0001]The present invention, carburizing steel, and a process for producing a carburizing steel.
BACKGROUND
[0002]Recently, automotive gear, a clutch plate, the mechanical structural components of the damper or the like, in addition to the high durability is required to be inexpensive to manufacture. In general, as a method for manufacturing these components, cutting and carburization using a hot forged material it has been made. However, in response to the growing demand for cost reduction, hot rolled steel sheets and cold-rolled steel sheet as a material, after molding to the shape of the cold working to member advances is the development of technology for carburizing It is. The cold working, punching the material, followed by bending, drawing, press forming machining such as spread well performed. At this time, if it is necessary to mold a complicated shape such as a damper parts and the like of the torque converter, ultimate deformability is obtained. Here, the "ultimate deformability" is a physical property value given by the natural logarithm of the cross-sectional shrinkage at break of the tensile test specimen, is known to exhibit a hole expansion positively correlated. From this point of view, in recent years, various techniques have been proposed.
[0003]
　For example, Patent Document 1 below, the structure of the hot rolled steel sheet composed of ferrite and pearlite, then technique spheroidizing carbides subjected to spheroidizing annealing is proposed.
[0004]
　In Patent Document 2 below, after controlling the particle size of the carbides, and controlling the ratio of the number of ferrite grain boundary carbide to the number of carbide in ferrite grains, further, the ferrite mother phase crystal grain by controlling the diameter, techniques for improving the impact properties of the member after carburization has been proposed.
[0005]
　In Patent Document 3 below, the particle size and aspect ratio of the carbide, and, after controlling the crystal grain size of the ferrite mother phase, by further controlling the aspect ratio of ferrite, the cold workability technology to improve have been proposed.
CITATION
Patent Document
[0006]
Patent Document 1: Patent No. 3094856 Patent Publication
Patent Document 2: WO 2016/190370
Patent Document 3: WO 2016/148037
Summary of the Invention
Problems that the Invention is to Solve
[0007]
　Mechanical structural parts as described above, hardenability is required to increase the strength. That is, a member having a complicated shape for molding by cold working, while maintaining the hardenability, to ensure the hole expansion (i.e., to achieve excellent intrinsic deformability) obtained .
[0008]
　However, in the manufacturing method mainly the microstructure control of the carbide of the Patent Document 1, the cold workability, in particular to improve the hole expansion sufficiently is difficult. In the above Patent Document 2, for improving the cold workability of the prior carburization, it has not been studied at all. Further, in the technique proposed in Patent Document 3, it is difficult to obtain a cold hole expansion withstand processing into complex shapes of members. Thus, in the technique conventionally proposed, it is difficult to improve the hole expansion carburized steel plate sufficiently, therefore, carburizing steel sheet to the component having a particularly complex shape such damper parts and the like of the torque converter application of has been limited.
[0009]
　The present invention has been made in view of the above problems, it is an object of the present invention is to provide a method of manufacturing a carburizing steel which exhibit better ultimate deformability prior carburizing .
Means for Solving the Problems
[0010]
　The present inventors have, how to resolve the above problems, intensive studies. As a result, as described in detail below, by appropriately controlling the X-ray random intensity ratios of a predetermined orientation component group in the ferrite grain by texture control of ferrite in the hot-rolled steel sheet, while maintaining the hardenability , improving the hole expansion (i.e., to impart excellent extreme deformability) inspired and is possible, thereby completing the present invention.
　Summary of the completed invention based on this idea is as follows.
[0011]
[1] by mass%, C: less than 0.02% or more 0.30%, Si: 0.005% or more and less than 0.5%, Mn: less than 0.01% or more 3.0%, P: 0. less than 1%, S: 0.1% or less, sol. Al: 0.0002% or more and 3.0% or less, N: contains 0.2% or less, and the balance consists of Fe and impurities, {100} of the ferrite grain <011> - {223} <110> mean value of X-ray random intensity ratio of the orientation group, is 7.0 or less, the average circle equivalent diameter of carbide, not more than 5.0 .mu.m, the ratio of the number of carbide aspect ratio of 2.0 or less, All carbides and 80% or more, the number ratio of cementite present in the ferrite grain is less than 60% relative to the total carbide, carburized steel plate.
[2] instead of the part of the remainder of Fe, in mass%, Cr: 0.005% to 3.0% or less, Mo: 0.005% to 1.0% or less, Ni: 0.010% or more 3.0% or less, Cu: 0.001% to 2.0% or less, Co: 0.001% to 2.0% or less, Nb: 0.010% or more 0.150% or less, Ti: 0.010 % or more 0.150% or less, V: 0.0005% to 1.0% or less, B: further containing one or more than 0.0005% 0.01% or less, according to [1] carburizing steel.
[3] in place of part of the remainder of Fe, by mass%, Sn: 1.0% or less, W: 1.0% or less, Ca: 0.01% or less, REM: 0.3% or less of 1 further contains more species or two or, carburizing steel sheet according to [1] or [2].
[4] [1] to a method for producing a carburizing steel sheet according to any one of [3], the steel having a chemical composition according to any one of [1] to [3] heating, rolling of one pass before the final hot rolling, carried out at 25% or less of reduction ratio of 15% or more in a temperature range of 900 ° C. or higher 980 ° C. or less, the hot finish rolling, 920 less than ° C. 800 ° C. or higher It ends in a temperature range of 6% or higher reduction ratio, and the hot rolling step of winding at 700 ° C. temperature below the steel sheet obtained by the hot rolling process, or cold rolling after the hot rolling step the is subjected steel, the nitrogen concentration in an atmosphere which is controlled by the volume fraction below 25% at 5 ° C. / h or higher 100 ° C. / h or less of the average heating rate, Ac defined by the following formula (1) 1 was heated to a temperature range below point, the Ac 1 annealing treatment which holds 10h or 100h following temperature range below point It was subjected to, including, and annealing step of performing cooling of the average cooling rate 5 ° C. / h or higher 100 ° C. / h or less in a temperature range of up to 550 ° C. the temperature in annealing termination method of carburizing steel .
[0012]
[Number 1]

[0013]
　Here, in the above formula (1), the notation of [X], the content of the element X: represents (unit weight%), if not containing the appropriate elements shall substituting zero.
Effect of the invention
[0014]
　According to the present invention described above, it is possible to provide a carburizing steel which exhibit better ultimate deformability before carburization.
DESCRIPTION OF THE INVENTION
[0015]
　The following describes in detail preferred embodiments of the present invention.
[0016]
(The present inventors have for the contents and the resulting idea study was performed)
　investigation before described carburizing steel plate and a manufacturing method thereof according to the present invention, the present inventors to solve the above problems is carried out for the contents, it will be described in greater detail below.
　Upon such investigations, the present inventors, first, the method for improving the hole expansion correlated with ultimate deformability was investigated.
[0017]
　In order to improve the hole expansion suppresses the generation of cracks at the time of spread hole, in addition, when a crack occurs, it is important to suppress the extension of the generated cracks. In order to suppress the occurrence of cracks, it is effective to control the aspect ratio of the carbide to be generated in the steel plate (major axis / minor axis), the spheroidizing annealing, it is important to reduce the aspect ratio of the carbide . In order to suppress the extension of the cracks, as well as suppress the formation of coarse carbides, it is effective to control the precipitation position of the carbides. That is, when carbides in grain boundaries of ferrite is formed, since the extension of the crack to propagate path grain boundaries is promoted, it is important to produce a carbide crystal grains of the ferrite. Carbide be to produce in the crystal grains of the ferrite, it is believed to be able to suppress the crack propagation at the grain boundaries.
[0018]
　The present inventors have found that on carrying out the tissue control as described above, further, focusing on the improvement of hole expansion by texture control of ferrite mother phase, investigate the function and effect of such texture control in detail and it was studied. As a result, by controlling the X-ray random intensity ratio of specific crystal orientation component group, hole expansion was found that remarkably improved.
[0019]
　Specifically, the present inventors have found that in carburizing steel, the ferrite grain {100} <011> - {223} <110> the mean value of X-ray random intensity ratio of orientation component group below 7.0 by controlling, hole expansion was found that remarkably improved. Reason that the X-ray random intensity ratio of crystal orientation component group as described above is critical to resistance spread hole is not necessarily clear, it assumed that there is a crack ease the relationship at the time of spread holes It is. In the present invention, the carburizing steel, after controlling the precipitation position of the aspect ratio and carbides of carbide, further, by controlling the X-ray random intensity ratio of specific crystal orientation component group in the ferrite grains, the hole expansion We were able to dramatically improve the sex.
[0020]
　Furthermore, the present inventors have found that by controlling the rolling conditions finishing in the hot rolling process, and have conceived the can control the X-ray random intensity ratio of specific crystal orientation component group in the ferrite grains. {100} <011> - {223} <110> orientation component group of the crystal orientation of the ferrite is the grain of ferrite to be generated when the phase transformation from austenite non-recrystallized. The control of this reason, the finish rolling conditions, by promoting recrystallization of austenite, it is possible to reduce the production of these specific crystal orientation component group, as a result, {100} in the ferrite grain <011> - {223 } <110> to X-ray random intensity ratio of orientation component group found that it is possible to control the 7.0.
[0021]
　Conventionally, including techniques disclosed in the above Patent Documents 1 to 3, for the purpose of increasing the intrinsic deformability of carburizing steel plate, to control the texture of ferrite in the hot-rolled steel sheet is noted It did not. Therefore, conventionally, the temperature and rolling reduction of one pass before the hot finish rolling, as described in detail below, even control of the temperature and reduction ratio of the hot finish rolling was not performed. In the present invention, conditions such as rolling finish between these heat by appropriately controlling, it was possible to obtain a carburizing steel sheet having more excellent extreme deformability.
[0022]
　Incidentally, the improvement of hole expansion by controlling the {100} <011> ~ {223} <110> X-ray random intensity ratio of orientation component group in the ferrite crystal grains to 7.0 or less, in a high hardenability steel the more, the higher the effect. For example, tensile strength 340 MPa class, such as 440MPa grade, tensile strength in the above high-strength steel sheet 340 MPa, the hole expansion properties are significantly improved. Therefore, the tissue control as outlined above, while maintaining the hardenability, it is possible to improve the hole expanding property. This makes it possible to obtain a carburized steel plate having both hardenability and hole expansion.
[0023]
　Carburizing steel plate and a manufacturing method thereof according to an embodiment of the present invention to be described below, has been completed based on the findings as described above. Hereinafter, it has been completed based on such findings, carburizing steel sheet according to the present embodiment and its manufacturing method will be described in detail.
[0024]
(For carburizing steel)
　First, the carburizing steel sheet according to the embodiment of the present invention will be described in detail.
　Carburizing steel sheet according to the present embodiment has a predetermined chemical components, such as described in detail below. In addition, carburizing steel sheet according to the present embodiment, the average value of the ferrite crystal grains of {100} <011> - {223} <110> orientation component group X-ray random intensity ratio of is 7.0 or less, the average circle equivalent diameter of carbide, not more than 5.0 .mu.m, the ratio of the number of carbide aspect ratio of 2.0 or less, at least 80% relative to the total carbide, carbides present in the ferrite grain number ratio, that is 60% or more of the total carbide has a specific microstructure. Thus, carburizing steel sheet according to the present embodiment, before carburization, exhibits a more excellent extreme deformability.
[0025]

　First, the chemical components with the carburizing steel sheet according to the present embodiment will be described in detail. In the following description, "%" relates to chemical components, in particular means "% by weight" unless otherwise specified.
[0026]
[C: less than 0.02% to
　0.30%] C (carbon) is an element necessary for securing the strength of the plate thickness center in the finally obtained carburized member. Further, the carburizing steel, C is, a solid solution in the grain boundary of ferrite increases the strength of the grain boundary, is an element contributing to the improvement of hole expansion.
[0027]
　When the content of C is less than 0.02%, the hole expansion effect of improving the above-mentioned can not be obtained. Therefore, the carburizing steel sheet according to the present embodiment, the content of C is 0.02% or more. The content of C is preferably 0.05% or more. On the other hand, if the content of C is 0.30% or more, the average circle equivalent diameter of the carbide generated during carburizing steel sheet exceeds 5.0 .mu.m, hole expansion is deteriorated. Therefore, the carburizing steel sheet according to the present embodiment, the content of C is less than 0.30%. The content of C is preferably 0.20% or less. In consideration of the balance of hole expansion and hardenability, the C content is more preferably 0.10% or less.
[0028]
[Si: 0.005% or more and less than
　0.5%] Si (silicon) is an element which forms an action of sound of the steel by deoxidation of molten steel. When the content of Si is less than 0.005%, it is impossible to sufficiently deoxidizing the molten steel. Therefore, the carburizing steel sheet according to the present embodiment, the Si content is 0.005% or more. The content of Si is preferably 0.01% or more. On the other hand, if the content of Si is 0.5% or more, Si solid-solved in carbides to stabilize the carbides, the average circle equivalent diameter of carbide exceeds 5.0 .mu.m, hole expansion is impaired It is. Therefore, the carburizing steel sheet according to the present embodiment, the content of Si is less than 0.5%. The content of Si is preferably less than 0.3%.
[0029]
[Mn: less than 0.01% to
　3.0%] Mn (manganese) is an element which forms an action of sound of the steel by deoxidation of molten steel. When the content of Mn is less than 0.01% can not sufficiently deoxidizing the molten steel. Therefore, the carburizing steel sheet according to the present embodiment, the content of Mn is 0.01% or more. The content of Mn is preferably 0.1% or more. On the other hand, if the content of Mn is 3.0% or more, Mn was dissolved in the carbide is used to stabilize carbides, exceed 5.0μm average circle equivalent diameter of carbide, hole expansion degradation the lead. Therefore, the content of Mn is less than 3.0. The content of Mn is preferably less than 2.0%, more preferably less than 1.0%.
[0030]
[P: 0.1% or less]
　P (phosphorus) is segregated at the grain boundaries of the ferrite is an element degrading the hole expansion. When the content of P exceeds 0.1%, the intergranular strength of the ferrite is significantly reduced, hole expandability deteriorates. Therefore, the carburizing steel sheet according to the present embodiment, the P content is 0.1% or less. The content of P is preferably not 0.050% or less, more preferably 0.020% or less. The lower limit of the content of P is not particularly limited. However, to reduce the content of P to less than 0.0001%, de P cost is considerably increased, it is economically disadvantageous. Therefore, practically steel, the P content, is substantially the lower 0.0001%.
[0031]
[S: 0.1% or less]
　S (sulfur) is to form inclusions, is an element that degrades the hole expansion. When the content of S exceeds 0.1%, the hole expanding property is lowered to generate coarse inclusions. Therefore, the carburizing steel sheet according to the present embodiment, the content of S is 0.1% or less. The content of S is preferably 0.010% or less, more preferably 0.008% or less. The lower limit of the content of S is not particularly limited. However, to reduce the content of S to less than 0.0005%, de S cost increases significantly, is economically disadvantageous. Therefore, practically steel, the S content, is substantially the lower 0.0005%.
[0032]
[Sol. Al: 0.0002% or more and 3.0% or
　less] Al (aluminum) is an element which forms an action of sound of the steel by deoxidation of molten steel. When the content of Al is less than 0.0002% can not sufficiently deoxidizing the molten steel. Therefore, the carburizing steel sheet according to the present embodiment, the content of Al (more specifically, the content of sol. Al) is 0.0002% or more. The content of Al is preferably 0.0010% or more. On the other hand, if the content of Al exceeds 3.0%, the hole expansion coarse oxides are generated is impaired. Therefore, the content of Al is 3.0% or less. The content of Al is preferably not more than 2.5%, more preferably 1.0% or less, more preferably 0.5% or less, even more preferably 0.1% or less.
[0033]
[N: 0.2% or less]
　N (nitrogen) is an impurity element, an element that inhibits hole expansion to form a nitride. When the content of N exceeds 0.2%, the hole expansion is significantly reduced and generates coarse nitrides. Therefore, the carburizing steel sheet according to the present embodiment, the content of N is 0.2% or less. The content of N is preferably 0.1% or less, more preferably 0.02% or less, further preferably 0.01% or less. On the other hand, the lower limit of the content of N is not particularly limited. However, to reduce the content of N to less than 0.0001%, de-N cost increases significantly, is economically disadvantageous. Therefore, practically steel, the N content, is substantially the lower 0.0001%.
[0034]
[Cr: 0.005% to 3.0% or
　less] Cr (chromium) is the finally obtained carburized member, together with an element having an effect of enhancing the hardenability, in the carburizing steel, crystal ferrite granulate an element which contributes to the further improvement of hole expansion is miniaturized. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, it may contain Cr. If the inclusion of Cr, in order to obtain the effect of improving further the hole expansibility, it is preferable that the content of Cr and 0.005% or more. The content of Cr is more preferably 0.010% or more. In consideration of the influence of the generation of carbides and nitrides, in order to obtain a further improvement in hole expansion, the content of Cr is preferably 3.0% or less. The content of Cr is more preferably 2.0% or less, more preferably 1.5% or less.
[0035]
[Mo: 0.005% to 1.0% or
　less] Mo (molybdenum), in finally obtained carburized member, together with an element having an effect of enhancing the hardenability, in the carburizing steel, crystal ferrite granulate an element which contributes to the further improvement of hole expansion is miniaturized. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, it may contain Mo. If the inclusion of Mo, in order to obtain the effect of improving further the hole expansibility, it is preferable that the content of Mo is 0.005% or more. The content of Mo is more preferably 0.010% or more. In consideration of the influence of the generation of carbides and nitrides, in order to obtain a further improvement in hole expansion, the content of Mo is preferably 1.0% or less. The content of Mo is more preferably 0.8% or less.
[0036]
[Ni: 0.010% to 3.0% or
　less] Ni (nickel), in the finally obtained carburized member, together with an element having an effect of enhancing the hardenability, in the carburizing steel, crystal ferrite granulate an element which contributes to the further improvement of hole expansion is miniaturized. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, it may contain Ni. If the inclusion of Ni, to obtain a further improvement in hole expansion, the content of Ni preferably set to 0.010% or more. The content of Ni is more preferably 0.050% or more. Moreover, when Ni is to consider the effect of segregating at the grain boundaries, in order to obtain a further improvement in hole expansion, the content of Ni is preferably 3.0% or less. The content of Ni is more preferably 2.0% or less, more preferably 1.0% or less, even more preferably 0.5% or less.
[0037]
[Cu: 0.001% to 2.0% or
　less] Cu (copper), in the finally obtained carburized member, together with an element having an effect of enhancing the hardenability, in the carburizing steel, crystal ferrite granulate an element which contributes to the further improvement of hole expansion is miniaturized. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, it may contain Cu. If the inclusion of Cu, in order to obtain a further improvement in hole expansion, the content of Cu preferably 0.001% or more. The content of Cu is more preferably 0.010% or more. Furthermore, when Cu is to consider the effect of segregating at the grain boundaries, in order to obtain a further improvement in hole expansion, the content of Cu is preferably 2.0% or less. The content of Cu is more preferably 0.80%.
[0038]
[Co: 0.001% to 2.0% or
　less] Co (cobalt), in the finally obtained carburized member, together with an element having an effect of enhancing the hardenability, in the carburizing steel, crystal ferrite granulate an element which contributes to the further improvement of hole expansion is miniaturized. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, it may be contained Co. If the inclusion of Co, in order to obtain a further improvement in hole expansion, the content of Co preferably 0.001% or more. The content of Co is more preferably 0.010% or more. Also, when Co is considering the effect of segregating at the grain boundaries, in order to obtain a further improvement in hole expansion, the content of Co is preferably 2.0% or less. The content of Co is more preferably 0.80%.
[0039]
[Nb: 0.010% or more 0.150% or
　less] Nb (niobium) is an element contributing to further improvement in hole expansion grain of the ferrite and fine. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, it may contain Nb. If the inclusion of Nb, in order to obtain a further improvement in hole expansion, the content of Nb preferably set to 0.010% or more. The content of Nb is more preferably 0.035% or more. In consideration of the influence of the generation of carbides and nitrides, in order to obtain a further improvement in hole expansion, the content of Nb is preferably set to 0.150% or less. The content of Nb is more preferably not more than 0.120%, further preferably 0.100% or less.
[0040]
[Ti: 0.010% or more 0.150% or
　less] Ti (titanium) is an element contributing to further improvement in hole expansion grain of the ferrite and fine. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, it may contain Ti. If the inclusion of Ti, in order to obtain a further improvement in hole expansion, the content of Ti preferably set to 0.010% or more. The content of Ti is more preferably 0.035% or more. In consideration of the influence of the generation of carbides and nitrides, in order to obtain a further improvement in hole expansion, the content of Ti is preferably not more than 0.150%. The content of Ti is more preferably not more than 0.120%, more preferably not more 0.100% or less, more is at more or less preferably 0.050% and still more preferably 0.020% or less is there.
[0041]
[V: 0.0005% to 1.0% or
　less] V (vanadium) is an element contributing to further improvement in hole expansion grain of the ferrite and fine. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, it may be contained V. If the inclusion of V, in order to obtain a further improvement in hole expansion, the content of V is preferably set to 0.0005% or more. The content of V is more preferably 0.0010% or more. In consideration of the influence of the generation of carbides and nitrides, in order to obtain a further improvement in hole expansion, the content of V is preferably set to 1.0% or less. The content of V is more preferably not more than 0.80%, more preferably not more than 0.10%, even more preferably at most 0.080%.
[0042]
[B: 0.0005% or more than 0.01%]
　B (boron) is to improve the strength of the grain boundaries by segregating at the grain boundaries of the ferrite is an element to further improve the hole expansion. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, it may be contained B. If the inclusion of B, in order to obtain a further improvement in hole expansion, the content of B is preferably set to 0.0005% or more. The content of B is more preferably 0.0010% or more. Further, even though the content exceeds 0.01% B, since the hole expansion of further improving the above effect is saturated, the content of B is preferably set to 0.01% or less . The content of B is more preferably not more than 0.0075%, further preferably 0.0050% or less, even more preferably 0.0020%.
[0043]
[Sn: 1.0% or
　less] Sn (tin) is an element which forms an action of further sound of the steel by deoxidation of molten steel. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, it may contain Sn 1.0% as the upper limit. Sn content is more preferably 0.5% or less.
[0044]
[W: 1.0% or less]
　W (tungsten) is an element which forms an action of further sound of the steel by deoxidation of molten steel. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, may contain W 1.0% as the upper limit. The content of W is more preferably 0.5% or less.
[0045]
[Ca: 0.01% or
　less] Ca (calcium) is an element which forms an action of further sound of the steel by deoxidation of molten steel. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, it may contain Ca as a maximum of 0.01%. The content of Ca is more preferably 0.006% or less.
[0046]
[REM: 0.3% or
　less] REM (rare earth metal) is an element which forms an action of further sound of the steel by deoxidation of molten steel. Therefore, in the carburizing steel sheet according to the present embodiment, if necessary, it may be contained REM 0.3% as the upper limit.
[0047]
　Incidentally, REM is Sc (scandium), Y is a generic name for total 17 elements consisting of elements of (yttrium) and lanthanoids, the content of REM means the total amount of the element. REM is, in many cases to be contained by using the misch metal, in addition to the La (lanthanum) and Ce (cerium), there is a case to be contained the elements of the lanthanide series in the complex. Even such a case, carburizing steel sheet according to the present embodiment exhibits excellent ultimate deformability. Moreover, even contain a metal REM such as metal La and Ce, carburizing steel sheet according to the present embodiment exhibits excellent ultimate deformability.
[0048]
: [Balance Fe and impurities]
　balance of the composition of the mid-thickness portion is Fe and impurities. As the impurity, steel material or scraps, and / or, inevitably mixed in steel making process, element permitted is illustrated in a range that does not inhibit the characteristics of carburizing steel sheet according to the present embodiment.
[0049]
　Although the chemical components with the carburizing steel sheet according to the present embodiment has been described in detail.
[0050]

　Next, the microstructure constituting the carburizing steel sheet according to the present embodiment will be described in detail.
　Microstructure of carburizing steel sheet according to the present embodiment is substantially composed of ferrite and carbides. More specifically, in a carburizing steel sheet microstructure of the present embodiment, the area ratio of ferrite is in the range of for example 80% to 95%, the area ratio of carbides, for example in the range of 5-20% there are, and configured such that the total area ratio of the ferrite and carbides does not exceed 100%.
[0051]
　The area ratio of ferrite and carbides, such as described above is measured using samples collected as an observation plane cross section perpendicular to the width direction of the carburized steel plate. The length of the sample, depending on the measurement device, may be about 10 mm ~ 25 mm. Samples, after polishing the observation surface to nital etching. Observation surface which is nital etching, sheet thickness 1/4 position (meaning a position of 1/4 thickness of the steel sheet in the thickness direction of the steel sheet from the surface of carburization steel.), Thickness 3/8 position, and, the range of the sheet thickness 1/2 position, observed by thermal field emission scanning electron microscope (e.g., JEOL Ltd. JSM-7001F).
[0052]
　For observation target range of each sample, 2500 [mu] m 2 was observed 10 field of view range, in each field, and measuring the ratio of the area occupied by the ferrite and carbides in the field area. Then, the average value of the entire field of view of the ratio of the area occupied by the ferrite, and the average value of the entire field of view of the ratio of the area occupied by the carbides, respectively, the area ratio of ferrite, and the carbide area ratio.
[0053]
　Here, the carbide in the microstructure according to the present embodiment is mainly cementite is a compound of iron and carbon (Fe 3 C), and, epsilon carbide (Fe 2 ~ 3 is C) an iron-based carbides such. Also, the carbide in the microstructure, in addition to iron-based carbides described above, or a compound obtained by substituting Fe atoms in cementite Mn, with Cr, an alloy carbides (M 23 C 6 , M 6 C, a MC or the like, M is either Fe and other metal elements, or, sometimes containing a metal element other than Fe.). Carbides in the microstructure according to the present embodiment is composed mostly of iron-based carbides. Therefore, the carbide as described above, when focusing on the number, such as described in detail below, the number thereof may be any of various total number of carbides as mentioned above, there in the number of only iron-based carbide it may be. That is, as described in detail below, various number ratio of about carbides to various carbide containing iron-based carbides may be one that as a population, be one that only iron carbide and population it may be. Iron-based carbides, for example, can be identified using the Diffraction analysis and EDS (Energy dispersive X-ray spectrometry ) to the sample.
[0054]
　When spread hole after punching the carburized steel plate processed, concentrated deformation stress is the punched edge cracking is, by further continuing the processing, the crack is extended. Generation of cracks, such as surfactants or the like in which the soft tissue and hard tissue adjacent prone in the region between tissues hardness difference is large. As described above, carburizing steel sheet according to the present embodiment, since it is composed of ferrite and carbides, at the time of spread holes, cracks are likely to occur from the interface between ferrite and carbides. At that time, the shape of the carbide is flat, stress is easily concentrated at the tip of the carbide, thus conducive to cracking. Therefore, it is important to reduce the aspect ratio of the carbide by spheroidizing annealing. Furthermore, in order to suppress the extension of cracks, as well to suppress the generation of coarse carbides, it is effective to control the precipitation position of the carbides. That is, when carbides in grain boundaries of ferrite is formed, since the extension of the crack to propagate path grain boundaries is promoted, it is important to produce a carbide crystal grains of the ferrite. Carbide be to produce in the crystal grains of the ferrite, it is believed to be able to suppress the crack propagation at the grain boundaries.
[0055]
　In addition, the present inventors have, for the crystal orientation of the ferrite were found to significantly affect the hole expansion. Hole expansion processing is deformed by azimuthal rotation grains of ferrite progresses, this time, the azimuthal rotation hardly crystal grains adjacent, crack occurs from the grain boundary not withstand the deformation. Therefore, by controlling the production amount of azimuthal rotation hard grain, it revealed that it is possible to improve the hole expanding property.
　Hereinafter, the reasons for limitation of the microstructure constituting the carburizing steel sheet according to the present embodiment will be described in detail.
[0056]
[The ferrite grain {100} <011> - {223} <110> mean value of X-ray random intensity ratio of orientation component group of 7.0 or less]
　result of studies by the present inventors, the ferrite grain {100 } <011> - {223} <110> mean value of X-ray random intensity ratio of orientation component group is equal to or below 7.0, it has been found that it is possible to obtain a good hole expansion. Above when the average value of the X-ray random intensity ratio exceeds 7.0, conducive occurrence of cracks during widened hole can not be obtained satisfactory hole expansion. Accordingly, the carburizing steel sheet according to the present embodiment, the average value of the X-ray random intensity ratio to 7.0 or less. The average value of the X-ray random intensity ratio, for further improvement of the ultimate deformability is preferably 5.5 or less. The lower limit of the X-ray random intensity ratio is not particularly limited, considering the current general continuous hot rolling process, 0.5 is the real lower limit.
[0057]
　Incidentally, the crystal orientation is usually perpendicular orientation to the plate surface displayed in [hkl] or {hkl}, displays a parallel orientation to the rolling direction (uvw) or . {Hkl}, is a generic term for equivalent planes. The main orientations included in the {100} <011> - {223} <110> orientation component group of ferrite crystal grains, {100} <011>, {116} <110>, {114} <110>, {113 } <110>, {112} <110>, {335} <110>, and a {223} <110>.
[0058]
　Next, a method of calculating the metal structure.
　First, from carburizing steel, as can be observed cross section perpendicular (thickness cross section) on the surface thereof is cut out a sample. The length of the sample, depending on the measurement device, may be about 10 mm ~ 25 mm. The thickness 1/4 position of the sample, the electron backscatter diffraction measurement interval 0.1μm (Electron Back Scattering Diffraction: EBSD ) was measured using to obtain the crystal orientation information. Here EBSD analysis, for example, using a thermal field emission scanning electron microscope (JEOL Ltd. JSM-7001F) and EBSD detector device constituted by (TSL manufactured DVC5 type detector), 15kV ~ 25 kV electron beam acceleration voltage , carried out in the analysis speed of 200 to 300 points / sec. EBSD using onboard "TEXTURE" function to the analysis device supplied software "OIM Analysis (registered trademark)", from the obtained crystal orientation information to calculate a three-dimensional texture calculated by the series expansion method. Then using the "ODF" function, in .phi.2 = 45 ° cross section of the three-dimensional texture (001) [1-10], (116) [1-10], (114) [1-10], ( 113) [1-10], (112) [1-10], (335) [1-10], it is used as (223) [1-10] X-ray random intensity ratio intensity as ferrite crystal grains of the good. {100} <011> and ~ {223} <110> average orientation group is arithmetic mean of the azimuth. Incidentally, when it is not possible to obtain all of the intensity of the orientation described above, for example, {100} <011>, {116} <110>, {114} <110>, {112} <110>, {223 } may be replaced by the arithmetic mean of the orientation <110>. In the crystallography, "- 1" orientation that is formally denoted denoted by the upper bar on the "1" but the constraints described herein, - denoted as "1" there.
[0059]
[The number ratio of the carbide is an aspect ratio of 2.0 or less of the total carbide: 80%]
　As prior mentioned, carbides in this embodiment, cementite (Fe 3 C) and ε carbide (Fe 2 ~ 3 C) mainly composed of iron-based carbides such. Result of studies by the present inventors, of the total carbide, if the number ratio of the carbide aspect ratio of 2.0 or less is 80% or more, that it is possible to obtain satisfactory hole expansion revealed . When the number proportion of the carbide aspect ratio of 2.0 or less of the total carbides is less than 80%, and the occurrence of cracks is promoted when spread well, it is impossible to obtain a good hole expansion. Therefore, in the carburizing steel sheet according to the present embodiment, the lower limit of the ratio of the number of carbide is an aspect ratio of 2.0 or less of the total carbides is 80%. The number ratio of the carbide aspect ratio of 2.0 or less of the total carbides, the purpose of further improving the hole expansion, preferably 85% or more. The upper limit of the number ratio of the carbide is an aspect ratio of 2.0 or less of the total carbides is not particularly defined. However, it is difficult to more than 98% in actual operation, it is a substantial upper limit of 98%.
[0060]
[The number ratio of cementite present in the crystal grains of the ferrite of the total carbide: 60%]
　result of studies by the present inventors, the ratio of the number of carbides present in the crystal grains of the ferrite of the total carbide 60% if more, that it is possible to obtain satisfactory hole expansion revealed. If the total number ratio of cementite present in the crystal grains of the ferrite of the carbide is less than 60%, and crack extension is promoted during widened hole, it is impossible to obtain a good hole expansion. Therefore, in the carburizing steel sheet according to the present embodiment, the lower limit of the ratio of the number of carbides present in the crystal grains of the ferrite of the total carbides is 60%. The number ratio of cementite present in the crystal grains of the ferrite of all carbides, the purpose of further improving the hole expansion is preferably 65% or more. The upper limit of the number ratio of cementite present in the crystal grains of the ferrite of all carbides is not particularly defined. However, it is difficult to more than 98% in actual operation, it is a substantial upper limit of 98%.
[0061]
: [Average circular equivalent diameter of the carbide 5.0 .mu.m or less]
　In the microstructure of carburizing steel sheet according to the present embodiment, the average circle equivalent diameter of carbide is required to be less 5.0 .mu.m. When the average circle equivalent diameter of the carbide is more than 5.0μm, the cracks during punching occurs, it is impossible to obtain a satisfactory hole expansion. As the average circle equivalent diameter of the carbide is small, cracking during punching is hard to occur, the average circle equivalent diameter of carbide is preferably not 1.0μm or less, more preferably 0.8μm or less, more preferably 0 .6μm is less than or equal to. The lower limit of the average circle equivalent diameter of carbide is not particularly defined. However, in actual operation, it is difficult to average circle equivalent diameter of the carbide and 0.01 [mu] m or less, 0.01 [mu] m is substantially the lower.
[0062]
　Next, a method for measuring various number ratio and average circle equivalent diameter of the carbide of the carbide in the microstructure will be described in detail.
　First, cut out the sample so that it can be observed a cross-section perpendicular (thickness cross section) on the surface of carburized steel plate. The length of the sample, depending on the measurement device, may be about 10 mm. Section was polished and corroded, and subjected to measurement of the average circle equivalent diameter and precipitation position and aspect ratio of the carbide. Here, polishing, for example, after polishing the measurement surface using silicon carbide paper size 1500 from size 600, the particle size is a 6μm diamond powder dispersed in a diluent or pure water such as an alcohol from 1μm use the liquid may be Shiagere to the mirror surface. Corrosion, if a technique capable of observing the shape and precipitation position of carbides, the present invention is not particularly limited, for example, as a means to corrode the grain boundary carbide and the base steel, a saturated picric acid - etched with an alcohol solution also may be non-aqueous solvent electrolyte by constant potential electrolysis etching method (Fumio Kurosawa et al., Japan Institute of Metals Journal, 43,1068, (1979)) by the like, carbides removed several micrometers base iron only how to leave may be adopted.
[0063]
　Calculation of the aspect ratio of carbide, using a thermal field emission scanning electron microscope (e.g., JEOL Ltd. JSM-7001F), the sheet thickness 1/4 position of the sample, 10000 2 performed by observing the range. For all carbides contained in the observed visual field, by measuring the major and minor axes calculates the aspect ratio (long axis / short axis) and calculate the average. The above observations were carried out in five fields, the average value of the five visual fields, and the aspect ratio of the carbide sample. Referring to the aspect ratio of the resulting carbide, and the total number of carbide aspect ratio of 2.0 or less, and the total number of carbides present in the five fields from the aspect ratio of the total carbides 2. calculating a ratio of the number of carbides is 0 or less.
[0064]
　Confirmation of deposition position of carbide, using a thermal field emission scanning electron microscope (e.g., JEOL Ltd. JSM-7001F), the sheet thickness 1/4 position of the sample, 10000 2 performed by observing the range. For all carbides contained in the observed field of view, observing the precipitation position, of all the carbides, it calculates the proportion of carbides precipitated in the grains of ferrite. The above observations were carried out in five fields, the average value of the five fields, the proportion of carbides formed in crystal grains of the ferrite of the carbides (i.e., the ratio of the number of carbides present in the crystal grains of the ferrite of the total carbide) to.
[0065]
　The average circle equivalent diameter of carbide, using a thermal field emission scanning electron microscope (e.g., JEOL Ltd. JSM-7001F), the sheet thickness 1/4 position of the sample, 600 .mu.m 2 performed by 4 field shooting range . For each field, the image analysis software (e.g., Media Cybernetics manufactured IMage-Pro Plus) is used to measure the major and minor axes of the fancy-through carbides, respectively. For each carbide in the field of view, the average value of the obtained major axis and a minor axis and a diameter of the carbides, for all the carbides fancy-through in the field of view, and calculates the average value of the obtained diameters. Thus obtained, further on average in field number average value of the diameters of the carbides in the 4 field, the average circle equivalent diameter of carbide.
[0066]
　Although the microstructure having the carburizing steel sheet according to the present embodiment has been described in detail.
[0067]

　For the thickness of carburizing steel sheet according to the present embodiment is not particularly limited, for example, it is preferable to 2mm or more. The thickness of the carburized steel plate by the above 2 mm, it is possible to further reduce the plate thickness difference in the coil width direction. Thickness of carburizing steel sheet, and more preferably is 2.3mm or more. Further, the thickness of carburizing steel sheet is not particularly limited, it is preferable to 6mm or less. The thickness of the carburized steel plate by a 6mm or less, it is possible to lower the load during the press molding, the molding of the part easier ones. Thickness of carburizing steel sheet, and more preferably not more than 5.8 mm.
[0068]
　Although the carburizing steel sheet according to the present embodiment has been described in detail.
[0069]
(Manufacturing method of carburizing steel)
　Next, a method for producing a carburizing steel sheet according to the present embodiment as described above, will be described in detail.
[0070]
　Manufacturing method, using a steel having a chemical composition as described (A) prior description, hot rolled steel sheet according to a specific condition for producing carburizing steel sheet according to the present embodiment as described above a hot rolling step of producing a, (B) obtained hot-rolled steel sheet, or, with respect to steel sheet cold-rolled is performed after hot rolling step, the annealing process according to a specific heat treatment conditions includes an annealing step of performing, the.
　Hereinafter, the above hot rolling process, and, for the annealing step will be described in detail.
[0071]

　hot rolling step to be described below, using a steel having a predetermined chemical composition, to produce hot-rolled steel sheet according to a specific condition.
[0072]
　Here, steel slab subjected to hot rolling (steel) may be any slab produced in conventional manner, for example, the use of a steel strip produced by continuous casting slab, a general method such as thin slab caster can.
[0073]
　More specifically, using a steel having a chemical composition as described prior, subjected to hot rolling by heating such steel, a path pre-rolling of the hot finish rolling, 900 ° C. or higher 980 ° C. temperature below carried out at 25% or less of reduction ratio of 15% or more in-band, then the hot finish rolling, terminates at a reduction rate of 6% or more in a temperature range below 800 ° C. or higher 920 ° C., wound at 700 ° C. below the temperature by taking, the hot rolled steel sheet.
[0074]
[Hot finish rolling one pass before the rolling temperature: 900 ° C. or higher 980 ° C. or less, reduction rate: 15% or more and 25% or less]
　The hot rolling step according to the present embodiment, by one pass before the rolling step of hot finish rolling, thereby promoting the recrystallization of austenite, to form a lattice defect is small austenite grains. If the rolling temperature is below 900 ° C., or, when the rolling reduction exceeds 25%, will be introduced excessively lattice defects in the austenite, unnecessarily recrystallization austenite in the subsequent finish rolling process inhibit, the average value of the ferrite crystal grains of {100} <011> - {223} <110> orientation component group X-ray random intensity ratio of can not be controlled to 7.0 or less. Further, if the rolling temperature exceeds 980 ° C., or, when the rolling reduction is less than 15%, becomes remarkable coarsening of austenite grains, as a result, recrystallization of austenite grains in the subsequent finish rolling step inhibitor is, it becomes impossible to control the average value of the ferrite crystal grains of {100} <011> - {223} <110> orientation component group X-ray random intensity ratio of 7.0 or less. From such viewpoints, the hot rolling step according to the present embodiment, the rolling temperature of one pass before the final hot rolling and 900 ° C. or higher 980 ° C. or less, the reduction ratio, and 15% or more 25% or less. To better control the average value of the ferrite crystal grains of {100} <011> - {223} <110> orientation component group X-ray random intensity ratio of rolling temperature of one pass before the hot finish rolling, it is preferably 910 ° C. or higher. Further, the ferrite grain {100} <011> to better control the average value of ~ {223} <110> orientation component group X-ray random intensity ratio of one pass before the rolling temperature of the hot finish rolling it is preferably 970 ° C. or less. To better control the average value of the ferrite crystal grains of {100} <011> - {223} <110> orientation component group X-ray random intensity ratio of rolling reduction is preferably 17% or more. Further, in order to better control the average value of the ferrite crystal grains of {100} <011> - {223} <110> orientation component group X-ray random intensity ratio of, that reduction ratio is 20% or less preferable.
[0075]
[Hot finish rolling rolling temperature: 800 ° C. or higher 920 than ° C., rolling reduction: 6% or more]
　In the hot rolling step according to the present embodiment, the hot finish rolling process, to promote the recrystallization of austenite. If the rolling temperature is below 800 ° C., or, when the rolling reduction is less than 6%, the recrystallization austenite is not sufficiently promoted, the ferrite grain {100} <011> - {223} <110> the mean value of X-ray random intensity ratio of orientation component group can not be controlled to 7.0 or less. Therefore, the hot finish rolling according to the present embodiment, the rolling temperature of 800 ° C. or higher, the reduction ratio is 6% or more. To better control the ferrite crystal grains of {100} <011> ~ { 223} <110> X -ray random intensity ratio of orientation component group, rolling temperature in the hot finish rolling is preferably 810 ° C. or higher . On the other hand, if the rolling temperature is 920 ° C. or higher, will significantly austenite grain coarsening of the austenite, resulting in a next step, the ferrite generation is hindered. Therefore, the hot finish rolling according to the present embodiment, the rolling temperature less than 920 ° C.. To better control the ferrite crystal grains of {100} <011> ~ { 223} <110> X -ray random intensity ratio of orientation component group, rolling temperature in the hot finish rolling is preferably less than 910 ° C. . Incidentally, in the hot finish rolling according to the present embodiment, the upper limit of the reduction ratio is not particularly defined. However, from the viewpoint of shape stability of the hot rolled steel sheet, 50% is a substantial upper limit.
[0076]
: Coiling temperature 700 ° C. or less]
　As mentioned prior microstructure of carburizing steel sheet, the average circle equivalent diameter of carbide, not more than 5.0 .mu.m, {100} of the ferrite grain <011> - { 223} <110> mean value of X-ray random intensity ratio of orientation component group is 7.0 or less, and a ratio of the number of carbide aspect ratio of 2.0 or less of the total carbides is 80% or more, and, the number ratio of the carbide formed in crystal grains of the ferrite of the total carbides is required to be 60% or more. To do this, (more specifically, spheroidizing annealing) subsequent annealing steel sheet structure before being subjected to (hot-rolled steel sheet structure) is 80% or less of ferrite of 10% or more in area ratio, the area ratio configuration and in 10% to 60% or less of pearlite, the total area ratio contained such that 100% or less, the balance being bainite, martensite, tempered martensite, and from at least one of retained austenite it is preferred that the.
[0077]
　In the hot rolling step according to the present embodiment, when the coiling temperature exceeds 700 ° C., the formation of pearlite ferrite generation is too accelerated is suppressed, and finally, in the steel sheet after annealing, carbides to control the percentage of the carbide is the aspect ratio is 2.0 or less to 80% or more becomes difficult of. Therefore, in the hot rolling step according to the present embodiment, the upper limit of the coiling temperature to 700 ° C.. For the coiling temperature of the hot rolling step according to the present embodiment, the lower limit is not particularly defined. However, the actual operation, since it is difficult to wind at or below room temperature, room temperature is substantially lower limit. Incidentally, the coiling temperature of the hot rolling step according to the present embodiment, from the viewpoint of smaller aspect ratio of carbides after subsequent annealing step is preferably 400 ° C. or higher.
[0078]
　Here, in the hot rolling step according to the present embodiment as described above, the total number of passes of the hot rolling is not particularly specified and may be any number of paths. As for the rolling reduction in the second pass before the previous hot finish rolling, not specifically defined, as the desired final thickness is obtained may be set as appropriate.
[0079]
　Incidentally, pickling rewind the hot wound steel sheet in the rolling step as described above (hot-rolled steel sheet) may be subjected to cold rolling. Pickling by by removing an oxide surface of the steel sheet, and the like can be achieved a further improvement in hole expansion. Incidentally, pickling may be a single, or may be performed a plurality of times. Cold rolling the usual reduction ratio (e.g. 30 to 90%) may be cold rolling conducted at. The hot-rolled steel sheets and cold-rolled steel sheet, other than those that remain hot rolling and cold rolling also also includes steel sheets subjected to temper rolling under normal conditions.
[0080]
　The hot rolling step according to the present embodiment, as described above, hot-rolled steel sheet is manufactured. Hot-rolled steel sheet produced, or, with respect to steel sheet cold rolling after the hot rolling step is performed, further, in the annealing step, as described in detail below, by performing a specific annealing treatment, the it can be obtained carburizing steel sheet according to the embodiment.
[0081]

　annealing step to be described below, the above hot rolling process by resulting heat rolled steel plate, or, with respect to steel sheet cold after hot rolling process rolling is performed, a predetermined a step of performing annealing treatment (spheroidizing annealing process) with reference to the heat treatment conditions. Such annealing, the resulting pearlite in the hot rolling step is spheroidized.
[0082]
　More particularly, hot rolled steel sheets obtained as described above, or, a steel sheet cold rolling is performed after hot rolling process, the nitrogen concentration in the volume fraction in the atmosphere is controlled to less than 25% Te, at an average heating rate of less than 5 ° C. / h or higher 100 ° C. / h, Ac defined by the following formula (101) 1 was heated to a temperature range of less points, Ac 1 10h or 100h following temperature range below point after performing annealing for holding, subjected to cooling to the average cooling rate 5 ° C. / h or higher 100 ° C. / h or less in a temperature range of up to 550 ° C. the temperature in annealing finished.
　Here, in the following formula (101), the notation of [X], the content of the element X: represents (unit weight%), if not containing the relevant elemental shall substituting zero.
[0083]
[Number 2]

[0084]
: Annealing atmosphere atmosphere with controlled nitrogen concentration at a volume fraction below 25%
　in the annealing process as described above, the annealing atmosphere is an atmosphere which is controlled to be less than 25% nitrogen concentration in the volume fraction. When the nitrogen concentration of 25% or more by volume fraction, and nitrides in the steel sheet is formed, because it causes hole expansibility degradation is undesirable. Such a nitrogen concentration, the lower desirable. However, to control to 1% or less of nitrogen concentration in the volume fraction are the costs disadvantageous, volume fraction of 1% is a substantial lower limit of the nitrogen concentration.
[0085]
　Atmospheric gas, for example, nitrogen, gas such as hydrogen, or at least one suitably selected from among inert gases such as argon, as the nitrogen concentration in the heating furnace used in the annealing step has a desired concentration, it may be used above various gases. Also, if a small amount, no problem contains a gas such as oxygen in the ambient gas. For example, the atmospheric gas is preferably higher hydrogen concentration. For example, the volume fraction of the hydrogen concentration by 60% or more, it is possible to increase the thermal conductivity in the annealing apparatus, it is possible to reduce the manufacturing cost. More specifically, as the annealing atmosphere, the hydrogen concentration at a volume fraction of 95% or more, the balance may be used as nitrogen. Atmospheric gas in the furnace used in the annealing step, for example, by suitably measuring the gas concentration inside the heating furnace while introducing the above-mentioned gas, it is possible to control.
[0086]
: Heating condition 5 ° C. / h or higher 100 ° C. / h or less of the average heating rate Ac 1 to a temperature range below point]
　The annealing process according to the present embodiment, the hot rolled steel sheet as described above, or, hot the steel sheet cold rolling is performed after the rolling step, at 5 ° C. / h or higher 100 ° C. / h or less of the average heating rate, a defined by the formula (101) C1 it is necessary to heat to a temperature range of below points. When the average heating rate is less than 5 ° C. / h, the average circle equivalent diameter of carbide exceeds the 5.0 .mu.m, hole expandability deteriorates. On the other hand, when the average heating rate exceeds 100 ° C. / h is without being spheroidized sufficiently promoted carbide, the ratio of the number of carbide aspect ratio of 2.0 or less of the total carbide 80% or more it is difficult to control. The heating temperature is, A defined by the formula (101) C1When it exceeds the point, the number proportion of carbides formed in crystal grains of the ferrite of the total carbides becomes less than 60%, it is impossible to obtain a good hole expansion. The lower limit of the temperature range of the heating temperature is not particularly defined. However, the temperature range of the heating temperature is less than 600 ° C., holding time in the annealing process is long, the manufacturing cost becomes disadvantageous. Therefore, the temperature range of the heating temperature is preferably set to 600 ° C. or higher. To better control the state of the carbide, the average heating rate in the annealing step according to this embodiment is preferably set to 20 ° C. / h or higher. Further, in order to better control the state of the carbide, the average heating temperature in the annealing step according to this embodiment is preferably set to less 50 ° C. / h. To better control the state of the carbide, the temperature range of the heating temperature in the annealing process according to the present embodiment, it is more preferable to 630 ° C. or higher. Further, in order to better control the state of the carbides, the temperature range of the heating temperature in the annealing process according to the present embodiment is more preferably set to 670 ° C. or less.
[0087]
[Retention time: Ac 1 or less temperature range 10h or 100h following points]
　In the annealing process according to the present embodiment, Ac as described above 1 below point (preferably, Ac 600 ° C. or higher 1 the temperature range below point) , it is necessary to hold 10h or 100h following. If the holding time is less than 10h are without being spheroidized sufficiently promoted carbide, difficult to control the ratio of the number of carbide aspect ratio of 2.0 or less of the total carbide 80% or more to become. On the other hand, if the retention time exceeds 100h, the average circle equivalent diameter of carbide exceeds 5.0 .mu.m, hole expandability deteriorates. To better control the state of the carbides, the holding time in the annealing step according to the present embodiment is preferably 20h or more. Further, in order to better control the state of the carbides, the holding time in the annealing step according to the present embodiment, it is preferable to 80h or less.
[0088]
: Cooling Conditions 5 ° C. / h or higher 100 ° C. / h cooling at below average cooling rate]
　In the annealing step according to the present embodiment, after the above-mentioned heating and holding, the steel sheet 5 ° C. / h or higher 100 ° C. / h or less the average is cooled at a cooling rate of. The average cooling rate (in other words, the temperature during annealing finished) heating and maintaining temperature is the average cooling rate from to 550 ° C.. When the average cooling rate is less than 5 ° C. / h, the carbides is too coarse, the hole expansion properties are deteriorated. On the other hand, if the average cooling rate exceeds 100 ° C. / h is without being spheroidized sufficiently promoted carbide, the ratio of the number of carbide aspect ratio of 2.0 or less of the total carbide 80% or more it is difficult to control. To better control the state of the carbide, the average cooling rate from the heating retention temperature to 550 ° C. is preferably set to 20 ° C. / h or higher. Further, in order to better control the state of the carbide, the average cooling rate from the heating retention temperature to 550 ° C. is preferably at most 50 ° C. / h.
[0089]
　Note that in the annealing step according to the present embodiment, the average cooling rate in the temperature range below 550 ° C. is not particularly defined, it may be cooled at any average cooling rate to a predetermined temperature range. The lower limit of the temperature for stopping the cooling is not particularly specified. However, cooling to room temperature or less is difficult on actual operation, room temperature is substantially lower limit.
[0090]
　Although the annealing step according to the present embodiment has been described in detail.
　Or hot as described rolling process and by carrying out the annealing step, as described prior, it is possible to produce a carburizing steel sheet according to the present embodiment.
[0091]
　The above prior to performing annealing step as described, the steel sheet after hot rolling, in air, at 40 ° C. or higher 70 ° C. below the temperature range may be held 72h or 350h following. By performing such holding, it is possible to form aggregates of carbon in solid solution in ferrite grain. Aggregates of such carbon is one in which carbon number of atoms are aggregated in crystal grains of the ferrite. By forming such agglomerates the carbon, carbide formation in the subsequent annealing step is further promoted. As a result, further improve the mobility of metastasis in the steel sheet after annealing, it is possible to further improve the formability of the steel sheet after annealing.
[0092]
　Further, with respect to carburizing steel sheets obtained as described above, for example, may cold working is performed as a post process. Further, with respect to the processed above carburized steel plate cold, for example, in the range carbon potential of 0.4 to 1.0 mass%, carburizing heat treatment may be performed. Conditions of carburizing heat treatment is not particularly limited, it can be appropriately adjusted so as to obtain desired characteristics. For example, a carburizing steel was heated to the austenite single phase region temperature, after carburizing treatment, may be cool to room temperature, once after cooling to room temperature, reheated, it may be rapidly cooled. Further, for the purpose of adjusting the intensity, for all or part of the member may be subjected to a tempering process. Further, for the purpose of obtaining a corrosion protection, it may be plated on the steel sheet surface, in order to improve the fatigue characteristics, may be subjected to shot peening the surface of the steel sheet.
Example
[0093]
　Next, a description will be given of an embodiment of the present invention. The conditions in the examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is not limited to this single example of conditions. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0094]
(Test Example)
　The steel having the chemical composition shown in Table 1 below, after hot rolling (and cold rolling) under conditions shown in Table 2 below, and annealed to obtain a carburized steel plate. Incidentally, after the hot rolling under the conditions shown in Table 2 below, in the atmosphere, after holding 105 hours at 55 ° C., it was annealed under conditions shown in Table 2 below. In Table 1 and Table 2 below, underline indicates that it is outside the scope of the present invention.
[0095]
[Table 1-1]

[0096]
[Table 1-2]

[0097]
[table 2-1]

[0098]
[Table 2-2]

[0099]
[Table 2-3]

[0100]
　For each of the obtained carburized steel plate, (1) the ferrite grain {100} <011> - {223} <110> mean value of X-ray random intensity ratio of orientation component group, (2) of the total carbide Aspect the number ratio of the carbide ratio is 2.0 or less, (3) the ratio of the number of carbides formed in crystal grains of the ferrite of the total carbides, and, (4) the average circle equivalent diameter of carbides was described prior It was measured by the method.
[0101]
　In order to evaluate the cold workability of the respective carburizing steel sheet obtained, with reference to JIS Z 2256 (hole expansion test method for metallic materials), it was subjected to hole expansion test. Hole expansion rate, resulting test pieces were taken from an arbitrary position of each of the carburizing steel sheet according to the test methods and formulas are defined in JIS Z 2256, it was calculated. In this test example, as excellent a case resulting hole expanding ratio is 80% or more extreme deformability was the "Examples". As for those cracks at the time of manufacture of the hole expanding test piece (when punching) occurs, - describing the "."
[0102]
　Further, as a reference to calculate the ideal critical diameter which is an index representing the hardenability after carburizing. Ideal critical diameter D i is an index calculated from the components of the steel sheet can be calculated Grossmann / Hollomon, according to equation (201) below using the method of Jaffe. Ideal critical diameter D i larger value of, the better the hardenability.
[0103]
[Number 3]

[0104]
　Table 3 below, the microstructure and properties of each of the carburizing steel sheet obtained are shown collectively.
[0105]
[Table 3-1]

[0106]
[Table 3-2]

[0107]
[Table 3-3]

[0108]
　As is apparent from Table 3, carburizing steel sheet corresponding to the embodiment of the present invention, JIS Z 2256 hole expansion ratio specified in (hole expansion test method for metallic materials) is 80% or more, excellent to have an ultimate deformability revealed. Further, the ideal critical diameter described by reference also becomes 5 or more, carburizing steel sheet corresponding to the embodiment of the present invention, it can be seen that combines excellent hardenability.
[0109]
　On the other hand, as is clear from Table 3, carburizing steel sheet corresponding to a comparative example of the present invention, the hole expansion ratio is less than 80%, was found to be inferior to the intrinsic deformability. In particular, No. 7,11-15,74,78,82,87, since cracking during the production of the hole expanding test piece (punching) occurs, it is impossible to calculate the hole expansion ratio was found to be poor in workability .
[0110]
　Having described in detail preferred embodiments of the present invention, the present invention is not limited to such an example. It would be appreciated by those skilled in the relevant field of technology of the present invention, within the scope of the technical idea described in the claims, it is intended to cover various changes and modifications , also such modifications are intended to fall within the technical scope of the present invention.

WE CLAIM

[Requested item 1]
　By
　mass%, C: less than 0.02% to 0.30%
　Si: less than 0.005% or more and 0.5%
　Mn: less than 0.01% to 3.0%
　P: 0.1% or less
　S: 0 .1% or less
　sol. Al: 0.0002% or more and 3.0% or less
　N: 0.2% or less
containing the balance consists of Fe and impurities,
　{100} of the ferrite grain <011> - {223} <110> orientation mean value of X-ray random intensity ratio of the group is, is 7.0 or less,
　the average circle equivalent diameter of carbide, not more than 5.0 .mu.m,
　the ratio of the number of carbide aspect ratio of 2.0 or less, the total at least 80% with respect to carbides,
　the ratio of the number of carbides present in the ferrite grain is less than 60% relative to the total carbide, carburized steel plate.
[Requested item 2]
　Instead a part of the remainder of Fe, in
　mass%, Cr: 0.005% to 3.0% or less
　Mo: 0.005% to 1.0% or less
　Ni: 0.010% to 3.0% or less
　Cu: 0.001% to 2.0% or less
　Co: 0.001% to 2.0% or less
　Nb: 0.010% or more 0.150% or less
　Ti: 0.010% or more 0.150% or less
　V: 0.0005% to 1.0% or less
　B: 0.0005% or more than 0.01%
further containing one or more, carburizing steel sheet according to claim 1.
[Requested item 3]
　Instead a part of the remainder of Fe, by
　mass%, Sn: 1.0% or less
　W: 1.0% or less
　Ca: 0.01% or less
　REM: 0.3% or less
of one or more further comprising, carburizing steel sheet according to claim 1 or 2.
[Requested item 4]
　A method of manufacturing a carburizing steel according to any one of claims 1 to 3,
　heating the steel having the chemical composition according to any one of claims 1 to 3, hot finish rolling rolling one pass before, carried out at 25% or less of reduction ratio of 15% or more at 900 ° C. or higher 980 ° C. or less of the temperature range, the hot finish rolling, more than 6% in a temperature range below 800 ° C. or higher 920 ° C. of the ends with rolling reduction, the hot-rolling process for winding at 700 ° C. temperature below
　the steel sheet obtained by the hot rolling process, or the steel sheet cold rolling is performed after the hot rolling step, the nitrogen concentration at a volume fraction in controlled atmosphere to less than 25%, an average heating rate of 100 ° C. / h or less 5 ° C. / h or higher, Ac defined by the following formula (1) 1 to a temperature range below point heating, the Ac 1 were subjected to annealing for holding 10h or 100h following temperature range below point , The annealing step of performing an average cooling rate of at most 5 ° C. / h or higher 100 ° C. / h cooling in a temperature range of up to 550 ° C. the temperature in annealing completion
including method of carburizing steel.
　Here, in the following formula (1), the notation of [X], the content of the element X: represents (unit weight%), if not containing the relevant elemental shall substituting zero.
[Number 1]