DESCRIPTION TITLE OF THE INVENTION High-Strength Hot-Dip Galvanized Steel Sheet and 5 Process for Producing the Same TECHNICAL FIELD [00011 The present invention relates to a high-strength 10 (for example, a tensile strength of 980 MPa or more) hotdip galvanized steel sheet with excellent bendability, which is used for an automotive structural material and the like, and a process for producing the same. 15 BACKGROUND ART [00021 For the purpose of enhancing the fuel efficiency of an automobile and achieving collision safety, application of a high tensile strength steel sheet to a vehicle body 20 frame structure is proceeding, but on the other hand, the increase in the strength of a material involves a decrease in formability of the material, and therefore, the steel sheet used may be required to satisfy both high press workability and high strength. 25 In a high-strength steel sheet, a retained (or residual) austenite steel having retained austenite in the steel structure may be known to, despite high strength, exhibit very high elongation by making use of a TRIP effect, In order to more increase the elongation of 30 this retained austenite steel, for example, Patent Document 1 discloses a technique of ensuring uniform elongation by controlling two kinds of ferrite (bainitic ferrite and polygonal ferrite) while keeping the retained austenite fraction high. 35 Meanwhile, in forming a high-strength steel sheet having a tensile strength of 980 MPa or more, the work may be often performed mainly by bend forming but not draw forming that has prevailed in forming a low-strength steel sheet having a tensile strength of 440 MPa or less. Similarly to elongation, enhanced bendability may be required of also a high-strength sheet steel. 5 [0003] Conventionally, it has been known that V-bendability correlates with local ductility, and as a technique for enhancing the local ductility, Patent Document 1 discloses a technique of making the structure uniform and 10 increasing the strength by adding a precipitation strengthening component to a ferrite single phase, and Patent Document 2 discloses a technique of similarly making the structure uniform by a structure mainly composed of bainite. 15 Also, Patent Document 3 discloses a high-strength high-ductility hot-dip galvanized steel sheet containing, in terms of volume fraction, from 30 to 90% of a ferrite phase, 5% or more of bainite, 10% or less of martensite, and from 5 to 30% of a retained austenite phase. Patent 20 Document 4 discloses a high-strength cold-rolled steel sheet, where the density of dislocations contained in the steel sheet is 8x10'' (dislocations/rnm2) or less, and the static/dynamic ratio (=FS2/FS1) as a ratio between a quasi-static strength (FS1) at a strain rate of 0.0067 (s- 25 I ) , and a dynamic strength (FS2) at a strain rate of 1,000 (s-') is 1.05 or more. However, at present, higher strength and higher workability are required of also in the case of a highstrength steel sheet, and a technique capable of 3 0 satisfying this requirement and also of satisfying both of the elongation and V-bendability at a sufficiently high level is not known. RELATED ART 35 PATENT DOCUMENTS [0004] [Patent Document 11 JP-A (Japanese Unexamined Patent Publication; KOKAI) No. 2003-306746 [Patent Document 21 JP-A No. 4-88125 [Patent Document 31 JP-A No. 2005-133201 [Patent Document 41 JP-A No. 2002-30403 5 SUMMARY OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION [0005] The present invention has been made to solve 10 conventional problems, and an object of the present invention is to provide a high-strength hot-dip galvanized steel sheet excellent in elongation and Vbendability, which is a technique found from many diligent studies to enhance the V-bendability of a 15 retained austenite steel having a tensile strength of 980 MPa or more, and a production process therefor. MEANS FOR SOLVING THE PROBLEM [0006] 2 0 As a result of earnest study, the present inventors have found that increasing the stability of retained austenite more than ever by imparting a residual compression stress to the retained austenite phase may effectively act on the local bending deformation of the 25 tensile stress part outside bending and the compression stress part inside bending. [0007] As a result of further study based on the above discovery, the present inventors have further made 30 studies based on the finding above, as a result, it has been found that when the roll diameter, tension and number of passes in repeated bending during an over-aging (OA) treatment are optimally controlled so as to impart a residual compression stress to the retained austenite 3 5 phase, a sufficiently high effect may be obtained on elongation and V-bendability. The present inventors have still further made studies based on the finding above, as a result, it has been found that when control of conditions in repeated bending during an over-aging (OA) treatment is conformed to control of the enrichment into austenite phase and the grain size, the stability of 5 retained austenite phase can be increased and this may be more effective for elongation and V-bendability. [0008] According to the knowledge and investigations of the present inventors, the mechanism for providing the above- 10 described effect in the present invention may be presumed as follows. Thus, the retained austenite steel may be a highstrength steel sheet obtained by controlling ferrite transformation and bainite transformation during 15 annealing to increase the C concentration in austenite and thereby retain austenite in the steel structure of a product, and thanks to TRIP effect of the retained austenite, capable of exhibiting high elongation. However, because of a mixed structure, such a retained 2 0 austenite steel may be presumed not to be a steel excellent in bendability. Meanwhile, the present inventors have made various studies on the method for obtaining desired tensile strength, ductility, V-bendability and plating property 25 by performing, in a laboratory, melting, hot rolling, cold rolling, annealing and hot-dip galvanization of various steels changed in the amounts of C, Si and Mn with an attempt to achieve an effective action of TRIP effect on bendability. 30 As a result of these earnest study, it has been found that when not only various components effective for the above-described purpose are specified but also a residual compression stress is imparted to the retained austenite phase, the stability of retained austenite may 35 be increased more than ever and at the same time, an effective action may be exerted on the local bending deformation of the tensile stress part outside bending and the compression stress part inside bending. [00091 The present inventors have accomplished the present invention, based on the above discoveries. The present 5 invention may include, for example, the following embodiments. [OOlOl [I] A hot-dip galvanized steel sheet, which is a steel sheet comprising, in mass%, 10 C: from 0.10 to 0.4%, Si: from 0.01 to 0.5%, Mn: from 1.0 to 3.0%, 0: 0.006% or less, P: 0.04% or less, S: 0.01% or less, Al: from 0.1 to 3.0%, N: 0.01% or less, and Si+A1>0.5%, with the balance being Fe and unavoidable impurities, 20 wherein the steel sheet is a high-strength hot-dip galvanized steel sheet comprising, as the main phase, in terms of volume fraction, 40% or more of ferrite and from 8 to 60% of retained austenite, and the balance structure is composed of any one member or trio or more members of 25 bainite, martensite and pearlite, wherein out of the austenite, an austenite grain having an average residual stress OR satisfying formula (1) accounts for 50% or more: -400 MPa I OR I200 MPa (1) 3 0 and wherein the steel sheet has, on the surface thereof, a hot-dip galvanized layer comprising Fe in an amount of less than 7 mass%, with the balance being Zn, A1 and unavoidable impurities. 35 [00111 [2] The hot-dip galvanized steel sheet according to [I], wherein the average particle size of the austenite is 10 pm or less, the average C concentration in the austenite is 0.7% or more to 1.5% or less in terms of mass %. 5 [00121 [3] The hot-dip galvanized steel sheet according to [I] or [2], wherein the average aspect ratio with respect to the rolling direction of the austenite grain is from 0.5 to 0.95. 10 [0013] [4] The hot-dip galvanized steel sheet according to [I] or [2], wherein the steel sheet further comprises one member or two or more members of, in mass%, Mo: from 0.02 to 0.5, Nb: from 0.01 to 0.10%, Ti: from 0.01 to 0.20%, V: from 0.005 to 0.10%, Cr: from 0.1 to 2.0%, Ca: from 0.0005 to 0.05%, Mg: from 0.0005 to 0.05%, REM: from 0.0005 to 0.05%, Cu: from 0.04 to 2.0%, Ni: from 0.02 to l.O%, B: from 0.0003 to 0.007%. 25 [0014] 151 A process for producing a hot-dip galvanized steel sheet, comprising subjecting a steel material comprising, in mass%, C: from 0.10 to 0.4%, Si: from 0.01 to 0.5%, Mn: from 1.0 to 3.0%, 0: 0.006% or less, P: 0.04% or less, S: 0.01% or less, Al: from 0.1 to 3.0%, N: 0.01% or less, and Si+Al>O.5%, with the balance being Fe and unavoidable impurities, to a hot rolling treatment at a hot-rolled slab temperature of 1,10O0C or more and a finishing temperature of 850 to 97OoC, cooling the steel sheet after the hot rolling to a 5 temperature region of 65OoC or less at 10 to 200°C/sec on average, and taking it up in a temperature range of 650°C or less, cold-rolling the steel sheet at a rolling reduction ratio of 40% or more, 10 annealing the steel sheet by setting the maximum temperature during annealing to be from 700 to 900°C, cooling the steel sheet to a temperature region of 350 to 550°C at an average cooling rate of 0 . 1 to 200°C/sec, and then holding it in the temperature region for 1 to 1,000 1 5 seconds, and immersing the steel sheet after holding in the temperature region in a hot-dip galvanizing bath and after the plating treatment, applying an alloying treatment at a temperature of 470 to 580°C, 20 wherein at the time of holding the steel sheet in a temperature region of 350 to 550°C, the steel sheet is repeatedly bent using a roll having a roll diameter of 50 to 2,000 mm to thereby impart a strain to the steel sheet, and 25 the longitudinal average stress applied to the steel sheet during the repeated bending is from 2 to 50 MPa. [61 The process for producing a hot-dip galvanized steel sheet according to 151, wherein the number of passes during the repeated bending is from 2 to 6. 30 EFFECT OF THE INVENTION [0015] The present invention can provide a hot-dip galvanized steel sheet having a high strength and being 35 excellent in the ductility and V-bendability. The production of the hot-dip galvanized steel sheet according to the present invention may be relatively easy and can be performed stably. Therefore, the hot-dip galvanized steel sheet may be optimally usable particularly as a steel sheet for automobiles in recent 5 years, which is intended for attaining weight reduction. As a result, the industrial value thereof may be remarkably high. [00161 10 [Fig. 11 Fig. 1 is a graph showing a relationship between the residual stress in retained austenite phase and the minimum bend radius R. [Fig. 21 Fig. 2 is a graph showing the range where the average residual stress OR of an austenite grain 15 satisfies formula (1). [Fig. 31 Fig. 3 is a graph showing a relationship between the percentage of austenite grain satisfying formula (1) and the minimum bend radius R. [Fig. 41 Fig. 4 is a graph showing a relationship 20 between the average grain size of retained austenite and the minimum bend radius R. [Fig. 51 Fig. 5 is a graph showing a relationship between the aspect ratio of retained austenite grain and the minimum bend radius R. 25 [Fig. 61 Fig. 6 is a graph showing a relationship between the C concentration and the minimum bend radius R. MODES FOR CARRYING OUT THE INVENTION 30 [00171 The high-strength thin steel sheet of the present invention may be the result of attention focused on increasing the stability of retained austenite phase in a retained austenite steel. The present invention has been 35 achieved, as described above, based on finding that by controlling the residual stress and aspect ratio of the retained austenite phase, the stability can be increased to an extreme and all of strength, elongation and Vbendability can be satisfied at a high level. [0018] The structure in the hot-dip galvanized steel sheet 5 of the present invention must be mainly composed of a ferrite phase and a bainite phase and contain 3% or more of a retained austenite phase. In the case of intending to achieve a higher strength, the structure may contain martensite, but if the structure is not mainly composed 10 of a ferrite phase and a bainite phase, elongation may be likely to significantly decrease. [00191 The residual stress in the retained austenite phase may be one of most important factors in the present 15 invention. As shown in Fig. 1, when the residual stress in the retained austenite phase is lower, particularly, is 15 MPa or less, the minimum bend radius R may become smallest. A residual compression stress may be imparted to individual retained austenite grains in the production 20 process, whereby martensite transformation during work may be retarded, as a result, the stability of the whole phase may be increased. In order to achieve this effect, as shown in Fig. 2, Xr may become minimum in the range where the average 25 residual stress OR of the austenite grain satisfies formula (1). Also, as shown in Fig. 3, when the austenite grain satisfying formula (1) accounts for 50% or more, the minimum bend radius R may stably become smallest. 30 -400 MPaO.5%, with the balance being Fe and unavoidable impurities, 15 wherein the steel sheet is a high-strength hot-dip galvanized steel sheet comprising, as the main phase, in terms of volume fraction, 40% or more of ferrite and from 8 to 60% of retained austenite, and the balance structure is composed of any one member or two or more members of 20 bainite, martensite and pearlite, wherein out of the austenite, an austenite grain having an average residual stress OR satisfying formula (1) accounts for 50% or more: -400 MPa < OR I200 MPa (1 25 and wherein the steel sheet has, on the surface thereof, a hot-dip galvanized layer comprising Fe in an amount of less than 7 mass%, with the balance being Zn, A1 and unavoidable impurities. 30 [Claim 21 The hot-dip galvanized steel sheet according to claim 1, wherein the average particle size of the austenite is 10 pm or less, the average C concentration in the austenite is 0.7% or more to 1.5% or less in terms 3 5 of mass %. [Claim 31 The hot-dip galvanized steel sheet according to claim 1 or 2, wherein the average aspect ratio with respect to the rolling direction of the austenite grain is from 0.5 to 0.95. 5 [Claim 41 The hot-dip galvanized steel sheet according to claim 1 or 2, wherein the steel sheet further comprises one member or two or more members of, in mass%, Mo: from 0.02 to 0.5, Nb: from 0.01 to 0.10%, Ti: from 0.01 to 0.20%, V: from 0.005 to 0.10%, Cr: from 0.1 to 2.0%, Ca: from 0.0005 to 0.05%, Mg: from 0.0005 to 0.05%, REM: from 0.0005 to 0.05%, Cu: from 0.04 to 2.0%, Ni: from 0.02 to 1.0%, B: from 0.0003 to 0.007%. 2 0 [Claim 51 A process for producing a hot-dip galvanized steel sheet, comprising subjecting a steel material comprising, in mass%, C: from 0.10 to 0.4%, Si: from 0.01 to 0.5%, Mn: from 1.0 to 3.0%, 0: 0.006% or less, P: 0.04% or less, S: 0.01% or less, Al: from 0.1 to 3.0%, N: 0.01% or less, and Si+A120.5%, with the balance being Fe and unavoidable impurities, to a hot rolling treatment at a hot-rolled slab temperature of 1,10O0C or more and a 35 finishing temperature of 850 to 970°C, cooling the steel sheet after the hot rolling to a temperature regioh of 650°C or less at 10 to 200°C/sec on average, and taking it up in a temperature range of 650°C or less, cold-rolling the steel sheet at a rolling reduction 5 ratio of 40% or more, annealing the steel sheet by setting the maximum temperature during annealing to be from 700 to 900°C, cooling the steel sheet to a temperature region of 350 to 550°C at an average cooling rate of 0.1 to 200°C/sec, and 10 then holding it in the temperature region for 1 to 1,000 seconds, and immersing the steel sheet after holding in the temperature region in a hot-dip galvanizing bath and after the plating treatment, applying an alloying 15 treatment at a temperature of 470 to 580°C, wherein at the time of holding the steel sheet in a temperature region of 350 to 550°C, the steel sheet is repeatedly bent using a roll having a roll diameter of 50 to 2,000 mrn to thereby impart a strain to the steel 2 0 sheet, and the longitudinal average stress applied to the steel sheet during the repeated bending is from 2 to 50 MPa. ~.~.. [Claim 61 The process for producing a hot-dip galvanized steel 25 sheet according to claim 5, wherein the number of passes during the repeated bending is from 2 to 6,~- ~-