DESCRIPTION TITLE OF INVENTION HEAT-ROLLED STEEL PLATE FOR TAILORED ROLLED BLANK, TAILORED ROLLED BLANK, AND METHODS FOR PRODUCING THESE TECHNICAL FIELD [0001] The present invention relates to a heat-rolled steel plate for a tailored rolled blank, a tailored rolled blank, and methods for producing these. BACKGROUND ART [0002] In recent years, the weights of various components that constitute automobiles are being reduced with the objective of improving the fuel consumption of the automobiles. The method ofreducing the weight differs depending on the performance requirements for the respective components. For example, for a framework component, wall thinning is carried out by enhancing the strength of a steel plate. For a panel component, measures such as substitution of a steel plate with a light metal plate such as an AI alloy are taken. [0003] However, a light metal plate such as an AI alloy is expensive in comparison to a steel plate. Therefore, utilization of light metal plates is mainly limited to luxury automobiles. The demand for automobiles is shifting from developed countries to emerging countries, and it is expected that from now there will be demands to achieve both weight reductions and price reductions. Accordingly, for every component, irrespective of the region, there is a demand to achieve increased strength using a steel plate and a weight reduction by wall thinning. [0004] When wall thinning is exhaustively carried out, it is necessary to meticulously set the plate thickness and material quality of component parts of each region. However, in this case the number of components increases and the production cost ;%- rises. From the viewpoint of enhancing the accuracy of the body shape and improving productivity and the like, it is preferable that the number of components is as small as possible. [0005] Application of tailored blanks is proceeding as a method that, as much as possible, can meticulously set the plate thickness and material quality of each region and also reduce the number of components. [0006] The term "tailored blank" refers to a press starting material in which a plurality of steel plates are joined together according to the purpose. Utilizing a tailored blank makes it possible to partially alter the characteristics of a single starting material and to also reduce the nu!Tiber of components. A tailored blank is normally produced by welding together a plurality of steel plates. Examples of the welding method include laser welding, mash seam welding, plasma welding and high-frequency induction welding. [0007] Tailored blanks produced by welding in this manner are called "tailored weld blanks". Technology relating to tailored weld blanks is proposed in, for example, Japanese Patent Application Publication No. 7-290 I82 (Patent Literature I) and Japanese Patent Application Publication No. 8-I74246 (Patent Literature 2). [0008] According to the technology disclosed in Patent Literatures I and 2, steel strips of different thicknesses are butted in the width direction and welded by laser welding or the like. However, in a case where tailored weld blanks are produce by applying these technologies, if there is a weld defect at one part of a weld zone, in some cases cracks arise in the weld zone in a pressing process that is after the welding process. In addition, even when a weld zone does not have a weld defect, a hardness difference arises between a weld zone and a base metal portion, and weld undercut portions arise. In such a case, in a subsequent press-forming process, in some cases the stress concentrates at the weld zone during press working, and cracks arise in a portion of the weld zone. [0009] 3 As described above, when welding together steel plates of different strengths that have different plate thicknesses by using a welding process that is currently in practical use such as laser welding, mash seam welding, arc welding or highfrequency welding, it is difficult to make the quality of the weld zone uniform, and a weld defect is liable to occur. [001 0] Therefore, tailored rolled blanks have been proposed as another kind of tailored blank that does not utilize welding. A tailored rolled blank is a steel plate of varying thickness on which partial wall thinning has been carried out by rolling. Technology relating to tailored rolled blanks is disclosed in Japanese Patent Application Publication No. 11-192502 (Patent Literature 3), Japanese Patent Application Publication No. 2006-272440 (Patent Literature 4), International Application Publication No. WO 2008/068352 (Patent Literature 5) and International Application Publication No. WO 2008/104610 (Patent Literature 6). [00 11] According to the technology discussed in Patent Literature 3, a steel strip is rolled with work rolls of a special shape to produce a steel strip in which the plate thickness varies in the width direction. However, when utilizing this technology, it is necessary to prepare a plurality of single-purpose work rolls that correspond to the shape of the steel strip for a tailored blank. [0012] According to technology discussed in Patent Literature 4, a steel plate of varying thickness is produced without using work rolls of a special shape. Specifically, at least at one location at an intermediate portion in the longitudinal direction of the plate thickness, rolling is performed by changing the setting of a rolling reduction position so that the plate thickness changes in a tapered shape within a predetermined length range, to thereby produce a tailored rolled blank. However, in Patent Literature 4, there is no discussion regarding the chemical composition and microstructure and the like of a steel strip to be used for a tailored rolled blank. [0013] In Patent Literatures 5 and 6, a chemical composition of a steel plate for a tailored rolled blank and a method for producing a steel plate for a tailored rolled blank are disclosed. According to the technology disclosed in Patent Literatures 5 and 6, using a steel strip having a specific chemical composition, rolling is performed while controlling a roll gap so that the plate thickness changes in the rolling direction. After rolling, a heat treatment is performed, and the yield strength of a thick-wall portion of the tailored rolled blank is made equal to or greater than the yield strength of a thin-wall portion. [0014] According to the technology disclosed in International Application Publication No. WO 2010/137317 (Patent Literature 7), a steel plate having a specific chemical composition is subjected to hot rolling under specific conditions to produce a heat-rolled steel plate. Cold rolling is executed at a draft of 0.1 to 5.0% on a heat-rolled steel plate to produce a cold-rolled steel plate. A heat treatment is executed under specific conditions on the cold-rolled steel plate to produce a highstrength steel plate that is excellent in elongation properties. CITATION LIST PATENT LITERATURE [0015] Patent Literature 1: Japanese Patent Application Publication No. 7-290182 Patent Literature 2: Japanese Patent Application Publication No. 8-174246 Patent Literature 3: Japanese Patent Application Publication No. 11-192502 Patent Literature 4: Japanese Patent Application Publication No. 2006-272440 Patent Literature 5: International Application Publication No. WO 2008/068352 Patent Literature 6: International Application Publication No. WO 2008/104610 Patent Literature 7: International Application Publication No. WO 2010/137317 Patent Literature 8: Japanese Patent Application Publication No. 2004-317203 5 NON PATENT LITERATURE [0016] Non Patent Literature 1: G. K. Williams and W. H. Hall: Act. Metal!., I (1953), 22 Non Patent Literature 2: G. K. Williams and R. E. Smallman: Philos. Mag., 8 (1956), 34 Non Patent Literature 3: T. Tsuchiyama: Heat Treatment 42 (2002), 163 [0017] However, according to the technology disclosed in Patent Literatures 5 and 6, if the strength of the steel strip is high, the rolling reaction force during cold rolling increases. In such a case, an excessive facility load and an increase in the number of rolling operations and the like are required in order to form a thin-wall portion by rolling. Consequently, the productivity decreases. The plate thickness accuracy and shape accuracy also decrease. In addition, when the yield strength of a thickwall portion is equal to or greater than the yield strength of a thin-wall portion, although it is considered preferable in terms of usability after pressing, if a difference between the yield strength of a thick-wall portion and a thin-wall portion is too large, a deformation will concentrate at the thin-wall portion during cold forming (cold pressing or the like) and a rupture is liable to occur. Further, even if cold rolling of around 5% is performed as in the case of the technology described in Patent Literature 7, a plate thickness difference between a thick-wall portion and a thin-wall portion that is required as a tailored rolled blank cannot be obtained. SUMMARY OF INVENTION [0018] An objective of the present invention is to provide a heat-rolled steel plate for a tailored rolled blank that is capable of producing a tailored rolled blank that has a tensile strength of 590 MPa or more and is excellent in cold formability, a tailored rolled blank produced using the heat-rolled steel plate, and methods for producing these. [0019] 6 A heat-rolled steel plate for a tailored rolled blank according to the present embodiment has a chemical composition consisting of, in mass%, C: 0.03 to 0.1 %, Si: 1.5% or less, Mn: 1.0 to 2.5%, P: 0.1% or less, S: 0.02% or less, AI: 0.01 to 1.2%, N: 0.01% or less, Ti: 0.015 to 0.15%, Nb: 0 to 0.1%, Cu: 0 to 1%, Ni: 0 to 1%, Mo: 0 to 0.2%, V: 0 to 0.2%, Cr: 0 to 1%, W: 0 to 0.5%, Mg: 0 to 0.005%, Ca: 0 to 0.005%, rare earth metal: 0 to 0.1 %, B: 0 to 0.005%, and one or more types of element selected from a group consisting of Zr, Sn, Co and Zn in a total amount of 0 to 0.05%, with the balance being Fe and impurities, and satisfying Formula (1), and has a microstructure containing, in terms of area ratio, 20% or more of bainite, with 50% or more in terms of area ratio of the balance being ferrite. At a depth position that is equivalent to one-half of a plate thickness from a surface of the heat-rolled steel plate, an average value of pole densities of an orientation group {100}<011> to {223 }<11 0> consisting of crystal orientations { 1 00}<011>, { 116}<11 0>, {114}<110>, {113}<110>, {112}<110>, {335}<110> and {223}<110> is four or less and a pole density of a {332}<113> crystal orientation is 4.8 or less. At a depth position that is equivalent to one-eighth of the plate thickness from the surface of the heat-rolled steel plate, a pole density of a { 110 }<00 1 > crystal orientation is 2.5 or more. In addition, a number density of fine Ti carbo-nitrides having a particle diameter of I 0 nm or less in the heat-rolled steel plate is l.Ox1 017 per cm3 , and a bake hardening amount is 15 MPa or more. [Ti]-48/14x[N]-48/32x[S] 2 0 (1) Where, a content (mass%) of a corresponding element is substituted for each symbol of an element in Formula (1). [0020] In a tailored rolled blank according to the present embodiment, a plate thickness changes in a tapered shape in a rolling direction. The tailored rolled blank includes a thick-wall portion, and a thin-wall portion that is thinner than the thickwall portion. In the tailored rolled blank, a ratio of an average hardness Htmax of a thickest wall portion at which the plate thickness is thickest to an average hardness Htmin of a thinnest wall portion at which the plate thickness is thinnest is in a range of more than 1.0 to 1.5. In addition, an average dislocation density of the thinnest wall portion is I xI 0 14m-2 or less, and a number density of fine Ti carbo-nitrides having a particle diameter of I 0 nm or less is more than 2x I 017 per cm3 • [002I] A method for producing a heat-rolled steel plate for a tailored rolled blank according to the present embodiment includes: a step of heating at not less than a temperature SRT min defined by Formula (2) a slab containing, in mass%, C: 0.03 to 0.1 %, Si: 1.5% or less, Mn: 1.0 to 2.5%, P: O.I% or less,. S: 0.02% or less, AI: 0.01 to 1.2%, N: O.OI% or less, Ti: 0.015 to O.I5%, Nb: 0 to 0.1 %, Cu: 0 to 1%, Ni: 0 to 1%, Mo: 0 to 0.2%, V: 0 to 0.2%, Cr: 0 to I%, W: 0 to 0.5%, Mg: 0 to 0.005%, Ca: 0 to 0.005%, rare earth metal: 0 to O.I %, B: 0 to 0.005%, and one or more types of element selected from a group consisting of Zr, Sn, Co and Zn in a total amount of 0 to 0.05%, with the balance being Fe and impurities, and satisfying Formula (I); a step of producing a rough bar by performing rough rolling with an overall draft of 60 to 90% with respect to the slab that is heated, and during the rough rolling, performing one rolling pass or more at a draft of 20% or more when a slab temperature is I 050 to II50°C; a step of producing a steel plate by starting finish rolling with respect to the rough bar within I 50 seconds after rough rolling ends, and performing finish rolling in which a temperature of the rough bar when starting the finish rolling is in a range of I 000°C to less than I 080°C, an overall draft is set in a range of75 to 95%, a total draft in a final two passes is set to 30% or more, a finish rolling ending temperature is set in a range from an An transformation temperature to I 000°C, and a shape ratio SR that is defined by Formula (3) is set to 3.5 or more; a step of starting cooling of the steel plate within three seconds after finish rolling ends, setting a cooling stopping temperature to 600°C or less, and setting an average cooling rate until the cooling stopping temperature as I5°C per second or more to thereby cool the steel plate, and making a total cumulative diffusion length Ltotai, that is defined by Formula (4), in a time period until coiling starts after the temperature of the steel plate passes an An transformation temperature O.I5 j.!m or less; and a step of coiling the steel plate after cooling at a coiling temperature of 600°C or less. [Ti]-48/14x[N]-48/32x [S];:::: 0% (I) SRT min= I 0780/ { 5.I3-Iog([Ti]x[C]) }-273 (2) SR = ld/hm (3) Ltotal = Lvf(D(T)L\tL) (4) Where, a content (mass%) of a corresponding element is substituted for each symbol of an element in Formula (1) and Formula (2). In Formula (3), "ld" represents a length of an arc of contact between a rolling roll that performs a final rolling reduction in the finish rolling and the steel plate, and is defined by the following formula. ld = v'(Lx(hin-hout)/2) Where, L (mm) represents a diameter of the rolling roll, hin represents a plate thickness (mm) of the steel plate at an entrance side of the rolling roll, and hout represents a plate thickness (mm) of the steel plate at an exit side of the rolling roll, and where hm is defined by the following formula. hm = (hin+hout)/2 In Formula (4), L\tL represents a time period until coiling starts after the temperature of the steel plate passes the An transformation temperature, and is a very small time period of 0.2 seconds. D(T) represents a volume diffusion coefficient ofTi at T°C, and is defined by the following formula when a diffusion coefficient of Ti is represented by DO, an activation energy is represented by Q, and a gas constant is represented by R. D(T) = DOxExp{-Q/R(T+273)} [0022] A method for producing a tailored rolled blank according to the present embodiment uses the aforementioned heat-rolled steel plate. The present method for producing a tailored rolled blank includes a step of producing a cold-rolled steel plate by performing cold rolling on the heat-rolled steel plate while changing a draft within a range of more than 5% to 50% so that a plate thickness changes in a tapered shape in a longitudinal direction of the heat-rolled steel plate, and a step of performing a precipitation hardening heat treatment on the cold-rolled steel plate. In the precipitation hardening heat treatment, a highest heating temperature T max is 600 to 750°C, a holding time period tK (sec) at 600°C or more satisfies Formula (5) with respect to the highest heating temperature T max, and a heat treatment index IN defined by Formula (6)is 16500 to 19500. 530-0.7xT max::; tK::; 3600-3.9xT max (5) IN= (Tn+273)(Iog(tn/3600)+20) (6) Where, tn (sec) in Formula (6) is defined by Formula (7). tn/3600 = J OX+.!\tiN/3600 (7) Where, X= ((Tn-I+273)/(Tn+273))(1og(tn-I/3600)+20)-20. Further, tl =.!\tiN, and .!\tiN is one second. T n(°C) in Formula (6) is defined by Formula (8). Tn=Tn-I+ as rectangular coordinates in an ODF (orientation distribution function). [FIG. 1 B] FIG. I B is a view illustrating main crystal orientation positions on a to {223 }<11 0> consisting of respective crystal orientations {100}<011>, {116}<110>, {114}<110>, {113}<110>, {112}<110>, {335}<110> and {223}<110> is made four or less and a pole density 02 of a {332}<113> crystal orientation is made 4.8 or less. [0040] In short, in the interior of the heat-rolled steel plate, the crystal orientation is made as random as possible. In a case where the average value of pole densities 01 ofthe orientation group {100}<011> to {223}<110> is four or less and the pole density 02 ofthe {332}<113> crystal orientation is 4.8 or less, the in-plane anisotropy of the tensile strength and breaking elongation decreases. Specifically, a value of IMI that is an index of the in-plane anisotropy of the tensile strength and breaking elongation is 0.6 or less. Specifically, in a case where an average of the tensile strength in the rolling direction, the plate-width direction, and a direction that is inclined by 45° relative to the rolling direction is 720 MPa, the standard deviation for the three directions is 12 MPa or less. Further, in a case where the average of the breaking elongation in the three directions is 17%, the standard deviation for the three directions is 0.8% or less. Because the in-plane anisotropy decreases, the plate thickness accuracy and plate width accuracy increase and cold formability is enhanced. [0041] \5 ft- On the other hand, in an outer layer in a range from the surface of the heatrolled steel plate to a depth equivalent to three-eighths of the plate thickness, a pole density D3 of a { 11 0}<001> crystal orientation is set to 2.5 or more. [0042] In short, while the crystal orientation in the interior is made as random as possible, on the outer layer, a proportion occupied by a { 11 0}<001> crystal orientation that is a specific crystal orientation is increased as much as possible. In the chemical composition of the present embodiment, grains of the { 110} <00 1 > crystal orientation are not susceptible to work hardening. When producing a tailored rolled blank, the draft is partially changed during cold rolling to produce a thick-wall portion and a thin-wall portion in the steel plate. Accordingly, the draft during the cold rolling differs between a thick-wall portion and a thin-wall portion. If the drafts are different, the amount of strain that is introduced will also be different. Therefore, a difference in work hardening arises between a thick-wall portion and a thin-wall portion, and thus a difference arises in the hardness. A difference in the hardness is liable to arise, in particular, between outer layer portions of a thick-wall portion and a thin-wall portion. [0043] As described above, the grains of the { 11 0} <00 1 > crystal orientation are not susceptible to work hardening. Further, as described later, in the present embodiment the cold-rolling rate is in a range from more than 5% to 50%. In this case, even after cold rolling, the { 110 }<00 1 > crystal orientation remains in the outer layer. Consequently, if the pole density D3 of the { 110}<001> crystal orientation is 2.5 or more, a hardness difference between a thick-wall portion and a thin-wall portion of the tailored rolled blank can be reduced, and variations in the hardness can be suppressed. As a result, the plate thickness accuracy and plate width accuracy are increased, and the cold formability is improved. [0044] If a tailored rolled blank is produced by subjecting the aforementioned heatrolled steel plate to cold rolling in which the draft is in a range of more than 5% to 50%, and performing precipitation hardening heat treatment under conditions that are described later, the aforementioned hardness ratio HR (= Htmax!Htmin =more than 1.0 -Yto 1.5) is obtained in the tailored rolled blank that is produced. In addition, the average dislocation density of a thinnest wall portion is 1 x 1 0 14m-2 or less and a number density n1 ofTi carbo-nitrides for which a circle-equivalent diameter is 0.5 to 10 nm is more than 2x1017 per cm3• [0045] A heat-rolled steel plate of the present embodiment that was completed based on the above described findings is a heat-rolled steel plate that is used for a tailored rolled blank. The heat-rolled steel plate has a chemical composition consisting of, in mass%, C: 0.03 to O.I%, Si: 1.5% or less, Mn: 1.0 to 2.5%, P: O.I% or less, S: 0.02% or less, AI: 0.01 to 1.2%, N: 0.01% or less, Ti: 0.015 to 0.15%, Nb: 0 to 0.1%, Cu: 0 to 1%, Ni: 0 to 1%, Mo: 0 to 0.2%, V: 0 to 0.2%, Cr: 0 to 1%, W: 0 to 0.5%, Mg: 0 to 0.005%, Ca: 0 to 0.005%, rare earth metal: 0 to 0.1 %, B: 0 to 0.005%, and one or more types of element selected from a group consisting of Zr, Sn, Co and Zn in a total amount of 0 to 0.05%, with the balance being Fe and impurities, and satisfying Formula (1 ), and has a microstructure containing, in terms of area ratio, 20% or more ofbainite, with 50% or more in terms of area ratio ofthe balance being ferrite. At a depth position that is equivalent to one-half of a plate thickness from a surface of the heat-rolled steel plate, an average value of pole densities of an orientation group {100}<011> to {223}<110> consisting of crystal orientations {100}<01 I>, {116}<110>, {114}<110>, {113}<110>, {112}<110>, {335}<110> and {223}<110> is four or less and a pole density of a {332}<113> crystal orientation is 4.8 or less. At a depth position that is equivalent to one-eighth of the plate thickness from the surface of the heat-rolled steel plate, a pole density of a {I I 0}<001> crystal orientation is 2.5 or more. In addition, a number density of fine Ti carbo-nitrides having a particle diameter of 10 nm or less among Ti carbonitrides in the heat-rolled steel plate is l.Ox I 017 per cm3 , and a bake hardening amount is 15 MPa or more. [Ti]-48/14x[N]-48/32x[S] ~ 0 (1) Where, a content (mass%) of a corresponding element is substituted for each symbol of an element in Formula (1). [0046] The above described chemical composition of the heat-rolled steel plate may contain one or more types of element selected from a group consisting ofNb: 0.005 to 0.1 %, Cu: 0.005 to 1%, Ni: 0.005 to 1%, Mo: 0.005 to 0.2%, V: 0.005 to 0.2%, Cr: 0.005 to 1% and W: 0.01 to 0.5%. The above described chemical composition may also contain one or more types of element selected from a group consisting of Mg: 0.0005 to 0.005%, Ca: 0.0005 to 0.005%, and rare earth metal: 0.0005 to 0.1 %. The above described chemical composition may also contain B: 0.0002 to 0.005%. The chemical composition may contain one or more types of element selected from the group consisting ofZr, Sn, Co and Zn in a total amount of0.005 to 0.05%. [0047] In a tailored rolled blank according to the present embodiment, a plate thickness changes in a tapered shape in a rolling direction. The present tailored rolled blank includes a thick-wall portion, and a thin-wall portion that is thinner than the thick-wall portion. In the tailored rolled blank, a ratio of an average hardness Htmax of a thickest wall portion at which the plate thickness is thickest to an average hardness Htmin of a thinnest wall portion at which the plate thickness is thinnest is in a range of more than 1.0 to 1.5. An average dislocation density ofthe thinnest wall portion is 1 x 10 14m-2 or less. A number density of fine Ti carbo-nitrides having a particle diameter of 1 0 nm or less is more than 2x 1 017 per cm3. [0048] Preferably, the aforementioned tailored rolled blank is produced using the aforementioned heat-rolled steel plate. The aforementioned tailored rolled blank may include a galvanized layer on the surface thereof. [0049] A method for producing a heat-rolled steel plate for a tailored rolled blank according to the present embodiment includes: a step of heating a slab having the above described chemical composition and satisfying Formula (1 ), at not less than a temperature SRTmin defined by Formula (2); a step of producing a rough bar by performing rough rolling with an overall draft of 60 to 90% with respect to the slab that is heated, and during the rough rolling, performing one rolling pass or more at a draft of 20% or more when the slab temperature is 1050 to 1150°C; a step of producing a steel plate by starting finish rolling with respect to the rough bar within 150 seconds after rough rolling ends, and performing finish rolling in which a temperature of the rough bar when starting the finish rolling is in a range of 1000°C to less than 1 080°C, an overall draft is set in a range of 75 to 95%, a total draft in a final two passes is set to 30% or more, a finish rolling ending temperature is set in a range from an An transformation temperature to 1 000°C, and a shape ratio SR that is defined by Formula (3) is set to 3.5 or more; a step of starting cooling ofthe steel plate within three seconds after finish rolling ends, setting a cooling stopping temperature to 600°C or less, and setting an average cooling rate until the cooling stopping temperature as 15°C per second or more to thereby cool the steel plate, and making a total cumulative diffusion length Ltotai, that is defined by Formula (4), in a time period until coiling starts after the temperature of the steel plate passes an An transformation temperature 0.15 )lm or less; and a step of coiling the steel plate after cooling at a coiling temperature of 600°C or less. [Ti]-48/14x[N]-48/32x[S] ~ 0% (1) SRT min= 10780/ { 5.13-log([Ti]x[C]) }-273 (2) SR = ld/hm (3) Ltotal = L..J(D(T)L\tL) (4) Where, a content (mass%) of a corresponding element is substituted for each symbol of an element in Formula (1) and Formula (2). In Formula (3), "ld" represents a length of an arc of contact between a rolling roll that performs a final rolling reduction in the finish rolling and the steel plate, and is defined by the following formula. ld = ..J(Lx(hin-hout)/2) Where, L (mm) represents a diameter of the rolling roll, hin represents a plate thickness (mm) ofthe steel plate at an entrance side of the rolling roll, and hout represents a plate thickness (mm) of the steel plate at an exit side ofthe rolling roll, and where hm is defined by the following formula. hm = (hin+hout)/2 In Formula (4), L\tL represents a time period until coiling starts after the temperature of the steel plate passes the An transformation temperature, and is a very small time period of 0.2 seconds. D(T) represents a volume diffusion coefficient ofTi at T°C, and is defined by the following formula when a diffusion coefficient of Ti is represented by DO, an activation energy is represented by Q, and a gas constant is represented by R. D(T) = DOxExp{-Q/R(T+273)} [0050] The method for producing a tailored rolled blank according to the present embodiment uses the aforementioned heat-rolled steel plate. The present method for producing a tailored rolled blank includes: a step of producing a cold-rolled steel plate by performing cold rolling on the heat-rolled steel plate while changing a draft within a range of more than 5% to 50% so that a plate thickness changes in a tapered shape in a longitudinal direction of the heat-rolled steel plate; and a step of performing a precipitation hardening heat treatment on the cold-rolled steel plate. In the precipitation hardening heat treatment, a highest heating temperature T max is 600 to 750°C, a holding time period tK (sec) at 600°C or more satisfies Formula (5) with respect to the highest heating temperature T max, and a heat treatment index IN defined by Formula (6) is 16500 to 19500. 530-0.7xT max:::; tK:::; 3600-3.9xT max (5) IN= (Tn+273)(log(tn/3600)+20) (6) Where, tn (sec) in Formula (6) is defined by Formula (7). tn/3600 = l OX+~tiN/3600 (7) Where, X= ((Tn-J+273)/(Tn+273))(log(tn-II3600)+20)-20. Further, t1 =~tiN, and ~tiN is one second. TnCOC) in Formula (6) is defined by Formula (8). Tn= Tn-J+a~tiN (8) Where, a represents the rate of temperature increase or a cooling rate (0 C/s) at the temperature T n-I . [0051] The above described method for producing a tailored rolled blank may further include a step of performing a galvanizing treatment before the step of heating the slab, before the step of cooling the steel plate after finish rolling, before the step of coiling the steel plate that is cooled, or after the step of performing a precipitation hardening heat treatment. The present method for producing a tailored rolled blank 2o may further include a step of performing an alloying treatment at 450 to 600°C after performing the galvanizing treatment. [0052] By using the heat-rolled steel plate of the present embodiment, a tailored rolled blank having a tensile strength of 590 MPa or more and having excellent cold formability can be obtained. The tailored rolled bl'ank can be used for uses such as framework components of automobiles as well as inner plate members, structural members and underbody members with respect to which a high level of performance is demanded with regard to collision absorption energy, rigidity, fatigue strength and the like. [0053] Hereunder, the heat-rolled steel plate for a tailored rolled blank, and a tailored rolled blank that is produced using the heat-rolled steel plate are described in detail. [0054] [Heat-rolled Steel Plate for Tailored Rolled Blank] [Chemical composition] The chemical composition of the heat-rolled steel plate for a tailored rolled blank of the present embodiment contains the following elements. Hereunder, the symbol "%"with respect to the content of each element denotes mass percent. [0055] C: 0.03 to 0.1% Carbon (C) increases the strength of steel by structural strengthening. In addition, when producing a tailored rolled blank using the present heat-rolled steel plate, C bonds with Ti to form Ti carbo-nitrides, and increases the strength of a tailored rolled blank by precipitation hardening. If the C content is too low, the above effects are not obtained, and the tensile strength of the tailored rolled blank will be less than 590 MPa. On the other hand, if the C content is too high, the strength becomes too high and elongation ofthe heat-rolled steel plate decreases. Accordingly, the C content is in a range of 0.03 to 0.1 %. A preferable lower limit of the C content is 0.06%. A preferable upper limit ofthe C content is 0.09%. [0056] Si: 1.5% or less Silicon (Si) is unavoidably contained. Si dissolves in steel to increase the strength of the steel. Si also improves the balance between tensile strength and elongation. However, if the Si content is too high, tiger-striped scale is formed and the surface properties ofthe heat-rolled steel plate deteriorate. In this case, the productivity of a pickling treatment that is performed with the objective of removing scale decreases. If the surface properties ofthe heat-rolled steel plate deteriorate, the chemical treatability will also decrease, and hence corrosion resistance after coating of the tailored rolled blank will decrease. Accordingly, the Si content is 1.5% or less (not including 0%). A preferable lower limit of the Si content is 0.02%. In this case, as well as the above described effects, the occurrence of scale defects as typified by fish-scale defects and spindle-shaped scale can also be suppressed. A preferable upper limit of the Si content is 0.07%. In this case, the occurrence of tiger-striped scale can be further suppressed. [0057] Mn: 1.0 to 2.5% Manganese (Mn) contributes to solid-solution strengthening of steel and also increases the hardenability of the steel. If the Mn content is too low, the strength of the steel will be too low, and the tensile strength will be less than 590 MPa. On the other hand, if the Mn content is too high, segregation is liable to occur and the workability and press formability will decrease. Accordingly, the Mn content is from 1.0 to 2.5%. An appropriate range ofthe Mn content depends on the tensile strength. A preferable Mn content in a tailored rolled blank having a tensile strength of 590 to 700 MPa is 1.0 to 1.8%. A preferable Mn content in a tailored rolled blank having a tensile strength of 700 to 900 MPa is 1.6 to 2.2%. A preferable Mn content in a tailored rolled blank having a tensile strength of 900 MPa or more is 2.0 to 2.5% [0058] Mn also suppresses the occurrence of hot cracking caused by S. In a case where the content of an element other than Mn for suppressing the occurrence of hot cracking caused by S is insufficient, a ratio of the Mn content ([Mn]) with respect to the S content ([S]) ([Mn]/[S]) is preferably 20 or more. [0059] P: 0.1% or less Phosphorus (P) is unavoidably contained. P contributes to solid-solution strengthening of steel. However, if the P content is too high, the workability and weldability of the steel plate decreases. Accordingly, the P content is 0.1% or less (not including 0%). A preferable lower limit of the P content is 0.005%. A preferable upper limit of the P content is 0.02%. [0060] S: 0.02% or less Sulfur (S) is an impurity that is unavoidably contained. S generates inclusions such as MnS and reduces the stretch-flange formability of steel, and also causes cracking during hot rolling. Accordingly, the S content is 0.02% or less (not including 0%). A preferable upper limit of the S content is 0.005%. In this case, the weldability and production stability during castirig and during heat rolling increases. Preferably, the S content is as low as possible. However, when production costs are taken into consideration, a lower limit of the S content is, for example, 0.0001%. [0061] AI: 0.01 to 1.2% Aluminum (AI) deoxidizes steel and reduces dissolved oxygen in molten steel. Therefore, AI can suppress the formation of alloy oxides that are formed by Ti, Nb, Mo and V bonding with dissolved oxygen. If the AI content is too low, this effect is not obtained. On the other hand, if the AI content is too high, a tundish nozzle is liable to clog at the time of casting. Furthermore, if the AI content is too high the chemical treatability and zinc plating properties will decrease. Moreover, if the AI content is too high, a large amount of non-metallic inclusions such as alumina are generated, and the local ductility of the steel decreases. Therefore, the AI content is in a range from 0.01 to 1.2%. A preferable lower limit ofthe AI content is 0.02%. In a case of further enhancing the chemical treatment and zinc plating properties, a preferable upper limit of the AI content is 0.6%. In a case of further suppressing generation of non-metallic inclusions such as alumina, a preferable upper limit ofthe · AI content is 0.3%. [0062] N: 0.01% or less Nitrogen (N) is an impurity that is unavoidably contained. N bonds with Ti, Nb and the like to form nitrides. In this case, if nitrides are formed, it is difficult for Ti and Nb to exhibit the actions that are described later. In addition, these nitrides precipitate at high temperature and tend to coarsen readily, and are liable to act as a starting point of burring cracking. Therefore, theN content is 0.0 I% or less (not including 0%). [0063] Note that, when using the tailored rolled blank of the present embodiment for a member in which aging deterioration becomes a problem, a preferable upper limit ofthe N content is 0.006%. Further, when using the tailored rolled blank of the present embodiment with respect to a member based on the premise that the member will be subjected to working after being left to stand at room temperature for two weeks or more after production, a preferable upper limit of theN content is 0.005%. In a case where the tailored rolled blank will be left to stand under a hightemperature environment in summer or will be exported using a marine vessel or the like to a region located across the equator, the preferable upper limit of the N content is less than 0.004%. [0064] Ti: 0.015 to 0.15% Among various kinds of precipitation hardening elements, titanium (Ti) is the element with the highest precipitation hardening capacity. This is because Ti is the element in which a difference between the solubility in a y-phase (austenite) and an a-phase (ferrite) is largest. In the present embodiment, precipitation ofTi carbonitrides (Ti(C, N)) in the heat-rolled steel plate is suppressed to the utmost, and Ti is caused to be present in a dissolved state or in a cluster state. Cold rolling is performed on the heat-rolled steel plate to produce an intermediate product in the shape of a tailored rolled blank. At such time, a large amount of dislocations are introduced into the intermediate product. The intermediate product is subjected to precipitation hardening heat treatment to produce a tailored rolled blank. At such time, Ti carbo-nitrides finely precipitate on the dislocations, and the tailored rolled blank undergoes precipitation hardening. In this way, the strength and elongation of the tailored rolled blank improves. [0065] When the Ti content is too low, the number density ofTi carbo-nitrides in the tailored rolled blank is less than 10 10 per mm3, and the tensile strength ofthe tailored rolled blank after precipitation hardening heat treatment is less than 590 MPa. In contrast, if the Ti content is too high, the above described effect saturates, and furthermore, a tundish nozzle is liable to clog up. Further, if the Ti content is too high, the austenite recrystallization speed is slow during hot rolling and an aggregate structure of the heat-rolled steel plate is liable to develop. In this case, in-plane anisotropy increases in the tailored rolled blank after the precipitation hardening heat treatment. In this case, because the cold formability of the heat-rolled steel plate decreases, the plate thickness accuracy and plate width accuracy of the tailored rolled blank becomes lower. Accordingly, the Ti content is from 0.015 to 0.15%. A preferable upper limit of the Ti content is 0.12%. [0066] [Regarding Formula (I)] The above described chemical composition also satisfies Formula (1). [Ti]-48/14x[N]-48/32x[S] ~ 0 (1) Where, a content (mass%) of the corresponding element is substituted for the respective symbols of elements in Formula (1 ). [0067] As described above, Ti finely precipitates as Ti carbo-nitrides (Ti(C, N)) when subjected to a precipitation hardening heat treatment, and thus the tailored rolled blank undergoes precipitation hardening and the tensile strength thereof is 590 MPa or more. However, Ti has a high affinity with Nand S. Therefore, if the Ti content is too low relative to the N content and S content, TiN and TiS are formed without forming Ti carbo-nitrides. Since TiN and TiS are coarse, TiN and TiS do not contribute to improving the strength of the steel. Therefore, Ti must be contained in an amount such that Ti sufficiently precipitates as Ti carbo-nitrides. [0068] :%- F1 is defined as equal to [Ti]-48/14x[N]-48/32x[S]. IfF1 is less than 0, the Ti content is too low relative to theN content and S content in the heat-rolled steel plate. In this case, even if a precipitation hardening heat treatment that is described later is performed on the heat-rolled steel plate, it will be difficult forTi carbonitrides to be formed. On the other hand, ifF 1 is 0 or more, a sufficient amount of Ti for precipitating as carbo-nitrides is contained. In this case, the strength of the tailored rolled blank can be raised to 590 MPa or more. [0069] The balance of the chemical composition of the heat-rolled steel plate of the present embodiment is Fe and impurities. Here, the term "impurities" refers to components that are contained in a raw material of ore, scrap or the like or that are mixed in due to some other cause when industrially producing the heat-rolled steel plate. [0070] The heat-rolled steel plate according to the present embodiment may further contain one or more types of element selected from the group consisting ofNb, Cu, Ni, Mo, V, Cr and Was a substitute for a part of Fe. Each of these elements is an optional element. Each of these elements increases the strength of the steel. [0071] Nb: 0 to 0.1% Niobium (Nb) is an optional element, and need not be contained. In a case where Nb is contained, the Nb increases the strength of the steel by precipitation hardening, similarly to Ti. If even a small amount ofNb is contained, the above described effect is obtained. However, ifthe Nb content is too high, the precipitation hardening saturates and the elongation and workability decreases. Therefore, the Nb content is from 0 to 0.1 %. A preferable lower limit of the Nb content for further effectively obtaining the above described effect is 0.005%, and more preferably is 0.02%. A preferable upper limit of the Nb content is 0.05%. [0072] Cu: 0 to 1% Copper (Cu) is an optional element, and need not be contained. In a case where Cu is contained, the Cu precipitates independently, and increases the strength ;26- of the steel. If even a small amount of Cu is contained, the above described effect is obtained. However, if the Cu content is too high, the steel becomes brittle during hot rolling. Therefore, the Cu content is from 0 to 1%. A preferable lower limit of the Cu content for further effectively obtaining the above described effect is 0.005%. [0073] Ni: 0 to 1% Nickel (Ni) is an optional element, and need not be contained. In a case where Ni is contained, similarly to Mn, the Ni increases the hardenability ofthe steel and raises the strength of the steel and also raises the toughness ofthe steel. In a case where Cu is contained, the Ni also suppresses hot brittleness of the steel. If even a small amount ofNi is contained, the above described effect is obtained. However, if the Ni content is too high, the production costs rise. Therefore, the Ni content is from 0 to 1%. A preferable lower limit of the Ni content for further effectively obtaining the above described effect is 0 .. 005%. [0074] Mo: 0 to 0.2% V: 0 to 0.2% Molybdenum (Mo) and vanadium (V) are each optional elements, and need not be contained. In a case where Mo and V are contained, similarly to Ti and Nb, the Mo and V cause the steel to undergo precipitation hardening. If even a small amount of Mo and V is contained, the above described effect is obtained. However, if the Mo and V content is too high, elongation ofthe steel decreases. Therefore, the Mo content is from 0 to 0.2%, and the V content is from 0 to 0.2%. For further effectively obtaining the above described effect, a preferable lower limit of the Mo content is 0.005% and a preferable lower limit of the V content is 0.005%. [0075] Cr: 0 to 1% Chromium (Cr) is an optional element, and n'eed not be contained. In a case where Cr is contained, similarly to Mn, the Cr increases the hardenability and raises the strength of the steel and also raises the toughness of the steel. If even a small amount of Cr is contained, the above described effect is obtained. However, if the Cr content is too high, Cr-based alloy carbides that are typified by Cr23C6 precipitate. IfCr-based alloy carbides precipitate at the grain boundary, the press formability decreases. Therefore, the Cr content is from 0 to I%. A preferable lower limit of the Cr content for further effectively obtaining the above described effect is 0.005%. [0076] W: 0 to 0.5% Tungsten (W) is an optional element, and need not be contained. In a case where W is contained, the W increases the strength of the steel by precipitation hardening or solid-solution strengthening. If even a small amount of W is contained, the above described effect is obtained. However, if theW content is too high, the above described effect saturates and the production costs rise. Therefore, theW content is from 0 to 0.5%. A preferable lower limit of theW content for further effectively obtaining the above described effect is 0.01 %. [0077] The heat-rolled steel plate according to the present embodiment may further contain one or more types of element selected from the group consisting of Mg, Ca and rare earth metals (REM) as a substitute for a part of Fe. Each of these elements increases the workability of the steel. [0078] Mg: 0 to 0.005% Ca: 0 to 0.005% Rare earth metal: 0 to 0.1% Magnesium (Mg), calcium (Ca) and rare earth metals (REM) are each optional elements, and need not be contained. If contained, each of these elements controls the form of non-metallic inclusions. Non-metallic inclusions are the starting points of fractures, and reduce the workability of steel. Therefore, if the form of non-metallic inclusions is controlled, the workability ofthe steel increases. If even a small amount of these elements is contained; the above described effect is obtained. However, if the content of these elements is too high, the above described effect saturates and the production costs rise. Therefore, the Mg content is from 0 to 0.005%, the Ca content is from 0 to 0.005%, and the REM content is from 0 to 0.1 %. For further effectively obtaining the above described effect, a preferable lower limit of the Mg content, a preferable lower limit of the Ca content and a preferable lower limit of the REM content are each 0.0005%. [0079] In the present description, the term "REM" is a generic term for a total of 17 elements of Sc, Y and lanthanoids, and the term "REM content" refers to the total content of the aforementioned elements. In many cases REM elements are added as a misch metal, and are contained in complex form with an element such as La or Ce. Metals such as La and Ce may also be added as an REM. [0080] The heat-rolled steel plate of the present embodiment may further contain B as a substitute for a part of Fe. [0081] B: 0 to 0.005% Boron (B) is an optional element, and need not be contained. If contained, B enhances the hardenability ofthe steel and increases a structural fraction of a lowtemperature transformation generating phase that is a hard phase. If even a small amount of B is contained, the above described effect is effectively obtained. However, if the B content is too high, the above described effect saturates and the production costs further rise. Therefore, the B content is from 0 to 0.005%. A preferable lower limit of the B content for further effectively obtaining the above described effect is 0.0002%. In a cooling step after continuous casting, a preferable upper limit of the B content for suppressing the occurrence of slab cracking is 0.0015%. [0082] The heat-rolled steel plate of the present embodiment may further contain one or more types of element selected from the group consisting of Zr, Sn, Co and Zn as a substitute for a part of Fe. [0083] One or more types of element selected from the group consisting of Zr, Sn, Co and Zn: 0 to 0.05% in total Zirconium (Zr), tin (Sn), cobalt (Co) and zinc (Zn) are each optional elements and need not be contained. If contained, these elements increase the strength of the ;,2!1- steel by solid-solution strengthening or precipitation strengthening. These elements also control the form of sulfides and oxides to increase the toughness of the steel. If even a small amount of these elements is contained, the above described effects are obtained. On the other hand, if the total content of these elements is too high, the ductility of the steel decreases. Therefore, the total content of one or more types of element selected from the group consisting of Zr, Sn, Co and Zn is 0 to 0.05%. A preferable lower limit of the total content of these elements is 0.005%. In a case where Sn is contained, if the Sn content is too high, flaws are liable to arise in the steel during hot rolling. Therefore, a preferable upper limit of the Sn content is 0.03%. [0084] [Microstructure] The microstructure ofthe heat-rolled steel plate of the present embodiment contains, in terms of the area ratio, 20% or more of bainite, and the balance is mainly ferrite. Here, the term "the balance is mainly ferrite" means that half (50%) or more of the balance in terms ofthe area ratio is ferrite. In addition to ferrite, the balance may contain martensite, retained austenite, pearlite and the like. Preferably, the area ratio of martensite in the microstructure is 5% or Jess, the area ratio of retained austenite is 2% or Jess, and the area ratio of pearlite is 2% or Jess. In this case, the local ductility increases and the stretch-flange formability is enhanced. [0085] If the area ratio of bainite in the microstructure is Jess than 20%, the area ratio of ferrite that is increased in strength by precipitation strengthening is too high, and hence the cold formability ofthe steel decreases. Specifically, in a case where a tailored rolled blank is produced using a heat-rolled steel plate in which the bainite area ratio is less than 20%, the strength ofthe steel plate excessively increases during cold rolling, and the rolling reaction force rises. In such a case, the dimensional accuracy (plate thickness accuracy and plate width accuracy) of the tailored rolled blank decreases and the cold formability also decreases. [0086] Furthermore, if the bainite area ratio is Jess than 20%, in some cases an overaging state arises in the heat-rolled steel plate. In such a case, the strength of the heat-rolled steel plate decreases. Therefore, the cold formability is maintained. However, an improvement in the strength ofthe steel plate by precipitation hardening during a heat treatment after cold rolling is not obtained. Therefore, in the microstructure of the heat-rolled steel plate, the bainite area ratio is 20% or more, and the balance is mainly ferrite. [0087] In the present embodiment, to dissolve or cluster Ti in the heat-rolled steel plate, as described later, a coiling temperature CT is set to 600°C or less. This coiling temperature CT comes close to a bainite transformation temperature for the aforementioned chemical composition. Therefore, the microstructure ofthe heatrolled steel plate of the present embodiment contains a large amount of bainite and also includes a large number of dislocations (transformation dislocations) that are introduced during bainite transformation. A transformation dislocation is a nucleation site of Ti carbo-nitrides. Therefore, an even greater amount of precipitation hardening can be obtained by the precipitation hardening heat treatment. [0088] The area ratio of bainite can be adjusted by controlling the cooling history during hot rolling. A preferable lower limit ofthe area ratio of bainite is more than 70%. In this case, the strength of the tailored rolled blank can be further enhanced by precipitation hardening, and coarse cementite for which the cold formability is low decreases in the microstructure. Hence, the cold formability increases. A preferable upper limit of the area ratio of bainite is 90%. [0089] The term "ferrite" as the balance in the microstructure that is mentioned above refers to polygonal ferrite (PF). More specifically, polygonal ferrite is a grain whose interior structure does not appear by etching using a nital reagent, and which also satisfies the formula lq/dq < 3.5 when the circumferential length of the target grain is represented by lq and the circle-equivalent diameter thereof is represented by dq. [0090] [Method of measuring area ratio of each phase] 3~- The area ratio of each phase in the aforementioned microstructure is measured by the following method. A sample is taken from the heat-rolled steel plate. Of the total surface of the sample, a plate-thickness cross section that is parallel to the rolling direction is taken as an observation surface. After polishing the observation surface, the observation surface is subjected to etching with nital. A visual field of 300 1-Lm x 300 1-Lm of the observation surface after etching is photographed using an optical microscope to generate a structural photograph at a position at a depth equivalent to one-quarter of the plate thickness. Image analysis is performed on the obtained structural photograph to determine the area ratio of ferrite (polygonal ferrite), the area ratio of pearlite, and the total area ratio of bainite and martensite, respectively. [0091] In addition, another sample is taken from the heat-rolled steel plate. On the surface of the sample, a plate-thickness cross section that is parallel to the rolling direction is taken as the observation surface. The observation surface is subjected to LePera corrosion after polishing the observation surface. A visual field of 300 1-Lm x 300 1-Lm of the observation surface after corrosion is photographed using an optical microscope to generate a structural photograph at a depth position equivalent to one-quarter of the plate thickness. Image processing is performed on the obtained structural photograph to determine the total area ratio of retained austenite and martensite. [0092] In addition, a different sample is prepared that is surface milled to a depth of one-quarter of the plate thickness from a rolling surface normal direction. Of the entire sample surface, X-ray diffraction measurement is performed with respect to the surface that underwent surface milling, and the volume ratio of retained austenite is thereby determined. Since the volume ratio of retained austenite is equal to the area ratio of retained austenite, the obtained volume ratio of retained austenite is defined as the area ratio of the retained austenite. [0093] The area ratio of bainite and the area ratio of martensite are determined based on the total area ratio of bainite and martensite, the total area ratio of retained austenite and martensite, and the area ratio of retained austenite that are obtained by the above described method. [0094] The respective area ratios of ferrite, bainite, martensite, retained austenite and pearlite can be determined by the above described method. [0095] [Number density no and bake hardening amount (BH amount) of fine Ti carbo-nitrides in heat-rolled steel plate] Preferably, the Ti is dissolved or is in clusters in the heat-rolled steel plate. In short, it is preferable that the amount ofTi carbo-nitride in the heat-rolled steel plate is as small as possible. Ti carbo-nitrides having a particle diameter exceeding 10 nm (hereunder, referred to as "coarse Ti carbo-nitrides") does not contribute to strengthening of the heat-rolled steel plate. On the other hand, if a large amount of Ti carbo-nitrides having a particle diameter of I 0 nm or less (hereunder, referred to as "fine Ti carbo-nitrides") precipitates, the strength ofthe heat-rolled steel plate will be too high. In this case, the rolling reaction force during cold rolling on the heatrolled steel plate becomes excessively high. [0096] In addition, in a case where coarse Ti carbo-nitrides and fine Ti carbo-nitrides are formed in the heat-rolled steel plate, even if a precipitation hardening heat treatment is performed on the steel plate after cold rolling (cold-rolled steel plate), it is difficult forTi carbo-nitrides to be formed and thus precipitation hardening is not obtained. Therefore, in the heat-rolled steel plate, it is preferable that the number of fine Ti carbo-nitrides and coarse Ti carbo-nitrides is small, and Ti is in a dissolved or clustered state. [0097] In a case where a number density no of fine Ti carbo-nitrides in the heat-rolled steel plate is l.Ox I 017 per cm3 or less, and a bake hardening amount (BH amount) is I5 MPa or more, Ti is adequately dissolved in the heat-rolled steel plate or is present . therein as cluster-shaped Ti carbo-nitrides. In this case, precipitation hardening does not occur in the heat-rolled steel plate, and breaking elongation increases. Consequently, a rolling reaction force during cold rolling can be suppressed to a low amount, and cold formability increases. In addition, a large number of dislocations are introduced into the steel plate by the decrease in the rolling reaction force. The introduced dislocations become precipitation sites ofTi carbo-nitrides during the precipitation hardening heat treatment after cold rolling. Therefore, a large amount of fine Ti carbo-nitrides precipitate, and the strength of the tailored rolled blank can be increased to 590 MPa or more. In addition, during the precipitation hardening heat treatment, restoration of dislocations occurs and the dislocation density decreases. As a result, the ductility of the tailored rolled blank increases. Therefore, the number density no of fine Ti carbo-nitrides in the heat-rolled steel plate is l.Ox 10 17 per cm3 or less, and the BH amount is 15 MPa or more. [0098] [Method of measuring number density no of fine Ti carbo-nitrides] The method of measuring the number density no of the fine Ti carbo-nitrides is as follows. An acicular sample is prepared from the heat-rolled steel plate by cutting and electropolishing. At this time, focused ion beam milling may be utilized together with electropolishing according to need. A three-dimensional distribution image of complex carbo-nitrides is acquired from the acicular sample by a threedimensional atom probe measurement method. [0099] According to the three-dimensional atom probe measurement method, integrated data can be reconstructed to acquire an actual three-dimensional distribution image of atoms in a real-space. With regard to measurement of the particle diameter of the Ti carbo-nitrides, a diameter when the relevant precipitate is regarded as a sphere is determined based on the number of atoms constituting the precipitate that is the observation object and the lattice constant thereof, and the diameter that is determined is defined as the particle diameter of the Ti carbo-nitride. [0100] In the present description, particles having a particle diameter in a range from 0.5 to 10 nm among the Ti carbo-nitrides are defined as fine Ti carbo-nitrides. In a case where the particle diameter is less than 0.5 nm, because the particle diameter is less than the lattice constant of the Ti carbo-nitrides, the Ti carbo-nitrides cannot be regarded as a precipitate. The number density no (particles/cm3 ) is determined based on the number of fine Ti carbo-nitrides. [0101] [Method of measuring bake hardening amount (BH amount)] The BH amount is an index that shows the amount of dissolved C. In a case where a large amount of coarse Ti carbo-nitrides precipitates, the BH amount in the heat-rolled steel plate is low. In this case, an adequate amount of carbo-nitride precipitation is not obtained in the precipitation hardening heat treatment after cold rolling. If the BH amount in the heat-rolled steel plate is 15 MPa or more, because the amount of coarse Ti carbo-nitrides contained in the heat-rolled steel plate is sufficiently suppressed, the steel plate after the precipitation hardening heat treatment is adequately hardened. A preferable BH amount is 25 MPa or m.o re, and a more preferable BH amount is 30 MPa or more. [01 02] The method of measuring the BH amount is as follows. A JIS No. 5 tensile test specimen for which the rolling width direction is taken as the longitudinal direction is extracted from the heat-rolled steel plate. A tension test is performed on the tensile test specimen, and given a tension prestrain of 4%. After being given the tension prestrain of 4%, the load is temporarily removed. The tensile test specimen from which the load is removed is subjected to heat treatment for 20 minutes at 180°C. The tensile test specimen after the heat treatment is subjected to a tension test once again. The BH amount is the margin of increase in the deforming stress at the time of the tension test after the heat treatment, and is determined by the following equation. BH amount (MPa) = UYa (MPa)- FSb (MPa) Where, UYa represents an upper yield point (MPa) when tension is reapplied after the heat treatment, and FSb represents the maximum deforming stress (MPa) when the tensile test specimen is given a tension prestrain of 4%. [0103] [Crystal orientation] With respect to the heat-rolled steel plate ofthe present embodiment, a range of a depth equivalent to three-eighths of the plate thickness to a depth equivalent to five-eighths of the plate thickness from the surface is defined as the "interior" of the heat-rolled steel plate. A result of a crystal orientation measurement at a depth position (center portion) equivalent to one-half of the plate thickness from the surface among the entire interior of the heat-rolled steel plate is defined as the crystal orientation of the interior. On the other hand, a range from the surface to a depth equivalent to one-quarter of the plate thickness is defined as an "outer layer" of the heat-rolled steel plate. Further, a result of a crystal orientation measurement at center position of the "outer layer", that is, a position at a depth equivalent to oneeighth of the plate thickness from the surface is defined as the crystal orientation of the outer layer. In the interior and the outer layer, the crystal orientation satisfies the following conditions. [0 I 04] [Crystal orientation of interior] In the interior, an average value of pole dens~ties 01 of a crystal orientation group (hereunder, referred to as "orientation group {I 00}<011> to {223 }<11 0>") consisting of crystal orientations {100}<011>, {116}<110>, {114}<110>, { 113 }<11 0>, { 112}<11 0>, {335}<11 0> and {223 }<11 0> is four or less and a pole density 02 of a {332}<113> crystal orientation is 4.8 or less. [01 05] In short, in the interior of the heat-rolled steel plate, the crystal orientation is made as random as possible to decrease the in-plane anisotropy. In a case where the average value of the pole densities 01 of the orientation group { 1 00}<011> to {223}<110> is four or less and the pole density 02 ofthe {332}<113> crystal orientation is 4.8 or less, the in-plane anisotropy ofthe tensile strength and breaking elongation decreases. Specifically, a value ofiMI that is an index ofthe in-plane anisotropy of the tensile strength and breaking elongation is less than 0.6. In this case, because the in-plane anisotropy is small, the dimensional accuracy (plate thickness accuracy and plate width accuracy) of an intermediate product after cold rolling increases, and excellent cold formability is obtained. [01 06] Ifthe average value ofthe pole densities 01 ofthe orientation group {100}<011> to {223}<110> exceeds 4, or ifthe pole density 02 ofthe {332}<113> crystal orientation exceeds 4.8, the value of IMI becomes 0.6 or more, and the inplane anisotropy becomes too large. In such case, the cold formability decreases. A preferable upper limit of the average value of the pole densities 01 of the orientation group {100}<011> to {223}<110> is 3.5. A further preferable upper limit is 3.0. A preferable upper limit of the pole density 02 of the {332}<113> crystal orientation is 4.0. A further preferable upper limit is 3.0. [01 07] [Crystal orientation of outer layer] On the other hand, in the outer layer, a pole density 03 of a { 11 0} <00 1 > crystal orientation is 2.5 or more. In short, although the crystal orientation is made as random as possible in the interior, in the outer layer the proportion thereof that is occupied by the { 11 0}<001> crystal orientation as a specific crystal orientation is made as high as possible. [0108] In plastic deformation (rolling deformation) of a bee metal, for grains of the {11 0} <00 1 > crystal orientation, there are few active slip systems and the orientation is not susceptible to work hardening. When producing a tailored rolled blank, the draft is partially changed during cold rolling to produce a thick-wall portion and a thin-wall portion in the steel plate. Accordingly, the draft during the cold rolling differs between a thick-wall portion and a thin-wall portion. If the drafts are different, the amount of strain that is introduced will also be different. Therefore, a difference in work hardening arises between a thick-wall portion and a thin-wall portion, and thus a difference arises in the hardness. A difference in the hardness is liable to arise, in particular, between the outer layer portions of a thick-wall portion and a thin-wall portion. In a case where the hardness of a steel plate differs depending on the region, the cold formability of a tailored rolled blank decreases. Accordingly, it is preferable to make a hardness difference as small as possible. [0109] As described above, the grains ofthe {110}<001> crystal orientation are not susceptible to work hardening. Further, as described later, in the present embodiment the cold-rolling rate is in a range frorri more than 5 to 50%. In this· case, even after cold rolling, the { 11 0}<001> crystal orientation remains in the outer -71'- layer. Therefore, in the outer layer of the heat-rolled steel plate, if the pole density of the { 11 0}<001> crystal orientation is high, specifically, if the pole density D3 of the {110}<001> crystal orientation is 2.5 or more, a hardness difference between a thick-wall portion and thin-wall portion of the tailored rolled blank can be reduced, and a variation in the hardness can be suppressed. As a result, the cold formability of the tailored rolled blank increases. [011 0] If the pole density D3 of the { 110}<001> crystal orientation is less than 2.5, the hardness difference between a thick-wall portion and a thin-wall portion of the tailored rolled blank becomes large. A preferable lower limit ofthe pole density of the {II 0 }<00 1 > crystal orientation is 3 .0, and further preferably is 4.0. [0 Ill] The term "pole density" refers to a value that indicates how many times higher the degree of accumulation of a test sample is relative to a reference sample that generally does not have accumulation in a specific orientation. In the embodiment of the present invention, values measured by an EBSP (Electron Back Scattering Pattern) method are used for the pole densities described hereunder. [0112] Measurement of a pole density by the EBSP method is performed as follows. A cross-section parallel to the rolling direction of the heat-rolled steel plate is adopted as the observation surface. Of the entire observation surface, a rectangular region of I 000 j..lm in the rolling direction and I 00 j..lm in the rolling surface normal direction that is centered on a depth position (t/8) that is equivalent to one-eighth of a plate thickness t from the steel plate surface is defined as an outer layer region. Similarly, a rectangular region of 1000 j..lm in the rolling direction and I 00 1-lm in the rolling surface normal direction that is centered on a depth position (t/2) that is equivalent to one-half of the plate thickness t from the steel plate surface is defined as an interior region. EBSD analysis is performed at measurement intervals of I j..lm with respect to the outer layer region and interior region to acquire crystal orientation information. [0113] ;$- The EBSD analysis is carried out at an analysis speed of200 to 300 points per second using an apparatus constituted by a thermal field emission scanning electron microscope (JSM-7001F; manufactured by JEOL Ltd.) and an EBSD detector (Hikari detector; manufactured by TSL). An ODF (orientation distribution function) is calculated with respect to the measured crystal orientation information using EBSD analysis software "OIM Analysis (registered trademark)". By this means, the pole density of each crystal orientation can be determined. [0 114] FIG. 1A is a schematic diagram of Euler space that takes angular variables cp1, cp2 and as rectangular coordinates in an ODF (orientation distribution function), and FIG. 1 B is a view illustrating main crystal orientation positions on a cp2 = 45° section in the Euler space shown in FIG. 1A. Regarding the orientations, normally, crystal orientations perpendicular to a plate plane are represented by (hkl) or {hkl}, and crystal orientation parallel to the rolling direction are represented by [uvw] or . The terms {hkl} and represent collective terms for equivalent planes, and (hkl) and [ uvw] represent individual crystal planes. [0 115] The crystalline structure of the heat-rolled steel plate of the present embodiment is a body-centered cubic structure (bee 'structure). Therefore, for example, (111), (-111), (1-11), (11-1), (-1-11), (-11-1), (1-1-1) and (-1-1-1) are equivalent and cannot be distinguished from each other. These orientations are collectively called { 111}. [0 116] Note that, ODF is also used for representing crystal orientations of lowsymmetry crystalline structures. In general, such crystal orientations are represented by cp 1 = 0 to 360°, = 0 to 180°, and cp2 = 0 to 360°, and individual crystal orientations are represented by (hkl)[uvw]. However, the crystalline structure of the heat-rolled steel plate ofthe present embodiment is a body-centered cubic structure that has a high degree of symmetry. Therefore, and cp2 can be represented with 0 to 90°. [0 117] ~- When performing a calculation,

are synonymous. Therefore, for example, a random strength ratio of an (001)[1-10] orientation ofthe ODF at a orientation. [0118] [Method for producing heat-rolled steel plate for a tailored rolled blank] An example of the method for producing a heat-rolled steel plate for a tailored rolled blank that is described above will now be described. The method for producing a heat-rolled steel plate for a tailored rolled blank according to the present embodiment includes a casting process and a hot rolling process. Hereunder, each process is described. [0 Il9] [Casting process] Molten steel is produced by a melting process using a shaft furnace, a converter, an electric furnace or the like, and the molten steel is then adjusted by various kinds of secondary refining processes so as to satisfy the aforementioned chemical composition and Formula (1). The molten steel that is produced is used to produce a slab by normal continuous casting, casting by an ingot method, or a thin slab casting method or the like. Note that, scrap may also be used for the raw material of the molten steel. In a case where a slab is obtained by continuous casting, a high-temperature slab may be directly transferred as it is to a hot rolling mill, or the slab may be cooled to room temperature and thereafter reheated in a heating furnace and subjected to hot rolling. [0120] [Hot rolling process] ).· 2 Hot rolling is carried out using the produced slab to thereby produce a heatrolled steel plate. The hot rolling process includes a heating step (S I), a rough rolling step (S2), a finish rolling step (S3), a cooling step (S4) and a coiling step (S5). [0 I2I] In the heat-rolled steel plate of the present embodiment, precipitation ofTi carbo-nitrides is suppressed as much as possible, and the Ti is dissolved or the Ti carbo-nitride is placed in a clustered state. In addition, the pole density 0 I of the interior orientation group {I OO} to {223 } and the pole density 02 of the {332} crystal orientation is reduced, and the pole density 03 ofthe {II 0}<00 I> crystal orientation of the outer layer is increased. By this means, the in-plane anisotropy of the heat-rolled steel plate is reduced, and the cold formability of the heat-rolled steel plate is increased. Furthermore, a hardness difference between a thick-wall portion and a thin-wall portion of the tailored rolled blank is decreased, and the cold formability ofthe tailored rolled blank is also increased. The respective steps are described in detail below. [0122] [Heating· step (S I)] First, the slab is heated in a heating furnace (heating step). The respective conditions in the heating step are as follows. [OI23] Heating temperature Ts 1: not less than temperature SR T min ( 0 C) defined by Formula (2) Heat the slab at the heating temperature Ts 1 that is not less than the heating temperature SRT min ( 0 C) defined by Formula (2). SRT min= 10780/ { 5.I3-log([Ti]x[C]) }-273 (2) The content of the corresponding element is substituted for the respective symbols of elements in Formula (2). [OI24] If the heating temperature Ts 1 is less than SRT min, coarse Ti carbo-nitrides in the slab do not dissolve sufficiently. In this case, a large amount of coarse Ti carbonitrides remain inside the heat-rolled steel plate, and as a result the BH amount decreases. Consequently, the strength ofthe heat-rolled steel plate decreases. In addition, an effect of precipitation hardening by the precipitation hardening heat treatment is not adequately obtained. If the heating temperature is SRT min or more, formability is adequately obtained at a time of cold rolling and the tensile strength of the tailored rolled blank is increased by precipitation hardening. A preferable lower limit of the heating temperature for further increasing the operational efficiency is 1100°C. [OI25] Heating time period ts 1 at temperature SRT min or more: 30 minutes or more A heating time period ts1 after the heating temperature becomes SRT min or more is 30 minutes or more. In this case, Ti carbo-nitrides can be sufficiently dissolved. A preferable heating time period ts1 is 60 minutes or more. In this case, the slab can be evenly heated to a sufficient degree in the thickness direction thereof. A preferable heating time period ts 1 is not more than 240 minutes. In this case, excessive generation of scale can be suppressed, and a decrease in the yield can be suppressed. [OI26] Note that, after casting the slab may also be directly transferred as it is without being reheated to a roughing mill, described later, to perform rough rolling. [OI27] [Rough rolling step (S2)] Rough rolling is promptly carried out on the slab extracted from the heating furnace to thereby produce a rough bar. The conditions for rough rolling are as follows. [OI28] Number of passes in which specific rolling is performed SPN: I or more In the rough rolling, rolling in which the draft 20% or more and the slab temperature is in a range from I 050 to II50°C is defined as "specific rolling". In the rough rolling, specific rolling is performed one time (one pass) or more. That is, the number of passes (specific passes number) SPN in which specific rolling is performed is one or more. We claim: 1. A heat-rolled steel plate for a tailored rolled blank comprising: a chemical composition coRsisting of, in mass%, C: 0.03 to 0.1 %, Si: 1.5% or less, Mn: 1.0 to 2.5%, P: 0.1% or less, S: 0.02% or less, AI: 0.01 to 1.2%, N: 0.01% or less, Ti: 0.015 to 0.15%, Nb: 0 to 0.1 %, Cu: 0 to 1%, Ni: 0 to 1%, Mo: 0 to 0 .. 2%, V: 0 to 0.2%, Cr: 0 to 1%, W: 0 to 0.5%, Mg: 0 to 0.005%, Ca: 0 to 0.005%, rare earth metal: 0 to 0.1 %, B: 0 to 0.005%, and one or more types of element selected from a group consisting of Zr, Sn, Co and Zn in a total amount of 0 to 0.05%, with the balance being Fe and impurities, and satisfying Formula (1); and a microstructure containing, in terms of area ratio, 20% or more of bainite, with 50% or more in terms of area ratio of the balance being ferrite; wherein: at a depth position that is equivalent to one-half of a plate thickness from a surface of the heat-rolled steel plate, an average value of pole densities of an orientation group { 1 00}<011> to {223 }<11 0> comprising crystal orientations { 1 00}<011>, { 116}<11 0>, { 114 }<11 0>, { 113}<11 0>, { 112}<11 0>, {335}<110> and {223}<110> is four or less and a pole density of a {332}<113> crystal orientation is 4.8 or less; at a depth position that is equivalent to one-eighth of the plate thickness from the surface ofthe heat-rolled steel plate, a pole density of a {110}<001> crystal orientation is 2.5 or more; a number density of fine Ti carbo-nitrides having a particle diameter of 1 0 nm or less among Ti carbo-nitrides in the heat-rolled steel plate is l.Ox 1017 per cm3; and a bake hardening amount is 15 MPa or more; [Ti]-48/14x[N]-48/32x[S];;:: 0 (1), where a content (mass%) of a corresponding element is substituted for each symbol of an element in Formula (1). 2. The heat-rolled steel plate according to claim 1, wherein: the chemical composition contains one or more types of element selected from a group consisting of: Nb: 0.005 to 0.1 %, Cu: 0.005 to I%, Ni: 0.005 to I%, Mo: 0.005 to 0.2%, V: 0.005 to 0.2%, Cr: 0.005 to 1%, and W: 0.01 to 0.5%. 3. The heat-rolled steel plate according to claim I or 2, wherein: the chemical composition contains one or more types of element selected from a group consisting of: Mg: 0.0005 to 0.005%, Ca: 0.0005 to 0.005%, and rare earth metal: 0.0005 to 0. I%. 4. The heat-rolled steel plate according to any one of claims 1 to 3, wherein the chemical composition contains: B: 0.0002 to 0.005%. 5. The heat-rolled steel plate according to any one of claims 1 to 4, wherein the chemical composition contains: one or more types of element selected from a group consisting of Zr, Sn, Co and Zn in a total amount of 0.005 to 0.05%. 6. A tailored rolled blank in which a plate thickness changes in a tapered shape in a rolling direction, comprising: a thick-wall portion, and a thin-wall portion that is thinner than the thick-wall portion; wherein: in the tailored rolled blank, a ratio of an average hardness Htmax of a thickest wall portion at which the plate thickness is thickest to an average hardness Htmin of a thinnest wall portion at which the plate thickness is thinnest is in a range of more than 1.0 to- 1.5, an average dislocation density ofthe thinnest wall portion is 1x1014m·2 or less, and a number density of fine Ti carbo-nitrides having a particle diameter of 10 nm or less is more than 2x 1017 per cm3• 7. The tailored rolled blank according to claim 6, wherein the tailored rolled blank is produced using a heat-rolled steel plate according to any one of claims 1 to 5. 8. The tailored rolled blank according to claim 6 or 7, further comprising a galvanized layer on a surface thereof. 9. A method for producing a heat-rolled steel plate for a tailored rolled blank, comprising: a step of heating at not less than a temperature SRT min that is defined by Formula (2) a slab containing, in mass%, C: 0.03 to 0.1 %, Si: 1.5% or less, Mn: 1.0 to 2.5%, P: 0.1% or less, S: 0.02% or less, AI: 0.01 to 1.2%, N: 0.01% or less, Ti: 0.015 to 0.15%, Nb: 0 to 0.1%, Cu: 0 to 1%, Ni: 0 to 1%, Mo: 0 to 0.2%, V: 0 to 0.2%, Cr: 0 to 1%, W: 0 to 0.5%, Mg: 0 to 0.005%, Ca: 0 to 0.005%, rare earth metal: 0 to 0.1 %, B: 0 to 0.005%, and one or more types of element selected from a group consisting of Zr, Sn, Co and Zn in a total amount of 0 to 0.05%, with the balance being Fe and impurities, and satisfying Formula (1); a step of producing a rough bar by performing rough rolling with an overall draft of 60 to 90% with respect to the slab that is heated, and during the rough rolling, performing one rolling pass or more at a draft of 20% or more when a slab temperature is 1050 to 1150°C; a step of producing a steel plate by starting finish rolling with respect to the rough bar within 150 seconds after rough rolling ends, and performing finish rolling in which a temperature of the rough bar when starting the finish rolling is in a range of 1 000°C to less than 1 080°C, an overall draft is set in a range of 75 to 95%, a total draft in a final two passes is set to 30% or more, a finish rolling ending temperature is set in a range from an An transformation temperature to 1 000°C, and a shape ratio SR that is defined by Formula (3) is set to 3.5 or more; a step of starting cooling of the steel plate within three seconds after finish rolling ends, setting a cooling stopping temperature to 600°C or less, and setting an average cooling rate until the cooling stopping temperature as 15°C per second or more to thereby cool the steel plate, and making a total cumulative diffusion length LtotaJ,that is defined by Formula (4), in a time period until coiling starts after the temperature of the steel plate passes an An transformation temperature 0.15 ).Lm or less; and a step of coiling the steel plate after cooling at a coiling temperature of 600°C or less; [Ti]-48/14x[N]-48/32x[S] ~ 0% (1) SRTmin = 10780/{5.13-log([Ti]x[C])}-273 (2) SR = ld/hm (3) Ltotal = I:~(D(T)~tL) (4) -.Jd'- where a content (mass%) of a corresponding element is substituted for each symbol of an element in Formula (1) and Formula (2), and ld in Formula (3) represents a length of an arc of contact between a rolling roll that performs a final roiling reduction in the finish rolling and the steel plate, and is defined by the following formula: ld = v'(Lx(hin-hout)/2) where L(mm) represents a diameter of the rolling roll, hin represents a plate thickness (mm) of the steel plate at an entrance side of the rolling roll, and hout represents a plate thickness (mm) of the steel plate at an exit side ofthe rolling roll, and where hm is defined by the following formula: hm = (hin+hout)/2 where ~tL in Formula (4) represents a time period until coiling starts after the temperature of the steel plate passes the An transformation temperature, and is a very small time period of 0.2 seconds, and D(T) represents a volume diffusion coefficient ofTi at PC, and is defined by the following formula when a diffusion coefficient of Ti is represented by DO, an activation energy is represented by Q, and a gas constant is represented by R: D(T) = DOxExp{-Q/R(T+273)}. 10. The method for producing a heat-rolled steel plate for a tailored rolled blank according to claim 9, wherein: the slab contains one or more types of element selected from a group consisting of: Nb: 0.005 to 0.1 %, Cu: 0.005 to 1%, Ni: 0.005 to 1%, Mo: 0.005 to 0.2%, V: 0.005 to 0.2%, Cr: 0.005 to 1%, and W: 0.01 to 0.5%. 11. The method for producing a heat-rolled steel plate for a tailored rolled blank according to claim 9 or 10, wherein: the slab contains one or more types of element selected from a group consisting of: Mg: 0.0005 to 0.005%, Ca: 0.0005 to 0.005%, and rare earth metal: 0.0005 to 0.1 %. 12. The method for producing a heat-rolled steel plate for a tailored rolled blank according to any one of claims 9 to 11, wherein: the slab contains: B: 0.0002 to 0.005%. 13. The method for producing a heat-rolled steel plate for a tailored rolled blank according to any one of claims 9 to 12, wherein: the slab contains: one or more types of element selected from a group consisting of Zr, Sn, Co and Zn in a total amount of 0.005 to 0.05%. 14. A method for producing a tailored rolled blank using a heat-rolled steel plate produced by a method for producing a heat-rolled steel plate for a tailored rolled blank according to any one of claims 9 to 13, comprising: a step of producing a cold-rolled steel plate by performing cold rolling on the heat-rolled steel plate while changing a draft within a range of more than 5% to 50% so that a plate thickness changes in a tapered shape in a longitudinal direction of the heat-rolled steel plate, and a step of performing a precipitation hardening heat treatment on the coldrolled steel plate; wherein: in the precipitation hardening heat treatment, a highest heating temperature T max is from 600 to 750°C, tqa holding time period tK (sec) at 600°C or more satisfies Formula (5) with respect to the highest heating temperature T max, and a heat treatment index IN defined by Formula (6) is 16500 to 19500, 530-0.7xT max:$ tK :$ 3600-3.9xT max (5) IN= (Tn+273)(log(tn13600)+20) (6) where tn (sec) in Formula (6) is defined by Formula (7): tn/3600 = I ox+~tiN/3600 (7) where X= ((Tn-I+273)/(Tn+273))(log(tn-JI3600)+20)-20, t1 =~tiN, and ~tiN is one second; Tn (0 C) in Formula (6) is defined by Formula (8): Tn = Tn-I+a~trN (8) where a represents a rate of temperature increase or cooling rate CC/s) at the temperature Tn-I. 15. The method for producing a tailored rolled blank according to claim 14, further comprising: a step of performing a galvanizing treatment before the step of heating the slab, before the step of cooling the steel plate after finish rolling, before the step of coiling the steel plate that is cooled, or after the step of performing a precipitation hardening heat treatment. 16. The method for producing a tailored rolled blank according to claim 15, further comprising: a step of performing an alloying treatment at 450 to 600°C after performing the galvanizing treatment.