The present invention relates to a steel sheet and an impact absorbing member, and to a method for its manufacture. More specifically, the present invention relates to a steel sheet having a high effective flow stress and being suitable as a starting material for an impact absorbing member in which occurrence 10 of cracking when subjected to an impact load is suppressed, a method for its manufacture, and an impact absorbing member made of the steel sheet. Background Art In recent years, in order to protect global environmental, weight reduction of 15 automobile bodies is demanded as a measure to decrease the amount of COz discharged from automobiles. For the purpose, increases in the strength of steel sheets for automobiles are required. This is because increase in the strength of steel sheet will allow decrease of the thickness of steel sheets for automobiles, thereby decreasing the weight of automotive bodies. 20 On the other hand, there are increased demands by society for safety of automobiles in collisions. Accordingly, there is a need for not only simply increasing the strength of steel sheet, but also developing a steel sheet having improved impact resistance upon collision during driving. Since each portion of a member for an automobile is subjected to deformation at a high strain rate of 25 several 10s to 103/s in collision, a high strength steel sheet having improved dynamic strength properties is required for automobile use. As a high strength steel sheet having improved dynamic strength properties, there are known high-strength multi-phase structure steel sheets including a lowalloy TRIP steel sheet (strain induced transformation type high-strength steel sheet) 30 which has a high static-dynamic difference (difference between static strength and dynamic strength) and a multi-phase structure steel sheet having a second phase primarily including martensite. Regarding the low-alloy TRIP steel sheet, for example, Patent Document 1 discloses a strain induced transformation-type high-strength steel sheet having improved dynamic deformation properties and for absorbing automobile collision energy. 5 Prior art examples relating to the high-strength multi-phase structure steel sheet having a second phase primarily including martensite include the following Patent Documents. Patent Document 2 discloses a high-strength steel sheet having improved balance of strength and ductility, and a static-dynamic difference of at least 170 10 MPa, and a method for its manufacture, wherein the steel sheet has a multi-phase structure comprising a ferrite phase and a hard second phase dispersed therein, in which the average grain diameter ds of nano crystal grains having a grain diameter of at most 1.2 pm and an average grain diameter dL of micro crystal grains having a grain diameter exceeding 1.2 pm in the ferrite phase satisfy a relationship of dL/ds 15 2 3 . Patent Document 3 discloses a hot-rolled steel sheet having a high staticdynamic ratio, and a method for producing the same, wherein the steel sheet has a dual-phase structure of martensite having an average grain diameter of at most 3 pm and ferrite having an average grain diameter of at most than 5 pm. 20 Patent Document 4 discloses a cold-rolled steel sheet having improving impact absorbing properties, and a method for its manufacture, wherein the steel sheet has a dual-phase structure, which contains at least 75% of ferrite phase having an average grain diameter of at most 3.5 pm, the remainder being tempered martensite. 2 5 Patent Document 5 discloses a cold-rolled steel sheet having a static-dynamic difference of at least 60 MPa at a strain rate of 5 x 1 o2 to 5 x 1 03/s, and a method for its manufacture, wherein the steel sheet is made to have a dual-phase structure of ferrite and martensite by pre-straining. Patent Document 6 discloses a high-strength hot-rolled steel sheet having 30 improved impact resistant properties, wherein the steel sheet has a dual-phase structure of at least 85% of bainite and a hard phase such as martensite. Citation List Patent Document Patent Document 1 : Japanese Patent Laid-Open No. 1 1-80879 Patent Document 2: Japanese Patent Laid-Open No.2006- 16 1077 5 Patent Document 3 : Japanese Patent Laid-Open No.2004-84074 Patent Document 4: Japanese Patent Laid-Open No.2004-27785 8 Patent Document 5 : Japanese Patent Laid-Open No.2000- 173 85 Patent Document 6: Japanese Patent Laid-Open No. 1 1-269606 10 Summary of Invention To improve impact absorbing properties of an impact absorbing member, it is effective to increase the strength of the steel sheet of the starting material for the impact absorbing member. That is, increasing the strength of steel sheet allows not only the decrease of thickness (decrease of weight), but also the increase of 15 absorbed impact energy. This is because as the strength of the starting material for the steel sheet increases, flow stress required for plastic deformation increases. Since, an impact absorbing member generally absorbs energy produced by a collision through its plastic deformation caused by the collision, increasing its strength tends to increase the impact absorbing capability. 20 However, the impact energy which can be absorbed by an impact absorbing member greatly depends on the thickness of the steel sheet of the starting material. This is obvious, for example, from the fact that the following relationship about an average load (Fave) that determines the absorption of impact energy of steel sheet holds as shown in Journal of Japan Society for Technology of Plasticity, vol. 46, 25 N0.534, p.641-645. Fave a (ayat2)/4 (Where, ay: effective flow stress, and t: sheet thickness.) The effective flow stress means a flow stress at a particular value of strain. That is, the average load (Fave) is in direct proportion to the square of the 30 sheet thickness t. Therefore, both a decreased thickness and a high impact absorbing capability for an impact absorbing member only by increasing the strength of steel sheet can be achieved only to some extent. On the other hand, the absorption of impact energy of an impact absorbing member also greatly depends on its shape. This is disclosed in, for example, International Publication Nos. 20051010396,200510 10397, and 200510 10398. Therefore, there is possibility to rapidly increase the absorption of impact 5 energy of an impact absorbing member to a level which cannot be achieved simply by increasing the strength of steel sheet, by optimizing the shape of the impact absorbing member so as to increase the plastic deformation work when subjected to impact by collision. However, even if the shape of the impact absorbing member is optimized so 10 as to increase the amount of plastic deformation work, a crack would have occurred in the impact absorbing member in an early period before the desired plastic deformation is completed upon collision of automobile unless the steel sheet has deformation capability to be able to endure the amount of plastic deformation work. As a result of that, it is not possible to increase the amount of plastic deformation 15 work of the impact absorbing member and therefore not possible to rapidly increase the absorption of impact energy thereof. Moreover, if a crack occurs in the impact absorbing member in an early period, an unexpected situation may be brought about in which another member disposed adjacent to this impact absorbing member is damaged. 20 As shown in the above described Patent Documents, conventionally, the dynamic strength of steel sheet has been increased based on the technical concept that the absorption of impact energy of the impact absorbing member depends on the dynamic strength (the static-dynamic difference or static-dynamic ratio) of steel sheet. However, simply increasing the dynamic strength of steel sheet may 25 significantly deteriorate deformation properties. For that reason, even if the shape of the impact absorbing member is optimized so as to increase the amount of plastic deformation work, it is not necessarily possible to dramatically increase the impact energy absorbed by the impact absorbing member. Further, since conventionally the shape of the impact absorbing member has 30 been studied on the assumption that the steel sheet manufactured based on the above described technical concept is used, the optimization of the shape of the impact absorbing member has been studied from the beginning on the assumption of deformation capability of conventional steel sheets. For that reason, sufficient study has not been done fiom the perspective of improving the deformation capability of steel sheet as well as optimizing the shape of the impact absorbing member so as to improve the amount of plastic deformation work. As described above, to improve the absorption of impact energy of the impact absorbing member, it is important to optimize the shape of the impact absorbing member, in addition to increase the strength of steel sheet so as to increase the amount of plastic deformation work. Regarding steel sheet, it is important to increase an effective flow stress to optimize the shape of the impact absorbing member, which can increase the amount of plastic deformation work. Increasing the effective flow stress of steel sheet will make it possible to increase the amount of plastic deformation work of steel sheet, while suppressing the occurrence of cracking when subjected to an impact load. In order to improve the absorption of impact energy of the impact absorbing member, the present inventors have studied on steel sheet regarding the means of suppressing the occurrence of cracking when subjected to an impact load, and concurrently allowing the effective flow stress to be increased, and thus obtained new findings listed below. (A) To improve the absorption of impact energy of an impact absorbing member, it is effective to increase the effective flow stress (hereafter, referred to as "5% flow stress") when a true strain of 5% is applied to the steel sheet of the starting material. (B) To suppress the occurrence of cracking in the impact absorbing member when subjected to an impact load, it is effective to improve uniform elongation and local ductility of the steel sheet of the starting material. (C) To increase the 5% flow stress of steel sheet, it is effective to increase yield strength thereof and a work hardening coefficient in a low strain region. @) To increase the yield strength and the work hardening coefficient in a low strain region of steel sheet, it is necessary that the steel structure of steel sheet has a multi-phase structure containing bainite as the main phase, and martensite which is harder than bainite and retained austenite in a second phase. (E) The martensite and retained austenite contained in the second phase of the multi-phase structure contribute to increases of the work hardening coefficient and uniform elongation in a low strain range of steel sheet. Therefore, it is necessary to set lower limits for the area fractions of martensite and retained austenite. (F) On the other, excessive large area fractions of martensite and retained austenite will lead to decrease of local ductility of steel sheet. Therefore, it is necessary to set upper limits for the area fractions of martensite and retained austenite. (G) If the ferrite which is a retained structure is coarse, strain is likely to be concentrated in soft ferrite, and thereby decreases the yield strength of steel sheet and the local ductility thereof. Therefore, it is necessary to specify upper limit for the average grain diameter of ferrite. (H) As described above, to improve the absorption of impact energy of an impact absorbing member, it is effective to increase the 5% flow stress of steel sheet; and to suppress the occurrence of cracking of the member when subjected to an impact load, it is effective to improve the uniform elongation and the local ductility of steel sheet. To respond to severe needs in recent years, as an index to realize these, it is necessary that the product of uniform elongation and hole expansion ratio is at least 300%~a,n d the effective flow stress when applied with 5% true strain is at least 900 MPa in steel sheet. (I) Appropriately suppressing the hardness ratio between the bainite which is the main phase and the martensite contained in the second phase will suppress mobile dislocation by plastic deformation, thus making it easy to achieve higher yield strength. Therefore, it is preferable to set upper limit for the hardness ratio between the bainite which is the main phase and the martensite. (J) On the other hand, appropriately improving the hardness ratio between the bainite which is the main phase and the martensite contained in the second phase will make it easy to increase the work hardening coefficient and uniform elongation in a low strain region by including martensite. Therefore it is preferable to set a lower limit for the hardness ratio between the bainite which is the main phase and the martensite. (IS) Suppressing strain concentration by plastic deformation only in bainite and work hardening in a multi-phase structure steel sheet containing bainite as the main phase will suppress the occurrence of cracking along a shear band and a grain boundary in the bainite, making it easy to improve the local ductility. On the other 5 hand, suppressing excessive hardening of the second phase caused by plastic deformation makes it possible to avoid that the hardness difference between the main phase and the second phase increases so that occurrence of cracking from an interface therebetween is suppressed, thus making it easy to increase the local ductility of steel sheet. 10 Therefore, to achieve even higher local ductility in a multi-phase structure steel sheet containing bainite as the main phase, it is preferable to cause strain to be appropriately distributed between bainite which is the main phase and the second phase. That is, it is preferable that bainite which is the main phase and the second phase are subjected to a same level of work hardening when plastically deformed. 15 As an index for this, it is appropriate to use a proportion of work hardening rates after 10% tensile deformation. That is, in a multi-phase structure steel sheet containing bainite as the main phase and martensite in a second phase, it is preferable to set a lower limit and an upper limit for the ratio between the work hardening rate of bainite after 10% tensile deformation and the work hardening rate 20 of martensite after 10% tensile deformation. (L) A steel sheet having the above described microstructure can be obtained by combining a specific chemical composition, a hot rolling condition, a cold rolling condition and an annealing condition as will be described later in detail. The present invention based on the above described new findings is a steel 25 sheet comprising: a chemical composition containing, by mass%, C: at least 0.08% and at most 0.30%, Mn: at least 1.5% and at most 3.5%, Si + Al: at least 0.50% and at most 3.0%, P: at most 0.10%, S: at most 0.010%, N: at most 0.010%, Cr: 0 to at most 0.5%, Mo: 0 to at most 0.5%, B: 0 to at most 0.0 1 %, Ti: 0 to less than 0.04%, Nb: 0 to less than 0.030%, V: 0 to less than 0.5%, Ca: 0 to at most 0.010%, Mg: 0 to 30 at most 0.010%, REM: 0 to at most 0.050%, and Bi: 0 to at most 0.050%, the remainder being Fe and impurities; a microstructure containing, by area%, bainite: more than 50%, martensite: at least 3% and at most 30%, and retained austenite: at least 3% and at most 15%, the remainder consisting of ferrite having an average grain diameter of less than 5 pm; and mechanical properties in which the product of uniform elongation and hole expansion ratio is at least 300%~a, nd an effective flow stress when 5% true strain is applied is at least 900 MPa. Here, the "effective flow stress when 5% true strain is applied" means the flow stress required to keep plastic deformation occurring when 5% true strain is applied and then plastic deformation is started. This effective flow stress can be determined fiom a true stress value at a true strain of 5% in a true stress-true strain curve obtained by a simple tension test. The microstructure preferably satisfies the following formulas (1) and (2): 1.2 5 HMo/HB5o 1.6 (1) 0.9 5 {(HMIo~Mo)/(HBIo5~ 1B.3o )) (2) where, HMOi:n itial average nano hardness of the martensite, HBOi:n itial average nano hardness of the bainite, HMlo: average nano hardness of the martensite after 10% tensile deformation, HBIOa:v erage nano hardness of the bainite after 10% tensile deformation. The average nano hardness can be determined by the method according to Examples. The initial average nano hardness means a nano hardness before tensile deformation is applied. The chemical composition may contain one or more selected from by mass%, Cr: at least 0.1% and at most 0.5%, Mo: at least 0.1% and at most 0.5%, B: at least 0.0010% and at most 0.010%, Ti: at least 0.01% and less than 0.04%, Nb: at least 0.005% and less than 0.030%, V: at least 0.010% and less than 0.5%, Ca: at least 0.0008% and at most 0.010%, Mg: at least 0.0008% and at most 0.010%, REM: at least 0.0008% and at most 0.050%, and Bi: at least 0.0010% and at most 0.050%. In another aspect, the present invention is an impact absorbing member having an impact absorbing portion, which absorbs impact energy by being axially crashed and buckled, wherein the impact absorbing portion is made of any of the above described steel sheets. In a further aspect, the present invention is a method for manufacturing a steel sheet, comprising following steps (A) to (c): (A) a hot rolling step in which a slab having the above described chemical composition is subjected to multi-pass hot rolling in which rolling is completed at a 5 temperature of at least AT3 point, the obtained steel sheet is cooled to a temperature ' range of at least 620°C and at most 720°C under a cooling condition in which cooling is started within 0.4 seconds after completion of rolling, and an average cooling rate is at least 600°C/sec, as well as a time required for cooling fiom completion of rolling in a rolling pass which is two passes before the last rolling 10 pass to 720°C is at most 4 seconds, and the steel sheet is held in the temperature range for at least 1 second and at most 10 seconds, thereafter being cooled to a temperature range of at least 300°C and at most 610°C at an average cooling rate of at least 10°C/sec and at most 100°C/sec, and being coiled to obtain a hot-rolled steel sheet; 15 (B) a cold rolling step in which the hot-rolled steel sheet obtained by the hot rolling step is subjected to cold rolling of a rolling reduction of at least 40% and at most 70% to be formed into a cold-rolled steel sheet; and (C) an annealing step in which the cold-rolled steel sheet obtained by the cold rolling step is subjected to a heat treatment in which the steel sheet is held in a 20 temperature range of at least (Ac3 point - 30°C) and at most (Ac3 point + 100°C) for at least 10 seconds and at most 300 seconds, and then is cooled at an average cooling rate of at least 1 5"CIsec in a temperature range of at least 500°C and at most 650°C, thereafter being held in a temperature range of at least 300°C and at most 500°C for at least 30 seconds and at most 3000 seconds. 25 The steel sheet relating to the present invention is suitable as a starting material of an impact absorbing portion in an impact absorbing member, the impact absorbing portion absorbing impact energy by being axially crashed and buckled, and especially suitable as a starting material for impact absorbing members of automobile. To be specific, the present steel sheet is, for example, preferably used 30 as a starting material for a crash box of automobile, which has a tubular main body having a closed section, (and which is mounted onto a body shell such as a side member while supporting a bumper reinforcement, and is configured to be axially crashed and plastically deformed into a bellows shape by an impact load applied fkom the bumper reinforcement). The steel sheet can also be advantageously used as a starting material for a side member, a fiont upper rail, a side sill, and a cross member of automobile. 5 . Manufacturing an impact absorbing member fiom a steel sheet involving to the present invention will make it possible to obtain an impact absorbing member which can suppress or eliminate the occurrence of cracking when subjected to an impact load, and which exhibits a high effective flow stress, thereby dramatically improving the absorption of impact energy of the impact absorbing member. 10 Since applying such an impact absorbing member to a product such as an automobile will allow further improvement of the collision safety of the product, the present invention is highly beneficial industrially. Brief Description of Drawings 15 Figure 1 is an explanatory diagram to show an example of regions where an impact absorbing member is applied. Figure 2 is a two-view diagram to show an example of the shape of an impact absorbing portion. Figure 3 is a two-view diagram to show an example of the shape of an impact 20 absorbing portion. Description of Embodiments Hereafter, the present invention will be described in more specifically. It is noted that in the following description, "%" relating to the chemical composition of 25 steel all represents "mass%". The following description is for the purpose of exemplifjing the present invention, and is not intended for limiting the present invention. 1. Chemical composition (1) C: at least 0.08% and at most 0.30% 30 C (carbon) has the function of promoting the formation of bainite which is the main phase, and martensite and retained austenite which are contained in a second phase. C also has the function of improving the tensile strength of steel sheet as a result of increasing the strength of martensite. Further, C has the function of strengthening steel through solid solution strengthening, thereby improving the yield strength and tensile strength of steel sheet. When C content is less than 0.08%, there may be cases where it is difficult to 5 achieve effects of the above described functions. Therefore, C content is at least 0.08%. It is preferably more than 0.12%, and more preferably more than 0.14%. On the other hand, when C content exceeds 0.30%, there may be cases where martensite and austenite are excessively formed, thereby causing a significant decrease in the local ductility of steel sheet. Moreover, the weldability is 10 significantly deteriorated. Therefore, C content is at most 0.30%. It is preferably less than 0.20%, and more preferably less than 0.19%. (2) Mn: at least 1.5% and at most 3.5% Mn (manganese) has the function of promoting the formation of bainite which is the main phase, and martensite and retained austenite which are contained 15 in a second phase. Moreover, Mn has the function of strengthening steel through solid solution strengthening, thereby improving the yield strength and tensile strength of steel sheet. Further, since Mn improves the strength of bainite through solid solution strengthening, it has the function of improving the local ductility of steel sheet by improving the hardness of bainite under a high strain load condition. 20 When Mn content is less than 1.5%, there may be cases where it is difficult to achieve effects of the above described functions. Therefore, Mn content is at least 1.5%. It is preferably more than 1.8%, more preferably more than 2.0%, and further preferably more than 2.2%. On the other hand, when Mn content is more than 3.5%, the bainite transformation is excessively delayed, and as a result of that, 25 the stabilization of retained austenite cannot be achieved, making it difficult to achieve a predetermined amount of retained austenite. Therefore, Mn content is at most 3.5%. It is preferably less than 3.1%, more preferably less than 2.8%, and fkther preferably less than 2.5%. (3) Si + Al: at least 0.50% and at most 3.0% 30 Si and A1 have the function of promoting the formation of retained austenite through the suppression of the formation of carbides in bainite, thereby improving the uniform ductility and the local ductility of steel sheet. Moreover they have the function of strengthening steel through solid solution strengthening, and thereby improving the yield strength and the tensile strength of steel sheet. Further, since the strength of bainite is improved by solid solution strengthening, they also have the function of improving the local ductility of steel sheet by improving the 5 hardness of bainite under a high strain load condition. When the total content of Si and A1 (also referred to as "Si + Al" content) is less than 0.50%, it is difficult to achieve effects of the above described functions. Therefore, (Si + Al) content is at least 0.50%. It is preferably at least 1.0%, and more preferably at least 1.3%. On the other hand, even when (Si + Al) content is 10 at least 3.0%, the effects of the above described functions reach a limit, which is disadvantageous in respect of cost. This also leads to increase in the temperature of transformation point, and thereby deteriorate the productivity. Therefore, (Si + Al) content is at most 3.0%. It is preferably at most 2.5%, more preferably less than 2.2%, and f.lurther preferably less than 2.0%. 15 Since Si has excellent solid solution strengthening capability, Si content is preferably at least 0.50%, and more preferably at least 1.0%. On the other hand, since Si has the function of reducing the chemical convertibility and weldability of steel sheet, Si content is preferably less than 1.9%, more preferably less than 1.7%, and further preferably less than 1.5%. 20 (4) P: at most 0.10% P (phosphorus), which is generally contained as an impurity and segregates at grain boundaries, has the function of embrittling the steel, and promoting the occurrence of cracking when subjected to an impact load. When P content is more than 0.10%, the embrittlement of steel due to the above described function becomes 25 significant, and it becomes difficult to suppress the occurrence of cracking when subjected to an impact load. Therefore, P content is at most 0.10%. It is preferably less than 0.020%, and more preferably less than 0.0 15%. (5) S: at most 0.010% S (sulfur), which is generally contained as an impurity, has the function of 30 forming sulfide-based- inclusions in steel and thereby deteriorating the formability thereof. When S content is more than 0.010%, the effects of the above described function becomes critical. Therefore, S content is at most 0.01 0%. It is preferably at most 0.005%, more preferably less than 0.003%, and further preferably at most 0.00 1 %. (6) N: at most 0.0 10% N (nitrogen), which is generally contained in steel as an impurity, has the 5 function of deteriorating the ductility of steel sheet. When N content is more than 0.010%, this ductility deterioration becomes significant. Therefore, N content is at most 0.0 10%. It is preferably at most 0.0060%, and more preferably at most 0.0050%. Elements to be described below are optional elements which can be 10 contained in steel as desired. (7) One or more selected fiom Cr: at most 0.5%, Mo: at most 0.5%, and B: at most 0.01% Cr, Mo, and B have the function of improving the hardenability, and promoting the formation of bainite. Moreover, they also have the function of 15 promoting formation of martensite and retained austenite. They further have the function of strengthening steel through solid solution strengthening, thereby improving the yield strength and the tensile strength of steel sheet. Therefore, one or two selected fiom Cr, Mo, and B may be contained. However, when Cr content exceeds 0.5%, Mo content exceeds 0.5%, or B 20 content exceeds 0.0 1%, there may be a case where the uniform elongation and the local ductility of steel sheet are significantly deteriorated. Therefore, it is set such that Cr content is at most 0.5%, Mo content is at most 0.5%, and B content is at most 0.01 %. To more surely achieve effects of the above described functions, it is preferable that any one of Cr: at least 0.1%, Mo: at least 0.1% and B: at least 25 0.0010% is satisfied. (8) One or more selected fiom Ti: less than 0.04%, Nb: less than 0.030%, and V: less than 0.5% Ti, Nb and V have the function of suppressing the grain growth of austenite being annealed such as by forming carbonitrides in steel, and thereby reducing 30 cracking sensitivity. Moreover, they also have the function of precipitating into bainite and improving the yield strength of steel sheet by the effect of precipitation strengthening. Therefore, one or more of Ti, Nb, and V may be contained. However, even when Ti content is at least 0.04%, Nb content is at least 0.030%, and V content is at least 0.5%, effects of the above described functions reach a limit, which is disadvantageous in respect to cost. Therefore, Ti content is less than 0.04%, Nb content is less than 0.030%, and V content is less than 0.5%. Ti content is preferably less than 0.020%. Nb content is preferably less than 0.020%, and more preferably at most 0.015%. V content is preferably at most 0.30%. To more surely achieve effects of the above described functions, it is preferable that any one of Ti: at least 0.01%, Nb: at least 0.005% and V: at least 0.010% is satisfied. When Nb is contained, Nb content is more preferably at least 0.010%. (9) One or more selected fiom Ca: at most 0.010%, Mg: at most 0.010%, REM: at most 0.050%, and Bi: at most 0.050% Ca, Mg, REM, and Bi all have the function of improving the local ductility of steel sheet: by controlling the shape of inclusions regarding Ca, Mb, and REM, and by making the solidification structure finer regarding Bi. Therefore, one or more of these elements may be contained. However, regarding Ca and Mg, when contained more than 0.010%, and regarding REM, when contained more than 0.050%, a large number of coarse oxides are produced in steel, and deteriorate the formability of steel sheet. Regarding Bi, when contained more than 0.050%, it segregates at grain boundaries, deteriorating the weldability. Therefore, the content of each element is specified as described above. The contents of Ca, Mg, and REM are preferably at most 0.0020% for each, and the content of Bi is preferably at most 0.010%. To more surely achieve effects of the above described functions, it is preferable to satisfy any of the conditions: Ca: at least 0.0008%, Mg: at least 0.0008%, REM: at least 0.0008%, and Bi: 0.0010%. Here, REM means 17 elements in total including Sc, Y, and lanthanoid, and regarding lanthanoid, industrially it is added in the form of misch metal. It is noted that in the present invention, the content of REM means a total content of these elements. 2. Microstructure (1) Multi-phase structure The steel structure of the steel sheet according to the present invention is configured to have a multi-phase structure containing bainite as the main phase, and martensite and retained austenite in a second phase to improve the effective flow stress by obtaining high yield strength and the high work hardening coefficient in a 5 low strain region. The remainder of the second phase is ferrite. (2) Area fraction of bainite: more than 50% In a multi-phase structure steel sheet having bainite as the main phase, the bainite area fraction affects the yield strength of the steel sheet. That is, the yield strength is improved by increasing the area fraction of bainite. When the area 10 fraction of bainite is less than 50%, it becomes difficult to obtain an impact absorbing member having excellent impact absorbing capability due to deficiency of yield strength. Therefore, the area fraction of bainite is more than 50%. (3) Martensite area fraction: at least 3% and at most 30% In the multi-phase structure steel sheet having bainite as the main phase, 15 martensite has the function of increasing 5% flow stress of steel sheet by improving the yield strength of steel sheet and the work hardening rate thereof in a low strain region. Moreover, it also has the function of improving the uniform elongation of steel sheet. When the martensite area fraction is less than 3%, it becomes difficult to obtain an impact absorbing member having excellent impact absorbing capability 20 due to deficiencies of 5% flow stress and uniform elongation. Therefore, the martensite area fraction is at least 3%. It is preferably at least 5%. On the other hand, when the martensite area fraction is more than 30%, the local ductility of steel sheet decreases so that cracking due to unstable buckling becomes likely to occur. Therefore, the area fraction of martensite is at most 30%. The area fraction of 25 martensite is preferably at most 25%, and more preferably at most 15%. (4) Retained austenite area fraction: at least 3% and at most 15% In the multi-phase structure steel sheet having bainite as the main phase, retained austenite has the function of increasing 5% flow stress of steel sheet by increasing the yield strength thereof and the work hardening rate in a low strain 30 region. Moreover, it also has the function of improving the uniform elongation of steel sheet. When the retained austenite area fraction is less than 3%, it becomes difficult to obtain an impact absorbing member having excellent impact absorbing capability due to deficiencies of 5% flow stress and uniform elongation. Therefore, the retained austenite area fiaction is at least 3%. On the other hand, when the retained austenite area fiaction is more than 15%, the local ductility of steel sheet decreases so that cracking due to unstable buckling becomes likely to occur. 5 Therefore, the area fiaction of retained austenite is at most 15%. (5) Average grain diameter of ferrite which is remaining structure: less than 5 Pm When the average grain diameter of ferrite which is the remaining structure is at least 5 pm, strain becomes likely to concentrate in soft ferrite and yield strength 10 decreases so that it becomes difficult to increase 5% flow stress of steel sheet. Moreover, the local ductility of steel sheet decreases and it becomes difficult to suppress the occurrence of cracking when subjected to an impact load. Therefore, the average grain diameter of ferrite is at most 5 ym. It is preferably less than 4.0 ym, and more preferably less than 3.0 p. There is no need to particularly specify 15 the lower limit of the average grain diameter of ferrite. Although there is no need to particularly specify the area fiaction of ferrite, the lower limit thereof is preferably at least 1 %, and more preferably at least 5%. On the other hand, the upper limit is preferably at most 20%, more preferably at most 15%, and Wher preferably at most 10%. 20 (6) Hardness ratio of bainite and martensite: 1.2 < HMo/HB