Specification [Title of the Invention] HIGH STRENGTH HOT-ROLLED STEEL SHEET EXHIBITING EXCELENT CHEMICAL CONVERSION PROCESSABILITY, FATIGUE . PROPERTY, AND HOLE EXPAND ABILITY, AND METHOD FOR PRODUCING THE 5 SAME [Technical Field] [0001] The present invention relates to a hot-rolled steel sheet that is suitably used to a component of a transport machine such as an automobile, and particularly has a tensile 10 strength of 780 MPa or more, and a method for producing the same. [Background Art) [0002] Accordin~ to a recent demand of society, a mass-reduction is strongly demanded in transport machines such as automobiles. A lot of steel sheets· are used in the transport 15 machines such as automobiles, and a use of high-strength materials for exterior sheets (body) or skeleton members is proceeded s~ as to fulfill the demand for mass-reduction. ---Hot-roBed steel sheets are used .for underbody components· such as anns and wheel disks,- with regard to these underbody components, there is a concern of an effect on ride quality due to a decrease in rigidity; and therefore, thinning through high strengthening has not 20 been positively examined. (0003] However, since the demand for the mass-reduction has further increased7 this demand is also made without exception to the underbody co:t:nponents. For example, the upper limit of the tensile strength of the hot-rolled steel sheet that is used in the related art 25 is 590 MPa class; however, a use of steel sheets of 780 MPa class begins to be examined. 2 . . Under this circumstance, a fatigue property and a corrosion resistance are required for the steel sheet in addition to a formability that commensurates with the strength. [0004] With regard to the corrosion resistance, a steel sheet having a sufficient sheet 5 thickness is usec;I to secure rigidity in the related art, therefore, even when the sheet thickness is· reduced due to corrosion, an effect on properties of the components is small, and that is not seen as a problem. However, as described above,. the thinning of a component has been directed, and a corrosion allowance to allow the reduction in sheet thickness due to corrosion has been reduced. In addition, simplification of chemical 1 0 conversion processing and coating is considered to reduce a manufacturing cost. Therefore, it is necessary to pay more attention to a property or state in a surface of a steel material as compared to the related art. [0005] When a hot-rolled steel sheet is applied to the underbody component, the 15 . hot-rolled steel sheet is shipped after being acid-pickled and coated with oil, thereafter, the hot-rolled steel sheet is processed into components, and then the processed steel sheet is ----- - - ---.. ---subjected-to a-chemical conversion-processing and .a coating process in many cases .. --.. i . Among these, particularly, a chemical conversion processability is most affected by the · property and the state in the surface of the steel sheet, and has a great effect on the · 20 corrosion resistance. In addition, since stress is repeatedly applied to ·strength members such as the underbody components, a fatigue property is required;·· Furthermore, since a sheared end portion is processed in· many cases, a stretch flangeability, that is, a hole expandability is also required in many cases. 25 [0006] --------------------------------- 3 Many te<;hnologies have been proposed which improve a fatigue property and a stretch flangeability of the steel sheet, and the present inventors also have promoted a research to optimize chemical components and a microstructure of the steel sheet. On the other hand, the chemical conversion processability of the steel sheet 5 depends on a Si content of the steel sheet, and it is well-known t4at the more the Si content is, the more inferior the chemical conversion processability becomes. [0007] However, In the case where the steel sheet is highly strengthened by making Si be solid-solubilized in ferrite phases, since a deterioration amount of ductility is not 1 0 remarkably large, therefore, Si is an element that is preferred to be used as much as 15 possible in the manufacturing of the high-strength steel sheet. In addition, particularly, in the case where a steel sheet having both .of high ductility and high strength is manufactured by combining the ferrite phases and hard phases such as martensite phases, Si is an element effective to secure a predetermined fraction ratio of the ferrite phases. [00081 As a method of responding to these contradicting demands, a technology in which - --a part of Si -is-substituted by A l-is proposed. ----For .example, it. is Patent Document-! .... -- ... --- -- -· ··- - ---The Patent Document discloses a hot-rolled steel sheet having a high tensile strength which contains less than 1% of Si and 0.005 to 1.0% of Al, and a method of 20 producing the same, but there is no description with respect to the production method 25 premised on the heating of the rough bar is special, as a result, there is a problem in that only limited business operators can execute the production me~od, and the chemical conversion properties of the steel sheet that is obtained. [0009] On the other hand, Patent Document 2 discloses a hot-rolled steel sheet that I ' i 4 contains· Si and Al and is superior in the chemical conversion processability, and a method of producing the same. However, in the Patent Document, the upper limit of anAl content is 0.1 %, and it is described. that in the case where the Al content exceeds this upper limit, the corrosion 5 resistance deteriorates, but there is no description with respect to the corrosion resistance different from the chemical conversion processability. [Related Art Documents] [Patent Documents] [0010] 10 [Patent Document 1] Japanese Unexamined Patent Application, First 15 Publication No. 2006-316301 [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2005-139486 [Non Patent Documents] [00 11] Non-Patent Document 1: . M. Nomura, I. Hashimoto, M. Kamura, S. Kozuma, Y. -------- -----Omiya:-Research and Development,-Kobe Steel Engineering Reports, Vol. 57, No.2---· (2007), 74 to 77 Non-Patent Document 2: S. Maeda, T. Asai, S. Arai, K. Suzuki: 20 Tetsu-to-Hagane, Vol. 68 (1982), 2497 to 2506 [Summary ofthe Invention] [Problems to be Solved by the Invention] [0012] As described above, a hot-rolled steel sheet that contains at least 0.3% or more of 25 Al together with Si and that is superi~r in the chemical conversion processability, and a 5 method of producing the same are not found. · The present invention has been made in consideration of these circumstances, and ·the mvention aims to provide a high strength hot-rolled steel sh~et that is superior in a chemical conversion processability, a fatigue property, and a hole expandability; and a 5 method for producing the hot-rolled steel sheet. [Means for Solving the Problem] [0013] The present inventors selected a DP steel sheet in which ferrite phases and martensite phases are combined as a steel sheet superior in a fatigue property, and they 10 changed chemical components and production conditions extensively, and then mechanical properties and a chemical conversion processability were evaluated. As a result, they found that in the case where a Si content and an AI content are controlled and combined within appropriate ranges, a steel sheet is obtained that is superior in not only the mechanical properties but also the chemical conversion processability, and they · 15 accomplished the invention. The gist of the present invention is as follows. ··------------- ----[001-4]----·----··---------------·-- ·-- ---·---. ·-- ----··-- (1) A high strength hot-rolled steel sheet that is superior in an acid pickling property, a chemical conversion processability, a fatigue property, and a hole expandability, · 20 the steel sheet including: in ten:rls of percent by mass, C: 0.05 to 0.12%; Si: 0.8 to 1.2%; Mn: 1.6 to 2.2%; AI: 0.30 to 0.6%; P: 0.05% or less; S: 0.005% or less; and N: 0.01% or less, with the remainder being Fe and unavoidable impurities, wherein a microstructure comprises: two phases of a ferrite phases and a martensite phases, or three phases of ferrite phases; martensite phases; and bainite phases, the ~ea ratio of the ferrite phase is 25 60% or more ; the area ration of the martensite phase is more than 1 0%; and the area 6 ration of the bainite phase is less than 5%; and a maxirimm concentration of AI as an oxide detected by a glow discharge· erri.ission spectroscopic analysis is in a range of 0. 75 mass% or less in a region fro in a surface of the steel sheet· to a thickness of 5 00 run after being hot rolled and acid-pickled. 5 [0015] (2) A method for producing a high strength hot-rolled steel sheet that is superior in a .c hemical conversion processabi.l ity, a fatigue property, and a hole e'Xpandability, the method including, during the hot rolling to obtain the hot-rolled steel sheet according to (1), heating a slab to a temperature in a range ofT! °C or less, rough rolling is performed 10 under conditions in which a ratio is in a range of 80% or more, after a temperature of T2°C or less, a descaling is performed, subsequent a fmish rolling is performed at a fmish temperature is set to be in a range of 700 to 950°C, and cooling is performed to a temperature in a range of 550 to 750°C at an average cooling rate of 5 to 90°C/s, further cooling the rolled speet to a temperature in a ~ange of 450 to 700°C at an average cooling · 15 rate of l5°C/s or less, and further cooling the rolled sheet to a temperature in a range of 250°C or less at an average cooling rate of30°C/s or more. Wherein Tl = 1215 + --------- ----- -------·-- ·- -~ ··---------------. ·------·--·--·----------·------·--· -- .. ··---- --·- 35x[Si] -70x[Al], and T2 = 1070 + 35x[Si] -70x[Al], here, [Si] and [AI] represent a Si content (mass%) in the slab, and an AI content (mass%) in the slab, respectively. (3) The method for producing a high strength hot-rolled steel sheet that is 20 superior in a chemical conversion processability, a fatigue property, and a hole expandability, the method including, after cooling according to (2),acid_ pickling is performed and the steel sheet surface is galvanized by dipping in a galvanizing bath. (4) The method for producing a high strength hot-rolled steel sheet that is superior in a chemical conversion processability, a fatigue property, and a hole 7 expandability, the method including, after cooling accorqing to (2), a Fn-Zn aHoy layer is formed on the steel sheet surface by acid picking, dipping in a galvanizing bath, and further heating to 600 °C or less. [Effect Of the Invention] 5 [0016] Since it is possible to provide a high strength hot-rolled steel sheet that is superior · in an acid pickling property, a chemical conversion processability, a fatigue property, and a hole expandability with the present invention, the thickness of the steel sheet can be decreased through measures such as reduction of a corrosion allowance, and the steel sheet 10 can contribute to a mass-reduction of the vehicle. fu addition, since a superior hole expandability can be obtained, the contribution in the industry is extremely remarkable such as a restriction in a process is small and an applicable range of the hot-rolled steel sheet is wide. [Brief Description of the Drawings] 15 [0017] [Figure 1] FIG. 1 is a schematic diagram illustrating a distribution of oxides in a · · --·· --------------surface of-a -steel--sheet after being hot-rolled-and acid-pickled:--- -- ·- [Embodiment of the Invention] [0018] 20 Upon completion of the present invention, the present inventors selected a DP steel sheet as a basic steel sheet, and the DP steel sheet is superior in a fatigue property. They performed experiments in which chemical components and production conditions were changed extensively, and evaluated mechanicalproperties and chemical conversion processability. 25 As a result thereof, they found that in the case where a Si content and a Al content ' . I' I I I i 8 are controlled within appropriate ranges and combined therewith, a steel sheet is obtained which is superior in not only the mechanical properties but also the chemical conversion processabili ty. First, findings obtained through such a research will be specifically described. 5 Here, in the following description, a unit in the content and concentration of~ component element is mass%, and when not particularly described, the unit is expressed only by %. [0019] Steels containing substantially 0.09% ofC, 0.85 to 1.15% ofSi, substantially 2% ofMn, 0.25 to 0.46% of AI, substantially 0.02% ofP, substantially 0.002% ofS, 10 substantially 0.002% ofN, and a remainder of Fe and unavoidable impurities were melted . to produce slabs. The obtained slabs were heated to 1130 to 1250°C, rough rolling was performed, and descaling was performed, subsequently, finish rolling was performed under a condition where a finish temperatUre was set to ·860°C. Subsequently, primary cooling 15 was performed to ·630°C at an average cooling rate of72°C/s, secondary cooling was performed to 593°C at an average cooling rate of 8°C/s, third cooling was performed to ,_ ----· --·---- ----:;-:;; 6s·oc "a£an aveni.gecooiillg raieo£"71 ocJs~ anresent Invention r--- 2 A 829 23.4 25 0.47 Present Invention r--- 3 821 23.5 23 0.47 Comparative Example 4 822 23.4 29 0.46 Present Invention r--- B 5 819 23.7 28 0.46 Comparative Example 6 c 830 22.1 37 0.43 Comparative Example 7 829 23.4 50 0.49 Comparative Example ~ 8 829 23.5 51 0.49 Comparative Example - D 9 822 23.9 53 0.48 Present Invention f-.-- 10 827 23.0 50 0.48 Comparative Example 11 828 23.2 52 0.49 Comparative Example - 12 830 23.3 53 0.49 Comparative Example - E 13 831 23.0 51 0.49 Present Invention - - --14----- -833--- ---23.2----50----- ----0.49- - -Present Invention---- 15 820 23.4 56 0.49 Present Invention r--- F 16 816 23.0 57 0.48 . Present Invention 17 832 23.6 53 0.46 Comparative Example r--- 18 835 23.4 53 0.47 Present Invention - G 19 831 23.6 52 0.47 Present Invention - 20 827• 23.9 54 0.46 Comparative Example 5 [Example 2] [0069] 34 Test specimens were collected from the hot-rolled steel sheets prepared in Example 1, and after acid pickling, were subjected to a galvanizing test. Acid pickling conditions were the same as those in Example 1.. A chemical composition of a molten galvanizing bath is set to be Zn-0.15% Al, 5 the temperature of the bath is set to be 460°C. 10 The test specimens were heated to 450 to 460°C, immersed in the galvanizing · bath for 3 seconds, removed, and then wiping was performed using nitrogen gas with a target adhesion arriount was set to 135 g/m2 • The processing from heating to cooling was carried out in a nitrogen atmosphere. [0070] Each of four test specimens for evaluation of30 x 30 rnm was collected from center of the test specimen. Both sides of the test specimens for evaluation were imaged with a scanner, and then a binarization processing in which a plated part is set to white and non-plated part is set to black is performed using photo retouching software. An area 15 ratio of the non-plated part occupied by the evaluation area (7200 mm2 ) is obtained as a non-plated area ratio, the ratio is shown in table 6. I . ·····- ---[00711-- --·--·· ------- ----------------------~ ···----------·-- ----------··· Non-platingwasnotobservedinallofsteelsheetNos.1,2,4,9, 13, 14, 15, 16, 18, and 19 in which the hot-rolling conditions are a range defined .in present invention. 20 The chemical conversion process was performed on the above mentioned hot dip 25 galvanizing steel sheet in which non,-platihg was not observed. The conditions were the same as those in Example 1. All of results, show an adhesion amount of 2 g/m2 or more with respect to the target adhesion amount of2 g/m2 , therefore, it was found that sufficient chemical conversion processability can be obtained. [0072] 35 [Table 6] I I Rough Slab Heating l Final Temperature Non-plated Area T1 T2 I Rolling No. Slab Temperature i of Rough Rolling Ratio coq (OC) Ratio coc) I (%) coq. (%) 1 1220 ! 80 1077 0 Present Invention - 2 A 1224 1079 1220 I 85 1075 0 Present Invention - I 3 1210 I 85 i 1085 2.6 Comparative Example 4 1210 I 85 1068 0 Present Invention 1-- B 1214 1069 I 5 1200 i 80 1080 4.0 Comparative Example 6 c 1231 1086 1230 85 1081 3.6 Comparative Example ' 7 1250 ' 80 1094. 3.2 Comparative Example I 1-- 8 1250 90. 1069 4.4 Comparative Example 1-- D 1226 1081 ! 9 1220 ' 85 1080 0 Present Invention - i 10 1220 I 75 1067 2.9 Comparative Example I I An underline represents a component be~ond a range defined in the present invention. -----------------------:------;------------------------------:--:--------------- 36 Table 6 (Co!itinued) I Rough Slab Heating ·: Final Temperature Non-plated Area T1 T2 ~ Rolling No. Slab Temperature / of Rough Rolling Ratio coq coq Ratio coq - (OC) (%) (%) 11 1230 ! 85 1082 4.8 Comparative Example r--- 12 1230 I I 85 1070 4.5 Comparative Example I-- E 1222 1077 i 13 1215 !I 85 1075 0 Present Invention I I-- ' 14 1160 ! , 80 1012 0 Present Invention I 15 1220 I 85 1072 0 Present Invention r----- F 1220 1075 I 16 1140 i 88 . 977 0 Present Invention i 17 1250 i 80 1051 4.1 Comparative Example I ~ ' 18 1230 ' i 80 1086 0 Present Invention r--- G 1234 1089 19 1155 I 85 1004 0 Present Invention I-- 20 1130 ! 76 995 3.7 Comparative Example : An underline represents a component be~ond a range defined in the present invention. ! ,· [Example 3] . [0073] 37 Test specimens were collected from the hot-rolled steel sheets prepared in Example 1, and after acid pickling, a test for producing a galvannealed steel sheet was 5 performed. The test specimens that were collected from the hot-rolled steel sheets that were acid-pickled under the same conditions as Example 1, were heated to 450 to 460°C, dipped for 3 seconds in a molten galvanizing bath (chemical composition: Zn-0.13% Al-0.03% Fe) that was maintained at 4600°C, and after the wiping was performed using 10 nitrogen gas by a adhesion value of 40 g/m2 with a target value, were immediately heated to 480°C. Heating was stopped after 30 seconds, and cooling to room temperature with nitrogen gas was performed. [0074] Each offive strip-shaped test specimens of 30 x 120 mm were collected from the 15 center portions of the test specimens, and then a re-:bending deformation and 5R U-bendingwere applied to the center portion of the long side. The mass of the plating --·-----------(powaer) thafis attached-to adhesion.' tape was measured by pedirigtne-mpe ·away·from _____ -·--- --- · -and attaching the tape to the inside (compression side) of the bend. Table 7 shows the total value of five peeling amounts (mass). 20 [0075] The peeling amount of all of steel sheet Nos. 1, 2, 4, 9, 13, 14, 15, 16, 18 and 19 in which the hot-rolling conditions are a range defined in present invention is 10 mg or less, and with respect to superior plating adhesion, the peeling amount of all of steel sheet Nos. 3, 5, 6, 7, 8, 10, 11, 12, 17 and 19 in which the hot-rolling conditions were not a 25 range defined in the present invention is 60 mg or more, it was clear that the plating I 38 adhesion was inferior. The chemical conversion pror;:ess was performed on the above mentioned galvannealed steel sheet that has superior plating adhesion. The conditions were the same as Example 1. The result of evaluating, all of results, show an adhesion amount of ·5 2 g/m2 or more with respect to the t~get adhesion amount of2 g/m2 , and thus show sufficient chemical conversion processability. [0076] 1···-- --------·-- ------- 1 [Table 7 I I Slab Heating : · Rough Final Temperature Plating Peeling Tl T2 Rolling - No. Slab (OC) ec) Temperature ; Ratio of Rough Rolling Amount (oC) (%) (°C) (mg) 1 1220 80 1077 4 Present Invention - 2 A 1224 1079 1220 85 1075 6 Present Invention - 3 1210 85 1085 61 Comparative Example 4 1210 ; 85 1068 8 Present Invention - B 1214 1069 5 1200 80 1080 84 Comparative Example 6 c 1231 1086 1230 85 1081 73 Comparative Example . 7 1250 80 1094 108 Comparative Example ---- 8 1250 90 1069 97 Comparative Example - D 1226 1081 9 1220 85 1080 5 Present Invention 1---- 10 1220 75 1067 90 Comparative Example I An underline represents a component berond a range defined in the present invention. I I I Table 7 (Continued) Slab Heating I Rough Final Temperature Plating Peeling T1 .T2 Rolling No. Slab Temperature of Rough Rolling Amount (OC) (OC) ! Ratio (OC) (°C) (mg) i (%) I I 11 1230 I' 85 1082 86 - I '12 1230 I 85 1070 74 - E 1222 1077 I 13 1215 I 85 1075 5 - 14 1160 : 80 1012 3 I 15 1220 I: 85 1072 9 - F 1220 1075 16 1140 ' 88 977 10 ! 17 1250 I 80 1051 89 - 18 1230 I 80' 1086 6 - G 1234 1089 19 1155 I 85 1004 7 - 20 1130 :' 76 995 93 An underline represents a component beyond a range defined in the present invention. I i I i I . II i Comparative Example Comparative Example Present Invention Present Invention Present Invention Present Invention Comparative Example .· Present Invention Present Invention Comparative Example 41 [Document Type J Claims [Claim 1] . A high strength hot-rolled steel sheet that is superior in an acid pickling property, a chemical conversion processability, a fatigue property, and a hole expandability, the steel 5 sheet comprising: in terms of percent by mass, C: 0.05 to 0.12%; Si: 0.8 to 1.2%; Mn: 1.6 to 2.2%; AI: 0.30 to 0.6%; 10 P: 0.05% or less; S: 0.005% or less; and N: 0.01% or less, with the remainder being Fe and unavoidable impurities, wherein a microstructure comprises: two phases of a ferrite phases and a 15 martensite phases, cir three phases of ferrite phases; martensite phases; and bainite phases, the area ratio of the ferrite phase is 60% or more ; the area ration of the martensite - -phase. is more than-10%; and the area-ration of the. bainite -phase.is less than-5%;-and-- --- ---------- . a maximum_ concentration of AI. as an oxide detected by a glow_ discharge emission spectroscopic analysis is in a range of0.75 mass% or less in a region froma 20 surface of the steel sheet to a thickness of 500 run after being hot rolled and acid-pickled. [Claim 2] A method for producing a high strength hot-rolled steel sheet that is superior in a chemical conversion processability, a fatigue property, and a hole expandability, the method comprising: 25 during the hot rolling to obtain the hot-rolled steel sheet according to Claim 1, 42 he·ating a slab to a tempeni.ture in a range ofTl °C or less . . . . rough ~oiling is performed under conditions in which a ratio is in a range of 80% or more, after a temperature of T2°C or less, a descaling is ·performed, 5 subsequent a finish rolling is performed at a finish temperature is set to be in a range of 700 to 950°C, arid cooling is performed to a temperature in a range of 550 to 7506C at an average cooling rate of 5 to 90°C/s, further cooling the rolled sheet to a temperature in a range of 450 to 700°C at an average cooling rate of l5°C/s or less, and further cooling the rolled 10 sheet to a temperature in a range of250°C or less at an average cooling rate of30°C/s or more, wherein T1 :;::: 1215 + 35x[Si]- 70x[Al], T2:;::: 1070 + 35x[Si]- 70x[Al], and here, [Si] and [AI] represent a Si content (mass%) in the slab, and an AI content 15 (mass%) in the slab, respectively. 20 [Claim 3] ----··-·-··- ----- --------·--- ····- --- ·---- ---··--------··· The method for producing a high strength hot-rolled steel sheet that is superior in a chemical conversion processability, .a fatigue property, and a hole expandability, the method comprising: after cooling according to claim 2,acid pickling is performed and the steel sheet surface is galvanized by dipping in a galvanizing bath. [Claim 4] The method for producing a high strength hot-rolled steel sheet that is superior in a chemical conversion processability, a fatigue property, arid a hole expandability, the 5 10 15 43 method comprising: after cooling according to claini 2, a Fn-Zn alloy layer is formed on the steel sheet surface by acid picking, dipping in a galvanizing bath, and further heating to 600 °C or less.