[Document Type] Specification [Title of the Invention] HIGH-STRENGTH LOW-SPECIFIC-GRAVITY STEEL SHEET HAVING SUPERIOR SPOT WELDABILITY [Technical Field of the Invention] [0001] The present invention relates to a high-strength low-specific-gravity steel sheet having superior spot weldability which is used for an automobile component or the like. Priority is claimed on Japanese Patent Application No. 2013-96428, filed on May 1, 2013, the content of which is incorporated herein by reference. [Related Art] [0002] Recently, as a countermeasure against environmental problems, reduction in the weight of a vehicle has been desired in order to reduce carbon dioxide emissions and fuel consumption. In order to reduce the weight of a vehicle, high-strengthening of steel is an effective means. Howeve1~ when the lower limit of the thickness of a steel sheet is limited due to rigidity required for a component, the thickness of the steel sheet cannot be reduced even after high-strengthening of steel, and it is difficult to reduce the weight of a vehicle. [0003] Therefore, for example, as disclosed in Patent Documents I to 5, some of the present inventors proposed a high Al-content steel sheet in which the specific gravity is reduced by adding a large amount of AI to steel. In the high Al-content steel sheets disclosed in Patent Documents 1 to 5, problems of a high Al-content steel sheet of the related mi including poor producibility such as cracking, which may occur during - I - rolling, and low ductility are solved. Further, in order to improve ductility, hot workability, and cold workability of a high Al-content steel sheet, for example, as disclosed in Patent Document 6, the present inventors proposed a method of adjusting a solidification structure after casting to be a fine equiaxed structure. Further, for example, as disclosed in Patent Document 7, the present inventors proposed a method of improving toughness of a high Al-content steel sheet by optimizing components. [Prior Art Document] [Patent Document] [0004] [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2005-15909 [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2005-29889 [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2005-273004 [Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2006-176843 [Patent Document 5] Japanese Unexamined Patent Application, First Publication No. 2006-176844 [Patent Document 6] Japanese Unexamined Patent Application, First · Publication No. 20D8-261023 · · [Patent Document 7] Japanese Unexamined Patent Application, First ... Publication No. 2010-270377 [Disclosure of the Invention] [Problems to be Solved by the Invention] - 2 - ;'it/ [0005] Recently, a high Al-content steel sheet having superior ductility, workability, and toughness has been able to be produced on an industrial scale. The high AIcontent steel sheet has, for example, superior arc weldability. However, the spot weldability of the high Al-content steel sheet is lower than that of a general automobile steel sheet having the same strength, and thus the use of the high Al-content steel sheet is limited. Accordingly, the improvement of spot weldability is an impmiant issue to increase the application range of the highAl-content steel sheet to automobile components. [0006] The present invention has been made in consideration of the above-described actual circumstances, and an object thereof is to provide a high-strength low-specificgravity steel sheet having superior spot weldability which is obtained by improving the spot weldability of a low-specific-gravity steel sheet to which AI is added. [Means for Solving the Problem] [0007] In order to improve the spot weldability of a high Al-content steel sheet, the present inventors investigated elements which decrease spot weldability. As a result, the present inventors found the following facts: that the spot weldability of a high AI- ·content steel· sheet is greatly affected by the Mn content in the high Al-content steel sheet; and that the spot w.eldability of a high Al-content steel sheet can be significantly improved by reducing the Mn content in the high Al-content steel sheet. The summary of the present invention-is as follows. [0008] - \< -. (1) According to an aspect of the present. invention, there is provided a steel - 3 - = sheet including, by mass%, C: more than 0.100% and 0.500% or less, Si: 0.0001% or more and less than 0.20%, Mn: more than 0.20% and 0.50% or less, AI: 3.0% or more and 10.0% or less, N: 0.0030% or more and 0.0100% or less, Ti: more than 0.100% and 1.000% or less, P: 0.00001% or more and 0.0200% or less, S: 0.00001% or more and 0.0100% or less, and a remainder including Fe and impurities, in which a sum of a C content and a Ti content satisfies 0.200