[0001]The present invention relates to a high-strength steel sheet and a high-strength galvanized steel sheet. BACKGROUND ART [0002]In recent years, in a high-strength steel sheet used for automobiles and the like, a demand for further improvement in impact resistance has been increasing. Further, in the high-strength steel sheet used for automobiles and the like, formability such as ductility or hole expandability has also been required in order to obtain a complicated member shape. [0003] For example, a high-strength cold-rolled steel sheet aiming at improvements in ductility and hole expandability has been described in Patent Literature 1, a high-strength steel sheet aiming at improvements in toughness and HAZ toughness has been described in Patent Literature 2, and a high-strength steel sheet aiming at improvements in shape fixability and workability has been described in Patent Literature 3. Further, a high-strength hot-dip galvanized steel sheet aiming at an improvement in bake hardenability while securing ductility has been described in Patent Literature 4, a high-strength hot-dip galvanized steel sheet aiming at an improvement in -mechanical cutting property while securing ductility has been described in Patent Literature 5, and a high-strength hot-dip galvanized steel sheet aiming at an improvement in workability has been described in Patent Literature 6. [0004] However, the conventional high-strength steel sheets fail to achieve excellent formability and impact resistance both that are required recently. CITATION LIST PATENT LITERATURE [0005] Patent Literature 1: Japanese Patent No.5463685 Patent Literature 2: Japanese Laid-open Patent Publication No. 2014-9387 Patent Literature 3; International Publication Pamphlet No. WO2013/018741 Patent Literature 4: International Publication Pamphlet No. WO2013/047821 Patent Literature 5: International Publication Pamphlet No. WO2013/047739 Patent Literature 6: Japanese Laid-open Patent Publication No. 2009-209451 SUMMARY OF INVENTION TECHNICAL PROBLEM [0006] An object oftire present invention is to provide a high-strength steel sheet and a high-strength galvanized steel sheet that are capable of obtaining excellent formability and impact resistance. SOLUTION TO PROBLEM [0007] The present inventors conducted earnest examination in order to solve the above-described problems. As a result, it became clear that it is important to arrange coarse retained austenite, which becomes a starting point of destruction, so as not to be adjacent to tempered martensite and fresh martensite as much as possible, as well as to make a chemical composition and volume fractions of a microstructure appropriate. Further, it became clear that suppression of uneven arrangement of Mn in a manufacturing process is extremely important for control of such an arrangement of retained austenite, tempered martensite, and fresh martensite. [0008] In general, in order to control the volume fractions of a microstructure of a high-strength steel sheet, a parent phase is composed of austenite grains during retention at around the maximum heating temperature of annealing after cold roiling, and cooling conditions thereafter and the like are adjusted. In a region where Mn is arranged unevenly, the austenite grains become coarse during retention, to obtain a structure in which coarse retained austenite and fresh martensite mixedly exist adjacently to each other during cooling. In tempering after annealing, almost the whole of the fresh martensite turns into tempered martensite, but the arrangement in the structure does not change in the tempering, so that in a microstructure after the tempering, the coarse retained austenite and the tempered martensite or the fresh martensite mixedly exist adjacently to each other. For example, the coarse retained austenite exists so as to be surrounded by the tempered martensite. In the high-strength steel sheet having such a microstructure, destruction starting from an interface between the coarse retained austenite andj the tempered martensite or the fresh martensite is likely to occur. [0009] Conventionally, conditions of the annealing and conditions of the tempering have been proposed in order to make the chemical composition and the volume fractions of the microstructure appropriate, but only adjustment of these conditions fails to control the uneven arrangement of Mn because the uneven arrangement of Mn progresses with a phase transformation at relatively high temperature. As a result that the present inventors conducted earnest examination in order to suppress the uneven arrangement of Mn, they found out that the uneven arrangement of Mn can be suppressed in a cooling process of hot rolling and a heating process of annealing, and the suppression of the uneven arrangement of Mn enables the parent phase to be composed of finely and homogeneously dispersed austenite grains during retention at around the maximum heating temperature. The parent phase is composed of finely and homogeneously dispersed austenite grains, and thereby the retained austenite and the tempered martensite or the fresh martensite are separated from each other by bainite, ferrite, and the like to make it difficult for the retained austenite to be adjacent to the tempered martensite or the fresh martensite after a phase transformation caused by cooling. Even when the retained austenite surrounded by the tempered martensite exists, the retained austenite does not easily become a starting point of destruction because of being fine. The present inventors devised the following various aspects of the invention based on such findings. [0010] (1) A high-strength steel sheet includes: a chemical composition represented by, in mass%, C: 0.075 to 0.400%, Si: 0.01 to 2.50%, Mn: 0.50^0-3.-50%, P: 0.1000% or less, S: 0.0100% or less, Al: 2.000% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0.000 to 0.200%, Nb: 0.000 to 0.100%, V: 0.000 to 0.500%, Cr: 0.00 to 2.00%, Ni: 0.00 to 2.00%, Cu: 0.00 to 2.00%, Mo: 0.00 to 1.00%, B: 0.0000 to 0.0100%, W: 0.00 to 2.00%, one type or two types or more selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM: 0.0000 to 0.0100% in total, a balance: Fe and impurities, and a parameter Q0 expressed by (Expression 1): 0.35 or more; and a microstructure represented by, in a 1/8 thickness to 3/8 thickness range with 1/4 thickness of a sheet thickness from a surface being a middle, in volume fraction, ferrite: 85% or less, bainite: 3% or more and 95% or less, tempered martensite: 1% or more and 80% or less, retained austenite: 1% or more and 25% or less, pearlite and coarse cementite: 5% or less in total, and fresh martensite: 5% or less, in which a solid-solution carbon content in the retained austenite is 0.70 to 1.30 mass%, in all grain boundaries of retained austenite grains having an aspect ratio of 2.50 or less and a circle-equivalent diameter of 0.80 jim or more, a proportion of interfaces with the tempered martensite or the fresh martensite is 75% or less. Q0 = Si + O.IMn + 0.6A1 « • • (Expression 1) (In (Expression 1), Si, Mn, and Al are set to the contents of the respective elements in mass%). [0011] (2) The high-strength steel sheet according to (1), further contains, in mass%, one type or two types or more selected from the group consisting of Ti: 0.001 to 0.200%, Nb: 0.001 to 0.100%, and V: 0.001 to 0.500%. [0012] (3) The high-strength steel sheet according to (1) or (2), further contains, in mass%, one type or two types or more selected from the group consisting of Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Mo: 0.01 to 1.00%, B: 0.0001 to 0.0100%, and W: 0.01 to 2.00%. [0013] (4) The high-strength steel sheet according to any one of (1) to (3), further contains, in mass%, 0.0001 to 0.0100% in total of one type or two types or more selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM. [0014] (5) The high-strength steel sheet according to any one of (1) to (4), in which a density of the retained austenite grains having an aspect ratio of 2.50 or less and a circle-equivalent diameter of 0.80 |im or more is 5.0 x 10 piece/m or less. [0015] (6) A high-strength galvanized steel sheet, includes: a galvanized layer formed on a surface of the high-strength steel sheet according to any one of (1) to (5). [0016] (7) The high-strength galvanized steel sheet according to (6), in which an Fe content in the galvanized layer is 3,0 mass% or less. [0017] (8) The high-strength galvanized steel sheet according to (6), in which an Fe content in the galvanized layer is 7.0 mass% or more and 13.0 mass% or less. ADVANTAGEOUS EFFECTS OF INVENTION [0018] According to the present invention, it is possible to obtain excellent formability and impact resistance because the relationship between retained austenite and tempered martensite or fresh martensite and the like are appropriate. DESCRIPTION OF EMBODIMENTS [0019] Hereinafter, there will be explained embodiments of the present invention. [0020] (First embodiment) First, there will be explained a chemical composition of a high-strength steel sheet according to a first embodiment of the present invention. Although details will be described later, the high-strength steel sheet according to the first embodiment is manufactured by going through a hot rolling step, a pickling step, a cold rolling step, an annealing step, a bainite transformation step, a martensite transformation step, and a tempering step. Thus, the chemical composition of the high-strength steel sheet is suitable for not only properties of the high-strength steel sheet but also these processes. In the following explanation, "%" being the unit of the content of each element contained in the high-strength steel sheet means "mass%" unless otherwise noted. The high-strength steel sheet according to this embodiment includes a chemical composition represented by, in mass%, C: 0.075 to 0.400%, Si: 0.01 to 2.50%, Mn: 0.50 to 3.50%, P: 0.1000% or less, S: 0.0100% or less, Al: 2.000% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0.000 to 0.200%, Nb: 0.000 to 0.100%, V: 0.000 to 0.500%, Cr: 0.00 to 2.00%, Ni: 0.00 to 2.00%, Cu: 0.00 to 2.00%, Mo: 0.00 to 1.00%, B: 0.0000 to 0.0100%, W: 0.00 to 2.00%, one type or two types or more selected from the group consisting of Ca, Ce, Mg, Zr, La, and rare earth metal (REM): 0.0000 to 0.0100% in total, a balance: Fe and impurities, and a parameter QO expressed by (Expression 1): 0.35 or more. Examples of the impurities include ones contained in raw materials such as ore and scrap and ones contained in a manufacturing step. QO = Si + O.IMn + 0.6A1 • • • (Expression 1) (In (Expression 1), Si, Mn, and Al are the contents of the respective elements in mass%). [0021] (C: 0.075 lo 0.400%) C stabilizes austenitc to obtain retained austenite, to thereby increase strength and formability. When the C content is less than 0.075%, it is impossible to obtain the retained austenite and it is difficult to secure sufficient strength and formability. Thus, the C content is 0.075% or more. In order to obtain more excellent strength and formability, the C content is preferably 0.090% or more and more preferably 0.100% or more. On the other hand, whenthe C content*is greater than 0,400%, spot voidability deteriorates greatly. Thus, the C content is 0.400% or less. In order to obtain good spot weldability, the C content is preferably 0.320% or less and more preferably 0.250% or less. [0022] (Si: 0.01 to 2.50%) Si suppresses generation of iron-based carbide in the steel sheet, to thereby stabilize the retained austenite and increase strength and formability. When the Si content is less than 0.01%, a large amount of coarse iron-based carbide is generated in the bainite transformation step, leading to deterioration in strength and formability. Thus, the Si content is 0.01% or more. In order to obtain more excellent strength and formability, the Si content is preferably 0.10% or more and more preferably 0.25% or more. On the other hand, Si embrittles the steel and reduces impact resistance of the steel sheet. When the Si content is greater than 2.50%, the embrittiement is prominent and a trouble such as cracking of a cast slab is likely to occur. Thus, the Si content is 2.50% or less. In order to obtain good impact resistance, the Si content is preferably 2.25% or less and more preferably 2.00% or less. [0023] (Mn; 0.50 to 3.50%) Mn increases hardenabilily of the steel sheet to increase the strength. When the Mn content is less than 0.50%, a large amount of soft structure is formed during cooling after annealing, and thus it is difficult to secure a sufficiently high maximum tensile strength. Thus, the Mn content is 0.50% or more. In order to obtain a higher strength, the Mn content is preferably 0.80% or more and more preferably 1.00% or more. On the other hand, Mn embrittles the steel and deteriorates spot weldability. When the Mn content is greater than 3.50%, a coarse Mn concentrated portion is generated in a sheet thickness center portion of the steel sheet and the embrittlement is likely to occur, resulting in that a trouble such as cracking of a cast slab is likely to occur. Thus, the'Mn content is 3.50% or less. In order to obtain good spot weldability, the Mn content is preferably 3.20% or less and more preferably 3.00% or less. [0024] (P: 0.1000% or less) P is not an essential element, and is contained in the steel, for example, as an impurity. P makes the steel brittle and makes a welded portion generated by spot welding brittle, and thus the lower the P content is, the better it is. When the P content is greater than 0.1000%, the embrittlement is prominent and a trouble such as cracking of a slab is likely to occur. Thus, the P content is 0.1000% or less. In order to obtain an excellent welded joint strength by suppressing the embrittlement of the welded portion, the P content is preferably 0.0400% or less and more preferably 0.0200% or less. Reducing the P content is expensive, and when the P content is tried to be reduced down to less than 0.0001%, the cost increases greatly. Therefore, the P content may be 0.0001% or more, and is preferably 0.0010% or more in view of cost. [0025] (S: 0.0100% or less) S is not an essential element, and is contained in the steel, for example, as an impurity. S couples with Mn to form coarse MnS to then reduce formability such as ductility, hole expandability, stretch flangeability, or bendability and deteriorate spot weldability, and thus the lower the S content is, the better it is. When the S content is greater than 0.0100%, the reduction in formability is prominent. Thus, the S content is 0.0100% or less. In order to obtain good spot weldability, the S content is preferably 0.0070% or less, and more preferably 0.0050% or less. Reducing the S content is expensive, and when the S content is tried to be reduced down to less than 0.0001%, the cost increases greatly. Therefore, the S content may be 0.0001% or more, and is preferably 0.0003% or more and more preferably 0.0006% or more in view of cost. [0026] (Al: 2.000% or less) - Al makes the steel brittle and deteriorates spot weldability, and thus the lower the Al content is, the better it is. When the Al content is greater than 2.000%, the embrittlement is prominent and a trouble such as cracking of a slab is likely to occur. Thus, the Al content is 2.000% or less. In order to obtain good spot weldability, the Al content is preferably 1.500% or less and more preferably 1.300% or less. Reducing the Al content is expensive, and when the Al content is tried to be reduced down to less than 0.001%, the cost increases greatly. Therefore, the Al content may be 0.001% or more. Al is effective as a deoxidizing material, and in order to sufficiently obtain the effect of deoxidation, the Al content is preferably 0.010% or more. Al suppresses generation of coarse carbide, and thus may be contained for the purpose of stabilizing the retained austenite. In order to stabilize the retained austenite, the Al content is preferably 0.100% or more and more preferably 0.250% or more. [0027] (N: 0.0100% or less) N is not an essential element, and is contained in the steel, for example, as an impurity. N forms coarse nitrides to reduce formability such as ductility, hole expandability, stretch flangeability, or bendability and cause blowholes at the time of welding, and thus the lower the N content is, the better it is. When the N content is greater than 0.0100%, the deterioration in formability is prominent. Thus, the N content is 0.0100% or less. In order to more securely suppress the blowholes, the N content is preferably 0.0075% or less, and more preferably 0.0060% or less. Reducing the N content is expensive, and when the N content is tried to be reduced down to less than 0.0001%, the cost increases greatly. Therefore, the N content may be 0.0001% or more, and is preferably 0.0003% or more and more preferably 0.0005% or more in view of cost. [0028] (O: 0.0100% or less) O is not ^n essential element, and is contained in the steel, for example, as an impurity. O forms oxides to reduce formability such as ductility, hole expandability, stretch flangeability, or bendability, and thus the lower the O content is, the better it is. When the O content is greater than 0.0100%, the deterioration in formability is prominent. Thus, the O content is 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0030% or less. Reducing the O content is expensive, and when the O content is tried to be reduced down to less than 0.0001%, the cost increases greatly. Therefore, the O content may be 0.0001% or more. [0029] (Parameter Q0: 0.35 or more) Although details will be described later, there is a concern that during a heat treatment in the annealing step after the martensite transformation step, the retained austenite decomposes into bainite, pearlite, or coarse ccmentite. Si, Mn, and Al are elements particularly important for suppressing the decomposition of the retained austenite to increase formability, and when the parameter Q0 expressed by (Expression 1) is less than 0.35, it is impossible to obtain the above-described effect. Thus, the parameter QO is set to 0.35 or more, preferably set to 0.60 or more, and more preferably set to 0.80 or more. Q0 = Si + O.IMn + 0.6A1 * • * (Expression 1) (In (Expression 1), Si, Mn, and Al are the contents of the respective elements in mass%). [0030] Ti, Nb, V, Cr, Ni, Cu, Mo, B, W, Ca, Ce, Mg, Zr, La, and REM are not essential elements, but are arbitrary elements that may be appropriately contained, up to a predetermined amount as a limit, in the high-strength steel sheet. [0031] (Ti: 0.000 to 0.200%) Ti contributes to strength increase of the steel sheet by precipitate strengthening, fine grain strengthening by growth suppression of ferrite crystal grains, and dislocation strengthening through suppression of reerystallization. A desired purpose is achieved unless Ti is contained, but in order to sufficiently obtain these effects, the Ti content is preferably 0.001% or more and more preferably 0.010% or more. However, when the Ti content is greater than 0.200%, a carbonitride of Ti precipitates excessively, leading to deterioration in formability in some cases. Therefore, the Ti content is 0.200% or less. In view of formability, the Ti content is preferably 0.120% or less. [0032] (Nb: 0.000 to 0.100%) Nb contributes to strength increase of the steel sheet by precipitate strengthening, fine grain strengthening by growth suppression of ferrite crystal grains, and dislocation strengthening through suppression of reerystallization. A desired purpose is aehieved unless Nb is contained, but in order to sufficiently obtain these effects, the Nb content is preferably 0.001% or more and more preferably 0.005% or more. However, when the Nb content is greater than 0.100%, a carbonitride of Nb precipitates excessively, leading to deterioration in formability in some cases. Therefore, the Nb content is 0.100% or less. In view of formability, the Nb content is preferably 0.060% or less. [0033] (V: 0.000 to 0.500%) V contributes to strength increase of the steel sheet by precipitate strengthening, fine grain strengthening by growth suppression of ferrite crystal grains, and dislocation strengthening through suppression of recrystallization. A desired purpose is achieved unless V is contained, but in order to sufficiently obtain these effects, the V content is preferably 0.001% or more and more preferably 0.010% or more. However, when the V content is greater than 0.500%, a carbonitridc of V precipitates excessively, leading to deterioration in formability in some cases. Therefore, the V content is 0.500% or less and more preferably 0.350% or less. [0034] (Cr: 0.00 to 2.00%) Cr increases hardenability and is effective for increasing strength. A desired purpose is achieved unless Cr is contained, but in order to sufficiently obtain these effects, the Cr content is preferably 0.01% or more and more preferably 0.10% or more. However, when the Cr content is greater than 2.00%, workability in hot working is impaired, leading to a decrease in productivity in some cases. Therefore, the Cr content is 2.00% or less and more preferably 1.20% or less. [0035] (Ni: 0.00 to 2.00%) Ni suppresses phase transformation at high temperature and is effective for increasing strength. A desired purpose is achieved unless Ni is contained, but in order to sufficiently obtain these effects, the Ni content is preferably 0.01% or more and more preferably 0.10% or more. However, when the Ni content is greater than 2.00%, weldability is sometimes impaired. Therefore, the Ni content is 2.00% or less and preferably 1.20% or less. [0036] (Cu: 0.00 to 2.00%) Cu exists as fine grains in the steel, to thus increase strength. A desired purpose is achieved unless Cu is contained, but in order to sufficiently obtain these effects, the Cu content is preferably 0.01% or more and more preferably 0.10% or more. However, when the Cu content is greater than 2.00%, weldability is sometimes impaired. Therefore, the Cu content is 2.00% or less and preferably 1.20% or less. [0037] (Mo: 0.00 to 1.00%) Mo suppresses phase transformation at high temperature and is effective for increasing strength. A desired purpose is achieved unless Mo is contained, but in order to sufficiently obtain these effects, the Mo content is preferably 0.01% or more and more preferably 0.05% or more. However, when the Mo content is greater than 1.00%, workability in hot working is impaired, leading to a decrease in productivity in some cases. Therefore, the Mo content is 1.00% or less and more preferably 0.50% or less. [0038] (B: 0.0000 to 0.0100%) B suppresses phase transformation at high temperature and is effective for increasing strength. A desired purpose is achieved unless B is contained, but in order to sufficiently obtain these effects, the B content is preferably 0.0001% or more and more preferably 0.0005% or more. However, when the B content is greater than 0.0100%, workability in-hot working is impaired, leading to a decrease in productivity in some cases. Therefore, the B content is 0.0100% or less and more preferably 0.0050% or less. [0039] (W: 0.00 to 2.00%) W suppresses phase transformation at high temperature and is effective for increasing strength. A desired purpose is achieved unless W is contained, but in order to sufficiently obtain these effects, the W content is preferably 0.01% or more and more preferably 0.10% or more. However, when the W content is greater than 2.00%, workability in hot working is impaired, leading to a decrease in productivity in some cases. Therefore, the W content is 2.00% or less and more preferably 1.20% or less. [0040] (One type or two types or more selected from the group consisting of Ca, Ce, Mg, Zr, la, and REM: 0.0000 to 0.0100% in total) REM refers to an element belonging to the lanthanoid series. For example, REM or Ce is added in misch metal, and sometimes contains elements of the lanthanoid series other than La and Ce in a complex form. The effects of the present invention are exhibited even when elements of the lanthanoid series other than La and Ce are contained. The effects of the present invention are exhibited even when metals La and Ce are contained. [0041] Ca, Ce, Mg, Zr, La, and REM are effective for an improvement in formability. A desired purpose is achieved unless Ca, Ce, Mg, Zr, La, and REM are contained, but in order to sufficiently obtain these effects, the total content of Ca, Ce, Mg, Zr, La, and REM is preferably 0.0001% or more and more preferably 0.0010% or more. However, when the total content of Ca, Ce, Mg, Zr, La, and REM is greater than 0.0100%, ductility is liable to be impaired. Therefore, the total content of Ca, Ce, Mg, Zr, La, and REM is 0.0100% or less and preferably 0.0070% or less. [0042] As the impurities, 0.0100% or less in total of H, Na, CI, Sc, Co, Zn, Ga, Ge, As, Se, Y, Zr, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, Cs, Hf, Ta, Re, Os, Ir, Pt, Au, and Pb is allowed to be contained. [0043] Next, there will be explained a microstructure of the high-strength steel sheet according to the first embodiment. The high-strength steel sheet according to this embodiment includes a microstructure represented by, in a 1/8 thickness to 3/8 thickness range with 1/4 thickness of a sheet thickness from a surface being a center, in volume fraction, ferrite: 85% or less, bainite: 3% or more and 95% or less, tempered martensite: 1% or more and 80% or less, retained austenite: 1% or more and 25% or less, pearlite and coarse cementite: 5% or less in total, and fresh martensite: 5% or less. [0044] (Ferrite: 85% or less) Ferrite has excellent ductility. However, the ferrite has low strength, and thus when the volume fraction of the ferrite is greater than 85%, it is impossible to obtain a sufficient maximum tensile strength. Therefore, the volume fraction of the ferrite is 85% or less. In order to obtain a higher maximum tensile strength, the volume fraction of the ferrite is preferably 75% or less and more preferably 65% or less. A desired purpose is achieved unless the ferrite is contained, but in order to obtain good ductility, the volume fraction of the ferrite is preferably 5% or more and more preferably 10% or more. [0045] (Bainite:-3% «r more -and 95% or less) Bainite is a structure excellent in balance between strength and formability. When the volume fraction of the bainite is less than 3%, it is impossible to obtain a good balance between strength and formability. Thus, the volume fraction of the bainite is 3% or more. The volume fraction of the retained austenite increases with generation of the bainite, and thus the volume fraction of the bainite is 7% or more and more preferably 10% or more. On the other hand, when the volume fraction of the bainite is greater than 95%, it becomes difficult to secure both the tempered martensite and the retained austenite, to thus fail to obtain a good balance between strength and formability. Therefore, the volume fraction of the bainite is 95% or less, and in order to obtain a more excellent balance between strength and formability, the volume fraction of the bainite is 85% or less and more preferably 75% or less. [0046] Incidentally, the bainite in the present invention includes bainitic ferrite made of lath-shaped body-centered cubic (bec) crystals and not containing iron-based carbides, granular bainite made of fine bcc crystals and coarse iron-based carbides, upper bainite made of lath-shaped bcc crystals and coarse iron-based carbides, and lower bainite made of plate-shaped bcc crystals and fine iron-based carbides aligned in parallel thereinside. [0047] (Tempered martensite: 1% or more and 80% or less) Tempered martensite greatly improves tensile strength of the steel sheet without impairing impact resistance. When the volume fraction of the tempered martensite is less than 1%, it is impossible to obtain a sufficient tensile strength. Thus, the volume fraction of the tempered martensite is 1% or more. In order to obtain a higher tensile strength, the volume fraction of the tempered martensite is preferably 5% or more and more preferably 10% or more. On the other hand, when the volume fraction of the tempered martensite is greater than 80%, interfaces between the tempered martensite and the retained austenite increase excessively, leading to deterioration in impact resistance. Thus, the volume fraction of the tempered martensite is 80% or less. In order to obtain more excellent impact resistance, the volume fraction of the tempered martensite is 73% or less and more preferably 65% or less. [0048] (Retained austenite: 1% or more and 25% or less) Retained austenite increases a balance between strength and ductility. When the volume fraction of the retained austenite is less than 1%, it is impossible to obtain a good balance between strength and ductility. Thus, the volume fraction of the retained austenite is 1% or more. In order to obtain good formability, the volume fraction of the retained austenite is 2.5% or more and more preferably 4% or more. On the other hand, in order to obtain greater than 25% of the volume fraction of the retained austenite, C of the degree to which weldability is impaired greatly is needed. Therefore, the volume fraction of the retained austenite is 25% or less. The retained austenite is transformed into hard martensite by receiving an impact, to work as a starting point of destruction. When the volume fraction of the retained austenite is greater than 21%, the martensite transformation is likely to occur. Thus, the volume fraction of the retained austenite is preferably 21% or less and more preferably 17% or less. [0049] As the solid-solution carbon content in the retained austenite is higher, stability of the retained austenite is higher, resulting in that excellent impact resistance can be obtained. When the solid-solution carbon content in the retained austenite is less than 0.70 mass%, it is impossible to sufficiently obtain this effect. Thus, the solid-solution carbon content in the retained austenite is 0.70 mass% or more. The solid-solution carbon content in the retained austenite is preferably 0.77 mass% or more and more preferably 0.84 mass% or more. On the other hand, when the solid-solution carbon content in the retained austenite increases excessively, the transformation from the retained austenite into the martensite caused by a tensile deformation does not sufficiently progress, and work hardening ability decreases on the contrary in some cases. When the solid-solution carbon content in the retained austenite is greater than 1.30 mass%5 it is impossible to obtain sufficient work hardening ability. Thus, the solid-solution carbon content in the retained austenite is 1.30 mass% or less, preferably 1.20 mass% or less, and more preferably 1.10 mass% or less. [0050] Retained austenite grains having an aspect ratio of 2.50 or less and a circle-equivalent diameter of 0.80 (im or more are transformed into hard martensite by receiving an impact to easily work as a starting point of destruction. Particularly, in the vicinity of an interface between the retained austenite grains corresponding to the above and the tempered martensite or the fresh martensite, the retained austenite is bound by the hard tempered martensite or the fresh martensite, and on the retained austenite side, a high strain is caused with deformation, and the retained, austenite is transformed into martensite easily. Therefore, the interface between the retained austenite and the tempered martensite or the fresh martensite peels off and destruction is tikely to occur. [0051] Then, when to all grain boundaries of the retained austenite grains having an aspect ratio of 2.SO or less and a circle-equivalent diameter of 0.80 jim or more, the proportion of interfaces with the tempered martensite or the fresh martensite, namely the proportion of portions in contact with the tempered martensite or the fresh martensite is greater than 75%, the destruction caused by destruction of the interface is prominent. Thus, in this embodiment, this proportion is 75% or less. In this embodiment, this proportion is 75% or less, so that it is possible to suppress the destruction starting from the above-described retained austenite grains and increase the impact resistance. In order to obtain more excellent impact resistance, this proportion is preferably 60% or less and more preferably 40% or less. [0052] The coarse retained austenite grains to be a starting point of destruction are reduced, and thereby the impact resistance further improves. From this view, the density of the retained austenite grains having an aspect ratio of 2.50 or less and a circle-equivalent diameter of 0.80 urn or more is preferably 5.0 x 1010 piece/in2 or less and more preferably 3.0 x 1010 piece/m or less. [0053] (Fresh martensite: 5% or less) Fresh martensite greatly improves tensile strength, but becomes a starting point of destruction, leading to deterioration in impact resistance. When the volume fraction of the fresh martensite is greater than 5%, the deterioration in impact resistance is prominent. Therefore, the volume fraction of the fresh martensite is 5% or less. In order to obtain excellent impact resistance, the volume fraction of the fresh martensite is preferably 1% or less and more preferably 0%. [0054] (Pearlite and coarse cementite: 5% or less in total) Pearlite and coarse cementite deteriorate ductility. When the volume fraction of the pearlite and the coarse cementite is greater than 5% in total, the deterioration in ductility is prominent. Therefore, the volume fraction of the pearlite and the coarse cementite is 5% or less in total. Here, in this embodiment, the coarse cementite means cementite having a circle-equivalent diameter of 1.0 pm or more. An electron microscope observation makes it possible to measure the circle-equivalent diameter of the cementite easily and judge whether or not this cementite is coarse. [0055] The volume fractions of the ferrite, the bainite, the tempered martensite, the fresh martensite, the pearlite, and the coarse cementite can be measured by using the following method. A sample is taken from an observation surface that is a thicknesswise cross section parallel to the rolling direction of the steel sheet, the observation surface is polished and nital etched, and the 1/8 thickness to 3/8 thickness range with 1/4 thickness of the sheet thickness from the surface being a center is observed with a field emission scanning electron microscope (FE-SEM) and area fractions are measured, which can be assumed as the volume fractions. [0056] The volume fraction of the retained austenite is evaluated by an X-ray diffraction method. In the 1/8 thickness to 3/8 thickness range of the sheet thickness from the surface, a surface parallel to the sheet surface is mirror-finished and an area fraction of fee iron is measured by the X-ray diffraction method, which can be assumed as the volume fraction of the retained austenite. [0057] The solid-solution carbon content (Cy [mass%]) in the retained austenite can be found by using the following expression after an X-ray diffraction test is performed under the same condition as that of the volume fraction measurement of the retained austenite and an average lattice constant a [nm] of the retained austenite is found. Cy = 2.264 x 102 x (a - 0.3556) [0058] The aspect ratio, the circle-equivalent diameter, and the interfaces of the retained austenite grains are evaluated by performing a high-resolution crystal orientation analysis by a transmission EBSD method (electron back scattering diffraction method) by using a FE-SEM. In the 1/8 thickness to 3/8 thickness range of the sheet thickness, a thin piece parallel to the sheet surface is cut out, the thin piece is subjected to mechanical polishing and electrolytic polishing, and a periphery around a hole made in the thin piece is observed, thereby making it possible to accurately observe the fine retained austenite grains. Incidentally, "OIM Analysis 6.0" manufactured by TSL Corporation can be used for analysis of data obtained by the transmission EBSD method. The observation by the transmission EBSD method is performed by five or more regions each having a size of 2;0 * 1 La, and REM. 5.The high-strength steel sheet according to any one of claims 1 to 4, wherein a density of the retained austenite grains having an aspect ratio of 2.50 or less and a circle-■equivalent diameter of 0.80 |xm or more is 5,0 x 1010 piece/m2 or less. 6.A high-strength galvanized steel sheet, comprising: a galvanized layer formed on a surface of the high-strength steel sheet according to any one of claims 1 to 5. 7.The high-strength galvanized steel sheet according to claim 6, wherein an Fe content in the galvanized layer is 3.0 mass% or less. 8.The high-strength galvanized steel sheet according to claim 6, wherein an Fe content in the galvanized layer is 7.0 mass% or more and 13.0 mass% or less.