DESCRIPTION HIGH BURRING, HIGH STRENGTH STEEL SHEET EXCELLENT IN SOFTENING RESISTANCE OF WELD HEAT AFFECTED ZONE AND METHOD OF PRODUCTION OF SAME TECHNICAL FIELD The present invention relates to high burring, high strength steel sheet having a tensile strength of 540 MPe or more excellent in softening resistance of ths weld heat affected zone and a method of production of the same, more particularly relates to high burring, high strength steel sheet excellent in softening resistance of the weld heat affected zone suitable as a material used for applications such as auto parts where both workability and weld zone strength are sought in the case of spot, arc, plasma, laser, or other welding after beinc formed or in the case of being formed after such welding and a method of production of the same. BACKGROUND ART In recent years, for lightening weight for improving the fuel efficiency of automobiles etc., Al alloys and other light metals or high strength steel sheet have beer increasingly used for auto parts and members. However, Al alloys and other light metals have the advantage of being high in relative strength, but are remarkably higher in price compared with steel, so their use has been limited to specialty applications. To promote reduction of the weight of automobiles in a broader area, use of inexpensive high strength steel sheet is being strongly sought. In general, materials become worse in formability the higher the strength. Ferrous metal materials are no exception. Attempts have been made to achieve both high strength and high ductility up until now. Further, another characteristic sought in a material used for auto parts is, in'addition to ductility, burring. However, burring also exhibits a tendency to fall along with higher strength, so the improvement of burring is also becoming a topic in use of high strength steel sheet fir auto parts. On the other hand, auto parts are comprised of press formed and other worked members assembled together by spot, arc, plasma, laser, and other welding. Further, recently, steel sheet has been welded together, then press formed in some cases. Whatever the case, the weld strength at the time of forming or the time of use assembled as a part is extremely important from the viewpoints of the forming limits and safety. Therefore, in application of high strength steel sheet to auto parts etc., the burring and the weld zone strength also become important issues for study. For high strength steel sheet excellent in burring, an invention adding Ti and Nb to reduce the second phase and cause precipitation strengthening by Tie and NbC in the main phase of polygonal ferrite so as to obtain high strength rolled steel sheet excellent in stretch flange formability has been proposed (Japanese Unexamined Patent Publication (Kokai) No. 6-20C351). Further, an invention adding Ti and Nb so as to reduce the second phase, make the microstructure acicular ferrite, and cause precipitation strengthening by TiC ..and NbC to obtain high strength, hot rolled steel sheet excellent in stretch flange formability has also been proposed (Japanese Unexamined Patent Publication {Kokai) No. 7-11382). On the other hand, as technology for improving the weld zone strength, an invention complexly adding Nb and Mo so as to suppress the softening of the weld zone in steel sheet has been proposed (Japanese Unexamined Patent Publication (Kokai) No. 2qpQ-87175). Further, an invention making active use of the precipitation of NbN to suppress softening of the weld zone so as to obtain steel sheet comprised of ferrite and martensite has also been proposed (Japanese Unexamined Patent Publication (Kokai) No. 2900-178654). However, in suspension arms, front side members, anci steel sheet for other parts, burring and other formability and the strength of the weld zone are very important. In the above prior art, the two characteristics could never simultaneously be satisfied. Further, for example, even if the two characteristics are' satisfied, provision of a method of production enabling production inexpensively and safely is important. The above prior art must be said to be insufficient. That is, in the invention described in Japanese Unexamined Patent Publication (Kokai) No. 6-200351, to obtain a high stretch flange formability, an area ratio of at least 85% of polygonal ferrate is essential, but to obtain a 85% or higher polygonal ferrite, the s.eel has to be held for a long time to promote the growth of the ferrite grains after hot rolling. This is not preferable in operating costs. Further, in the invention described in Japanese Unexamined Patent Publication (Kokai) No. 7-11382, due tc the microstructure with the high dislocation density and the precipitation of fine Tic and/or NbC, just a ductility of about 17% at 80 kgf/mm2 is obtainec: and the formability is insufficient. Further, these inventions do not allude at all to softening of the weld zone. On the other hand, the invention described in Japanese Unexamined Patent Publication (Kokai) No. 2000-87175 does not describe anything regarding the improvement of burring. Further, the invention described in Japanese Unexamined Patent Publication (Kokai) No. 2000-178654 relates to a complex ferrite-martensite structure steel, which is clearly different from the technology of the present invention for obtaining a microstructure of steel sheet excellent in burring. DISCLOSURE OF THE INVENTION The present invention solves these problems and provides high burring, high strength steel sheet excellent in softening resistance of the weld heat affected zone suitable as a material for use in applications such as auto parts where both workability and weld zone strength are demanded in the case of spot, arc, plasma, laser, or other welding after being formed or the case of being formed after welding, and a method of production of the same. That is, the present invention has as its object the provision of high burring, high strength steel sheet having a tensile strength of 540 MPa or more excellent in softening resistance of the weld heat affected zone and a method of production enabling that steel sheet to be produced inexpensively and stably. The inventors kept in mind the process of production of thin steel sheet being produced on an industrial scale by production facilities currently ordinarily employed and engaged in intensive studies to improve the softening resistance of the weld heat affected zone of high burring, high strength steel sheet. As a result, they discovered that high burring, high strength stee>l sheet containing C: 0.01 to 0.1%, Si: 0.01 to 2%, Mn: 0.05 to 3%, PSO.1%, S^O.03%, Al: 0.005 to 1%, N: 0.0005 to 0.005%, and Ti: 0.05 to 0.5%, further containing C, S, M, and Ti in ranges satisfying 00.2%, Cr<0.5%, and MoiO.5%, the balance comprising Fe and unavoidable impurities, and having a microstructure comprised of ferrite or ferrite and bainite, is extremely excellent in burring, but has a weld heat affected zone which remarkably softens. Further, they pinpointed the cause of the softening of the weld heat affected zone of eaid high burring, high strength steel sheet as being the tempering of the microstructure due to the welding thermal history and newly discovered that to improve the softening resistance, complex addition of Cr and Mo was extremely effective, and thereby completed the present invention. That is, the gist of the present invention is as fellows: (1) High burring, high strength steel sheet excellent in softening resistance of the weld heat affected zone characterized by containing, by wt%, C: 0.01 to 0.1%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P<0.1%, S<0.03%, Al: 0.005 to 1%,N: 0.0005 to 0.005%, and Ti: 0.05 to 0.5% and further containing C, S, N, Ti, Cr, and Mo in ranges satisfying 0%0.2%, CrSO.5%, and Mo<0.5%, the balance comprising Fe and unavoidable impurities, wherein the microstructure is comprised of ferrite or ferrite and bainite. (2) High bur-ring, high strength steel sheet , excellent in softening resistance of the weld heat affected zone characterized in that said steel further contains, by wt%, Nb: 0.01 to 0.5% and further contains Nb in a range satisfying 020 is preferable. On the other hand, if adding over 3H, slab cracking occurs, so 3% or less. P is an impurity and is preferably as low as possible. If contained in an amount over 0.1%, :.t has a detrimental effect on the workability and weldability and causes a drop in the fatigue characteristics as well, so is made 0.1% or less, S, if too great in content, causes cracking at the time of hot rolling, so should be reduced as much as possible, but 0.3% or less is an allowable range. Al has to be added in an amount of 0.005% or more for deoxidation of the molten steel, but invites a rise in cost, so its upper limit is made 1%. Further,, if added in too large an amount, it causes nonmetallic inclusions to increase and the elongation to deteriorate, so preferably the amount is made 0.5% or less. N forms precipitates with Ti and Mb at higher temperatures than C and causes a reduction in the Ti and Nb effective for fixing the desired C. Therefore, it should be reduced as much as possible, but 0.005% or less is an allowable range. Ti is one of the most important elements in the present invention. That is, Ti contributes to the rise in strength of the steel sheet due to precipitation strengthening. However, with less than 0.05%, this effect is insufficient, while even if contained in over 0.5%, not only is the effect saturated, but also a rise in the alloy cost is incurred. Therefore, the content of Ti is made 0.05% to 0.5%. Further, to fix by precipitation the C causing cementite or other carbides causing burring to deteriorate so as to improve the burring, it is necessary to meet the condition C-{12/48Ti-12/14N-12/32S)£0.05%. On the other hand, from the viewpoint of. suppression of softening of the weld heat affected zone, enough solid solution C for causing Mo or Cr to cluster or precipitate is required, so 0y->a transformation has a low Cep, so is not hardened and is liable to soften. In this case, by adding B for improving the hardenability, the softening at that location is suppressed. There is the effect that the fracture behavior of the joint is shifted from the weld zone to the matrix, so this is added in accordance with need. However, addition of less than 0.0002% is insufficient for obtaining these effects, while addition of over 0.002% causes slab cracking. Accordingly, B is added in an amount of 0.0002% to 0.002%. Further, to impart strength, it is also possible to add one or two or more types of V and Zi precipitation strengthening or solution strengthening elements. However, with less than 0.02% and 0.02%, respectively, this effect cannot be obtained. Further, even if added in amounts over 0.2% and 0.2% re'spectively, the effect is saturated. Note that the steel having these as main ingredients may also contain Sn, Co, Zn, W, and Mg in a total of 1% or less. However, Sn is liable to cause defects at the time of hot rolling, so 0.05% or less is preferable. Next, the reasons for limitation of the method of production of the present invention will be explained in detail below. The present invention can be obtained as Ccist, hot rolled, then cooled; as hot rolled; as hot rolled, then cooled, pickled, cold rolled, then heat treated; or as hot rolled steel sheet or cold rolled steel sheet heat treated by a hot dip line; and further as these steel sheets given separate surface treatment. The method of production preceding the hot rolling in the present invention is not particularly limited. That is, after melting in a blast furnace or electric furnace etc., it is sufficient to perform various types of secondary refining to adjust the ingredients to give the target contents of ingredients, then cast this by the usual continuous casting, casting by the ingot method, thin slab casting/ or another method. For the material, scrap may also be used. In the case of a slab obtained by continuous casting, the slab may be directly conveyed as a hot slab to the hot rolling mill or may be cooled to room temperature, then reheated in a heating furnace, then hot rolled. The reheating temperature is not particularly limited, but if 1400°C or more, the scale off becomes large and the yield falls, so the reheating temperature is preferably less than 1400°C. Further, heating at less than 1000°C seriously detracts from the operational efficiency in schedules/ so the reheating temperature is preferably 1000°C or more. Further, heating at less than 1100CC not only results in precipitates including Ti and/or Nb not redissolving in the slab, but roughening and causing a loss of the precipitation strengthening, but also the precipitates including Ti and/or Nb in the sizes and distributions desirable for burring no longer precipitate, so the reheating temperature is preferably 1100°C or more. The hot rolling process comprises rough rolling, then finish rolling, but after rough rolling or after its: succeeding descaling, it is also possible to bond a sheet, bar and consecutively finish roll it. At that time, it if also possible to coil a rough bar once into a coil shape, store it in a cover having a heat retaining function in accordance with need, again uncoil it, then bond it. Further, the subsequent finish rolling is preferably performed within 5 seconds so as to prevent the formation of scale again after descaling. The finish rolling has to end in a temperature region where the final pass temperature (FT) is the Ars transformation point + 30°C°C or more. This is because to obtain the bainitic ferrite or ferrite and bainite desirable for burring in the cooling process after the hot rolling, the y->cc transformation must occur at a low temperature, but in a temperature region where the final pass temperature (FT) is less than the Ar3 transformation point + 30°C, stress induced ferrite transformation nuclei are formed and polygonal coarse ferrite is liable to end up being produced. The upper limit of the finish temperature does not have to be particularly set so far as obtaining the effects of the present invention, but there is a possibility of occurrence of scale defects in operation, so making it 1100°C or less is preferable. Here, the Ar3 transformation point temperature is simply shown in relation with the steel ingredients by for example the following calculation formula: Ars « 910-310 x IC+25 x %iJi-80 x %Mn After the finish rolling ends/ the steel is cooled to the designated coiling temperature (CT). The time until the start of cooling is made within 10 seconds. This is because if the time until the start of cooling is over 10 seconds, right after rolling, the steel is liable to recrystallize and the austenite grains to end up becoming coarser and the ferrite grains after the y->a transformation are liable to become coarser. Next, the average cooling rate until the end of cooling has to be at least 50cC/sec. This is because if the average cooling rate until the end of cooling is less than 50°C/sec, the volume fraction of the bainitic ferrite or ferrite and bainite desirable for burring is liable to end up decreasing. Further, the upper limit of the cooling rate is made 500°C/sec or less considering the actual capabilities of plant facilities. The cooling end temperature has to be in the temperature region of 700°C or less. This is because if the cooling end temperature is over 700'C, a microstructure other than the bainitic ferrite or ferrite and bainite desirable for burring is liable to end up being formed. The lower limit of the cooling end temperature does not have to be particularly defined to obtain the effect of the present invention. However, the coiling temperature or less is impossible in view of the process of the present invention. The processes from after cooling ends to coiling are not particularly defined, but in accordance with need/ it is possible to cool to the coiling temperature, but. in this case springback of the sheet due to thermal stress is a concern, so 300°C/sec or less is preferable. Next, with a coiling temperature of less than 350°c, sufficient precipitates containing Ti and/or Nb are no longer formed and a drop in strength is feared, while if over 650°C, the precipitates containing Ti and/or Nb become coarser in size and not only no longer contribute to the rise in strength by precipitation strengthening, but if the precipitates become too large, voids will easily occur at the interface between the precipitates and the matrix phase and the burring is liable to drop. Therefore, the coiling temperature is made 350°C to 650°C. Further, the cooling rate after coiling is not particularly limited, but when adding Cu in an amount of 1% or more, if the coiling temperature (CT> is over 450°C, Cu will precipitate after coiling and the workaoility will deteriorate. Not only this, the solid solution state: Cu effective for improving the fatigue resistan-e is liable to be lost, so when the coiling temperatare (CT) exceeds 450°C, the cooling rate after coiling is preferably at least 30ec/sec until 200°C. After the end of the hot rolling process, in accordance with need, the steel is pickled, then may be processed in-line or off-line by skin pass rolling with a reduction ratio of 10% or less or cold rolling until a reduction ratio of 40% or so. Next, when the cold rolled steel sheet is -he final product, the hot finish rolling conditions are not particularly limited. Further, the final pass temperature (FT) of the finish rolling may be less than the Ar3 transformation point temperature, but in this case a strong worked structure remains before the rolling or during the rolling, so restoration and recrystallization are preferable in the following coiling or heat treatment. The cold rolling process after the following pickling is not particularly limited'for obtaining the effect of the present invention. The heat treatment of this cold rolled steel sheet assumes a continuous annealing process. First, this is performed at a temperature region of 800°C or more for 5 to 150 seconds. When this heat treatment temperature is less than 800eC, in the later cooling, the bainitic ferrate or ferrite and bainite desirable for burring are liable not to be obtained, so the heat treatment temperature is made 800°C or more. Further, the upper limit of the heat treatment temperature is not particularly defined, but due to restrictions of the continuous annealing facilities, is substantially 900°C o.r less. On the other hand, a holding time at this temperature region of less than 5 seconds is insufficient for the Ti and Nb carbides to completely redissolve. Even with over 150 seconds of heat treatment, not only is the effect saturated, but also the productivity is lowered, so the holding time is made 5 to 150 seconds. Next, the average cooling rate until the end of cooling has to be 50°C/sec or more. This is because if the average cooling rate until the end of cooling is less than 50"C/sec, the volume fraction of the bainitic ferrito or ferrite and bainite desirable for burring is liable to end up falling. Further, the upper limit of the cooling rate, considering the capabilities of actual plant facilities etc. is 200°C/sec or less. The cooling end temperature has to be in the temperature region of 700°C or less, but when using a continuous annealing facility, the cooling end temperature usually never exceeds 550*C, so no special consideration is required. Further, the lower limit of the cooling end temperature does not have to be particularly set to obtain the effect of the present invention. Further, after this, if necessary, skin pass rolling can be applied. To coat with zinc the hot rolled steel sheet after pickling or said cold rolled steel sheet after the heat treatment process, the sheet may be dipped in a zinc coating bath. It may also be alloyed in accordance with need. EXAMPLES Below, examples will be used to further explain the present invention. Each of the steels A to M having the chemical ingredients shown in Table 1 w.ss melted in a converter, continuously cast, reheated at the heating temperature shown in Table 2, rough rolled, then finish rolled to a thickness of 1.2 to 5.5 ran, then coiled. Note that the chemical compositions in the tables are expressed in wt%. Note that as shown in Table 2, some steels were pickled, cold rolled, and heat treated after the hot roiling process. The sheet thicknesses were 0.7 to 2.3 run. On the other hand, among said steel sheets, the steel H and steel c-7 were zinc coated. Details of the production conditions are shown in Table 2. Here, "SRT" indicates the slab heating temperature, "FT" the final pass finish rolling temperature, "start time" the rime from the end of rolling to the start of cooling, "cooling rate" the average cooling rate from the start of cooling to the end of cooling, end "CT" the coiling temperature. However, when rolling later by cold rolling, the steels are not limited in this way, so "-" is indicated. The tensile test for each of the thus obtained hot rolled sheets was conducted/ as shown in FIG. 3 0.02%, Cr < 0.05%, and Mo < 0.05%, the balance comprising Fe, one or more optional ingredients such as herein described and unavoidable impurities such as herein described, wherein the microstructure is comprised of fertile or ferrite and bainite. 2. A high burring, high strength steel sheet as claimed in claim 1 wherein the steel contains Nb in an amount in the range of 0.01 to 0.5%, the amount of Nb present in a range satisfying the formula 0