rolling step, was followed by a second cooling. Thereafter, the tempering treatment was conducted by heating to a tempering temperature at a rate of 10 ° C. / s. Thereafter, it was heated, subjected to alloying treatment at GA conditions shown in Table 5, then at a rate of 10 ° C. / s (third cooling rate), and cooled to third cooling stop temperature.
[0150]
[table 5]

[0151]
　Then, perform metallographic observation of the obtained galvannealed steel sheet was subjected to measurement of the volume ratio and the C content in the residual austenite of each tissue.
[0152]
　The volume fraction of retained austenite (V.gamma) from the data obtained using Mo-K [alpha line was calculated by the following equation.
　= V? (2/3) {100 / (0.7 × alpha (111) / gamma
　　　　(200) +1)} + (1/3) {100 / (0.78 × alpha (211) / gamma (311) +1)}
　However, α (211), γ ( 200), α (211), and gamma (311) represents the surface strength.
[0153]
　Further, C of the residual austenite (C gamma) is the X-ray analysis by Cu-K [alpha line, (200) plane of the austenite, (220) plane, and (311) plane lattice constant from the angle of reflection (in Angstroms) look, was calculated according to the following equation.
　Cγ = (lattice constant -3.572) /0.033
[0154]
　Then, FSLepera: Journal of Metals 32, No. 3, by the method described in (1980) 38-39, and corrode the rolling direction of the cross-section was made to appear fresh martensite and retained austenite. Thereafter, the sheet thickness 1/4 position, and observed at a magnification 1000 times using an optical microscope, the structure photographs with image processing, to measure the total area ratio of the fresh martensite and retained austenite (%), total it to a volume rate.
[0155]
　Then, from the value of the total volume fraction of fresh martensite and residual austenite, by subtracting the volume fraction of retained austenite was measured by the method described above, was determined volume fraction of fresh martensite.
[0156]
　Moreover, cut out a cross section perpendicular to the rolling direction, after mirror polishing, the samples electropolished was measured at 0.1step intervals 100 [mu] m × 100 [mu] m or more regions in the SEM-EBSD. Then, using the analysis software Inc. TSL Solutions, the average value of Image Quality in the grains in each of the crystal grains (Grain Average Image Quality: GAIQ value) was calculated.
[0157]
　Then, the fraction of grain GAIQ value of 5,000 or less and the total volume ratio of the low-temperature tempered martensite and fresh martensite. From this value, by subtracting the volume fraction of fresh martensite was determined volume ratio of the low-temperature tempered martensite.
[0158]
　Also, cut out a cross section perpendicular to the rolling direction, after mirror polishing, subjected to corrosion by nital. Perform SEM observation for the sample, at a lath-shaped tissue, and obtains the fraction of those containing cementite as total area ratio of the high-temperature tempered martensite and the low temperature tempered martensite, and it and the total volume ratio. SEM observation was performed at a magnification of 5000 times, the measurement region was a region of 25 [mu] m × 20 [mu] m 4 or more visual fields. From this value, by subtracting the volume fraction of the low-temperature tempered martensite was determined volume ratio of the high temperature tempered martensite.
[0159]
　The sum of the bainite and tempered martensite was also determined by SEM observation. Those blocks of bainite or martensite is observed, and the bainite or tempered martensite. Then, by measuring the total area ratio of bainite and tempered martensite was the same as the total volume ratio.
[0160]
　Similarly, the ferrite and pearlite, after nital corrosion, was observed by SEM, no infrastructure, and ferrite regions which are hollowed, and what lamellar structure is visible and pearlite. Then, a respective area ratios of ferrite and pearlite, and it the volume ratio.
[0161]
　Also, it was measured Fe content in the plated layer of the galvannealed steel sheet. Specifically, starting from the interface between the galvanized layer and the base material, the Fe concentration (mass%) in the region of the (1/8 × plated layer thickness) ~ (7/8 × plated layer thickness) It was measured using an energy dispersive X-ray analyzer (EDX). Then, to calculate the average value, and a Fe content in the plated layer.
[0162]
　Furthermore, using the galvannealed steel sheet, the measurement of the thickness of the surface layer softer layer was carried out by the following procedure.
[0163]
　First, cut out a cross section perpendicular to the rolling direction, and mirror polishing. Then, from 10μm position from the interface between the plated layer and the steel sheet of the sample to a thickness of the center (sheet thickness 1/2 position), and the micro Vickers hardness was measured sequentially at 10μm pitch. Test force, depending on the hardness of the tissue was adjusted in the range of 2 ~ 25 gf. Also, if the indentation overlap, it was measured by shifting in a direction perpendicular to the plate thickness.
[0164]
　From the above measurement results, an average hardness in the region from the sheet thickness 1/4 position to a thickness of 1/2 position and identified 0.9 times a position of the hardness. Then, the distance from the interface between the plated layer and the steel sheet to a position of 0.9 times the average hardness of above was determined as the thickness of the surface layer the soft layer.
[0165]
　However, when the hardness at 10μm position from the interface between the plated layer and the steel sheet, which exceeds 0.9 times the average hardness in the region to a thickness of 1/2 position from the sheet thickness 1/4 position, investigated changes in tissue fraction by SEM observation was determined the thickness of the surface layer the soft layer.
[0166]
　Specifically, the surface layer of the tissue was measured at 500 to 1000 times magnification were observed metallographic over a range of 100 ~ 200 [mu] m in a direction perpendicular to the thickness direction. Then, the fraction of hard microstructures in the position of 2,4,6,8,10μm from the interface between the plated layer and the steel sheet was determined, respectively. Alternatively, it is acceptable to obtain the average fraction of hard structures in the region from the sheet thickness 1/4 position to a thickness of 1/2 position to identify 0.9 times a position of the average fraction, plating and its position the distance between the interface layer and the steel sheet and a thickness of the surface layer the soft layer.
[0167]
　The above observations and measurements are shown in Table 6-9.
[0168]
[Table 6]

[0169]
[Table 7]

[0170]
[Table 8]

[0171]
[Table 9]

[0172]
　Next, we measured the mechanical properties of the resulting galvannealed steel sheet. From heat-treated test materials, were taken JIS5 No. Tensile test specimen such that the tensile direction perpendicular to the rolling direction and the transverse direction, yield strength (YS), tensile strength (TS), uniform elongation ( uEL), to measure the total elongation (tEL). Further, the difference between the total elongation and uniform elongation, and a local elongation (lel).
[0173]
　Further, using each galvannealed steel sheet were evaluated for fatigue properties. Fatigue limit, using JIS1 No. 5 test piece was measured by a plane bending fatigue test. Then, the stress ratio -1, was the repetition frequency and 25Hz. First, for each steel plate was subjected to fatigue test multiplied by 0.6 times the stress of tensile strength. As a result, all the steel sheets × 10 2 6 due to broken at the number of repetition less than once lowered from the stress by 20 MPa, was repeated the same fatigue test. Then, 2 × 10 6 when the broken at times became stress does not occur, the stress tests were carried out by raising 10MPa a. If that does not break, stress were tested by raising 5MPa, and if you break, stress tests were carried out by lowering 5MPa a. In fatigue tests carried out in the above manner, the maximum number of repeated × 10 2 6 a maximum stress rupture did not occur when the times was fatigue limit.
[0174]
　Furthermore, using the galvannealed steel sheet, the evaluation of the liquid resistant metal embrittlement crack resistance was conducted by the following procedure.
[0175]
　Overlapping two identical steel was bonded by spot welding, the cross-section of the junction was observed by SEM, it was investigated the mode of liquid metal embrittlement cracks. Spot welding, using a Cr-Cu electrode was carried out with respect to two stacked steel sheets in hitting the corner of 5 °. Energization pattern, using the power supply of 50 Hz, a pressure and 250 ~ 750 kgf, and the energization pattern for energizing the current nugget diameter is 5.5 ~ 6.0 mm at 40 cycles.
[0176]
　Aspect of the liquid metal embrittlement cracks, polished steel plate section including the center of the nugget was observed by SEM, the degree of cracking was evaluated by the following cracking score.
[0177]
　1: have cracked inside the plate pairs, the length of the crack is greater than 10 [mu] m.
　2: a plate set of cracks on the inside has occurred, the length of the crack is 10μm or less.
　3: cracking until the nugget is progressing, or cracks on the outer side of the plate set of positions away 300μm from the nugget has occurred.
　4: cracks between the surface portion and the nugget steel sheet electrodes are in contact only occurs.
　5: There is no crack.
[0178]
　The measurement results and fatigue property and liquid resistant metal embrittlement cracking resistance evaluation results of the mechanical properties shown in Tables 10-13.
[0179]
[Table 10]

[0180]
[Table 11]

[0181]
[Table 12]

[0182]
[Table 13]

[0183]
　An invention example Test No. 1 ~ In the 4,9-16,23,24,28 ~ 30, 33 ~ 71 and 80 ~ 85, TS exceeds 1470 MPa, the product of the tensile strength and the uniform elongation is 13000 mPa% or more, tensile strength and local elongation product of is at least 5000MP%, has excellent moldability, and it can be seen that adequate alloying have been made in the plating layer.
[0184]
　On the other hand, test No. 5,18,19 and 25, since the second cooling stop temperature is higher, not obtained hot tempered martensite, the product of the tensile strength and the uniform elongation was low. Test No. 6, for maximum annealing temperature is low, the ferrite - pearlite transformation is a large amount occurs, the tensile strength was low. Test No. 7, since the second cooling rate is slow, in the middle of cooling, since the pearlite transformation, the retained austenite fraction is low, the product of the tensile strength and the uniform elongation was low.
[0185]
　Test No. 8, a long time of tempering, the residual austenite to decompose into bainite containing carbides, the amount of retained austenite is reduced, the product of the tensile strength and the uniform elongation was low. Test No. 17, the second cooling stop temperature is low, because this temperature martensitic transformation in the have proceeded a large amount, austenite hardly remain, the product of the tensile strength and the uniform elongation was low.
[0186]
　Test No. 20, tempering treatment temperature is high, since the austenite is decomposed into bainite containing carbides, residual austenite is small, the product of the tensile strength and the uniform elongation was low. Test No. 21, tempering treatment temperature is low, and Test No. 22 because it was not subjected to tempering treatment, none C not is concentrated into austenite, the number of fresh martensite, the product of the tensile strength and local elongation is lowered.
[0187]
　Test No. 26, although excellent in mechanical properties, due to the alloying temperature is low, alloying of the plating layer becomes insufficient. Test No. 27, high alloying temperatures, since pearlite has a large amount occurs, residual austenite fraction is low, the product of the tensile strength and the uniform elongation was low.
[0188]
　Test No. 31, the third high cooling stop temperature, tempered martensite amount is small, further, then the bainite transformation may not proceed, As a result, the C occurs finally fresh martensite without enrichment in austenite , the product of the product and the tensile strength and local elongation and tensile strength and uniform elongation becomes both low. In addition, test No. 32, the third cooling stop temperature is low, cause a large amount of martensite at this point, since the austenite is reduced, the product of the product and the tensile strength and local elongation and tensile strength and uniform elongation becomes both low.
[0189]
　Test No. 72, C content is lower than the specified range, the tensile strength was low. Test No. 73 is higher than the C content is specified range, residual austenite becomes excessive, broke immediately after starting tensile test. Test No. 74, Si content is lower than the specified range, can not be secured retained austenite, the product of the tensile strength and the uniform elongation was low.
[0190]
　Test No. 75, Si content is higher than the specified range, it broke during cold rolling. Test No. 76 is lower than the Mn content is specified range, pearlite transformation proceeds during a second cooling can not be ensured residual austenite, the tensile strength was low. Test No. 77-79, respectively, Mn, higher than the content specified range of Cr and Mo, does not proceed bainite transformation does not C is concentrated into austenite remain in a large amount martensite, the tensile strength and the product of the local elongation is lowered.
[0191]
　Test No. 86-89 are examples of performing an alloying treatment after tempering treatment in accordance with conventional practice. Test No. In 86 and 87, although the alloying for alloying temperature is sufficient in progress, the volume fraction of retained austenite and the low-temperature tempered martensite is lowered, the product of the tensile strength and the tensile strength and the uniform elongation is lowered It was. Test No. Even 88, although alloying is somewhat advanced, the lower the volume fraction of the low-temperature tempered martensite, the product of the tensile strength and the tensile strength and the uniform elongation was low. Test No. In 89, since the alloying temperature is low, in addition to the alloying becomes insufficient, due to the low volume fraction of the low-temperature tempered martensite, the tensile strength was low. As described above, in the conventional technique, intensity - it is difficult to achieve both ductility balance and sufficient alloying.
[0192]
　Further, in the present invention example was a dew point -25 ° C. or less in the annealing step Test No. 2,12,24,29,66-68,84 and 85, the thickness of the surface layer softer layer becomes 10μm or less, the ratio of fatigue limit for the tensile strength is increased, it resulted in superior fatigue properties.
[0193]
　On the other hand, the test was dew point -10 ° C. or higher in the annealing step No. 1,3,4,9 ~ 11, 13 ~ 16,23,28,30,33 ~ 65, 69 ~ 71 and 80 ~ 83, it is 10μm ultra next thickness of the surface layer soft layer, cracking score becomes 4 or more , was the result of excellent liquid resistant metal embrittlement cracking resistance.
Industrial Applicability
[0194]
　According to the present invention, while having a high tensile strength of over 1470 MPa, it is possible to obtain a uniform deformation property (uniform elongation) and local deformability excellent galvannealed steel sheet (local elongation).

claims

[Claim 1]A galvannealed steel sheet on the surface of the steel sheet comprising a hot-dip galvannealed layer,
　the chemical composition of the steel sheet contains, by
　mass%, C: 0.25
　~ 0.70%, Si: 0.25 ~
　% 2.50,
　Mn: 1.00 ~ 5.00%, Al: 0.005 ~
　3.50%, P: 0.15% or
　less, S: 0.03% or
　less, N: 0.02% or less ,
　O: 0.01% or
　less,
　Ti:
　0 ~ 0.50%, Nb: 0 ~
　0.50%, V: 0 ~ 0.50%, Cr: 0 ~
　1.50%, Mo: 0 ~ 1
　%
　.50,
　Cu: 0 ~
　5.00%, Ni: 0 ~ 5.00%, B: 0 ~
　0.003%, Ca: 0 ~ 0.05%, REM: 0 ~
　0.05%, Mg :
　0
　~
　0.05%, W: 0 ~ 0.50%, Zr: 0 ~
　0.05%, Sb: 0 ~ 0.50%, Sn: 0 ~
　0.50%, As: 0 ~ 0. 0.05%, 　Te:
　　0 ~ 0.05%, Y: 0 ~ 0.20%, 　Hf: 0 ~ 0.20%, Co: 0 ~ 1.00%, 　the balance is Fe and impurities, 　the plate thickness of the steel plate 1 / 4 metal structure at positions, by volume%, 　retained austenite: 10.0 to 60.0%, 　high-temperature tempered martensite: 5.0% or more, 　low-temperature tempered martensite: 5.0% or more, 　fresh martensite: 10.0% or less, 　ferrite: 0 to 15.0%, 　perlite 0 to 10.0%, 　the balance: a bainite 　hot tempered martensite and the low temperature tempered martensite and the total volume fraction of the bainite 30.0 % or more, 　a tensile strength of more than 1470 MPa, 　the tensile strength and the product of the uniform elongation is 13000 mPa% or more, 　the product of the tensile strength and local elongation is not less than 5000 MPa%, 　galvannealed Me Can steel sheet.

[Claim 2]
　The chemical composition, in
　mass%, Si + Al: 0.80% or more,
　galvannealed steel sheet according to claim 1.
[Claim 3]
　The chemical composition, by
　mass%,
　Ti: 0.005
　~ 0.50%, Nb: 0.005 ~ 0.50%, V: 0.005
　~ 0.50%, Cr: 0.01 ~ 1.
　%
　50,
　Mo: 0.01 ~ 1.50%, Cu: 0.01 ~
　5.00%, Ni: 0.01 ~ 5.00%, B: 0.0001 ~
　0.003%, Ca: 0
　~
　0.05% .0001,
　REM: 0.0005 ~ 0.05%, Mg: 0.0001 ~ 0.05%,
　W: 0.005 ~ 0.50%, Zr: 0.005 ~ 0.05
　%,
　Sb:
　0.005 ~ 0.50%, Sn: 0.005 ~
　0.50%, As: 0.005 ~ 0.05%, Te: 0.001 ~
　0.05%, Y: 0.
　~ 0.20% 001, Hf: 0.001 ~ 0.20%,
　and, Co: 0.001 ~ 1.00%,
　1 or more selected from Containing,
　galvannealed steel sheet according to claim 1 or claim 2.
[Claim 4]
　C amount contained in the residual austenite is not less than 0.85 wt%,
　galvannealed steel sheet according to any one of claims 1 to 3.
[Claim 5]
　Fe content in the galvannealed layer is 3.0 to 20.0 mass%,
　galvannealed steel sheet according to any one of claims 1 to 4.
[Claim 6]
　The steel sheet surface layer comprises a surface layer soft layer having an average hardness of 0.9 times or less of the hardness of the region from the sheet thickness 1/4 position to a thickness of 1/2 position,
　the surface soft layer, wherein the thickness of the interface between the steel sheet and the galvannealed layer is 10μm greater,
　galvannealed steel sheet according to any one of claims 1 to 5.
[Claim 7]
　The steel sheet surface layer comprises a surface layer soft layer having an average hardness of 0.9 times or less of the hardness of the region from the sheet thickness 1/4 position to a thickness of 1/2 position,
　the surface soft layer, wherein the thickness of the interface between the steel sheet and the galvannealed layer has a 10μm or less,
　the ratio of fatigue limit for the tensile strength of the steel sheet is 0.30 or more,
　of claims 1 to 5 galvannealed steel sheet according to any one.
[8.]
　A method of manufacturing a galvannealed steel sheet according to any one of claims 1 to 5,
　having a chemical composition according to any one of (a) above (1) to (3) melting step of melting a steel ingot or slab,
　(b) heating the steel ingot or slab subjected to hot rolling, hot rolling step of hot-rolled steel sheet,
　the cooling of the (c) said hot-rolled steel sheet first cooling step,
　(d) taking up winding process the hot-rolled steel sheet,
　(e) unwinding the hot rolled steel sheet, after pickling, subjected to cold rolling, cold rolling step of cold-rolled steel sheet,
　(f) the cold-rolled steel sheet to Ac 1 annealing step of holding 5s or more in a temperature range of point ~ 920 °
　C., (g) the cold-rolled steel sheet at 1 ° C. / s or more average cooling rate, a temperature of 100 ~ 600 ° C. second cooling step of cooling to pass,
　(h) cooling or pressurizing said cold-rolled steel sheet to hot-dip galvanizing bath temperature Pretreatment step of,
　(i) by immersing the cold-rolled steel sheet in a molten zinc plating bath, galvanized steel sheet, plating process to hot-dip galvanized steel sheet,
　480 to the (j) the hot-dip galvanized steel sheet was heated to 600 ° C., galvanizing alloyed, the alloying step of the galvannealed steel sheet,
　in (k) average cooling rate of 1 ° C. / s or higher the galvannealed steel sheet, 80-300 the third step of cooling to a temperature range of ° C., and,
　tempering step, which holds less 48h or 1s in a temperature range of 100 ~ 450 ° C. the (l) the galvannealed steel sheet
　wherein the above (a) performed in order to process up to (l) from the method of manufacturing a galvannealed steel sheet.
[Claim 9]
　A method of manufacturing a galvannealed steel sheet according to claim 6,
　which melted the steel ingot or slab having a chemical composition according to any one of up to (a) above (1) to (3) melting step,
　(b) subjecting the steel ingot or slab is heated to hot rolling, hot rolling step of hot-rolled steel sheet,
　a first cooling step of cooling the (c) said hot-rolled steel sheet,
　(d) the winding step of winding the hot-rolled steel sheet,
　rewind the (e) the hot-rolled steel sheet, after pickling, subjected to cold rolling, cold rolling step of cold-rolled steel sheet,
　the (f) the cold-rolled steel sheet , in an atmosphere of higher dew point -25 ° C., Ac 1 annealing step of holding 5s or more in a temperature range of point ~ 920 °
　C., at (g) the cold-rolled steel sheet of 1 ° C. / s or more average cooling rate, 100-600 second cooling step of cooling to a temperature range of ° C.,
　the (h) the cold-rolled steel sheet to hot-dip galvanizing bath temperature Pretreatment step for retirement or heating,
　(i) the cold-rolled steel sheet is immersed in molten zinc plating bath, galvanized steel sheet, plating process to galvanized steel sheet,
　(j) the hot-dip galvanized steel sheet was heated to 480 ~ 600 ° C., galvanizing alloyed, the alloying step of the galvannealed steel sheet,
　in (k) the galvannealed steel sheet 1 ° C. / s or more average cooling rate, the third step of cooling to a temperature range of 80 ~ 300 ° C.,
　and, tempering step, which holds 1s or 48h or less in a temperature range of 100 ~ 450 ° C. the (l) the galvannealed steel sheet
　wherein the above the steps (a) to (l) carried out sequentially, the production method of the galvannealed steel sheet.
[Claim 10]
　A method of manufacturing a galvannealed steel sheet according to claim 7,
　which melted the steel ingot or slab having a chemical composition according to any one of up to (a) above (1) to (3) melting step,
　(b) subjecting the steel ingot or slab is heated to hot rolling, hot rolling step of hot-rolled steel sheet,
　a first cooling step of cooling the (c) said hot-rolled steel sheet,
　(d) the winding step of winding the hot-rolled steel sheet,
　rewind the (e) the hot-rolled steel sheet, after pickling, subjected to cold rolling, cold rolling step of cold-rolled steel sheet,
　the (f) the cold-rolled steel sheet , in an atmosphere of a dew point of -15 ° C. or less, Ac 1 annealing step of holding 5s or more in a temperature range of point ~ 920 °
　C., at (g) the cold-rolled steel sheet of 1 ° C. / s or more average cooling rate, 100-600 second cooling step of cooling to a temperature range of ° C.,
　the (h) the cold-rolled steel sheet to hot-dip galvanizing bath temperature Pretreatment step for retirement or heating,
　(i) the cold-rolled steel sheet is immersed in molten zinc plating bath, galvanized steel sheet, plating process to galvanized steel sheet,
　(j) the hot-dip galvanized steel sheet was heated to 480 ~ 600 ° C., galvanizing alloyed, the alloying step of the galvannealed steel sheet,
　in (k) the galvannealed steel sheet 1 ° C. / s or more average cooling rate, the third step of cooling to a temperature range of 80 ~ 300 ° C.,
　and, tempering step, which holds 1s or 48h or less in a temperature range of 100 ~ 450 ° C. the (l) the galvannealed steel sheet
　wherein the above the steps (a) to (l) carried out sequentially, the production method of the galvannealed steel sheet.