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.
[0003]
　When applying such technology, the steel sheet, hardenability after carburizing heat treatment cold workability are both determined. Generally, in order to increase the hardenability, tensile strength of carburizing steel sheet higher preferred. However, by increasing the strength of the steel sheet, cold workability is deteriorated. Therefore, techniques to achieve both these contradictory properties are required.
[0004]
　The cold working, punching the material, followed by bending, drawing, molding the member through the hole expansion processing or the like. Forming into complex shapes of members such as the damper parts, etc. of the torque converter is comprised of a combination of various deformation modes. Therefore, a method capable of improving the stretch flange formability such bendability and hole expandability, etc., or by methods which can significantly improve the ductility of the steel sheet, it is possible to improve the cold workability. From this point of view, in recent years, various techniques have been proposed.
[0005]
　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.
[0006]
　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.
[0007]
　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
[0008]
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
[0009]
　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, it is required to secure the moldability.
[0010]
　However, the microstructure control which mainly forms control of carbides proposed in Patent Document 1, poor ductility of the resulting steel sheet, it is difficult to process into complex shapes of members. Further, it proposed in Patent Document 2, in the manufacturing method mainly the microstructure control of carbide and ferrite, although the improved formability of the resulting steel sheet, in order to process into complex shapes of members it is difficult to ensure the necessary ductility. Further, in the method proposed in Patent Document 3, although the improved formability of the resulting steel sheet still, it is difficult to ensure the ductility required for processing into complex shaped members . Thus, in the conventional proposed techniques, it is difficult to increase the ductility of carburizing steel, therefore, is particularly applicable hardenability high steel sheet into parts of complex shapes such as damper parts and the like of the torque converter It has been limited.
[0011]
　The present invention has been made in view of the above problems, it is an object of the present invention is to provide an excellent carburizing steel and its manufacturing method by ductility.
Means for Solving the Problems
[0012]
　The present inventors have, how to resolve the above problems, intensive studies. As a result, as described in detail below, to reduce the number density of carbides produced within the steel sheet, and by refining the crystal grains of the ferrite in the steel sheet, while maintaining the hardenability, the ductility inspired and it is possible to achieve excellent carburizing steel, and have completed the present invention.
　Summary of the completed invention based on this idea is as follows.
[0013]
[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% to 3.0% or less, N: 0.2% or less, Ti: containing 0.010% or more 0.150% or less, and the balance consists of Fe and impurities, 1000 .mu.m 2 per number of carbides is 100 or less, the ratio of the number of carbide aspect ratio of 2.0 or less is 10% or more of the total carbide, the average circle equivalent diameter of carbide, below 5.0μm There, the average crystal grain size of the ferrite is 10μm or less, carburizing steel.
[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, V: 0.0005 % to 1.0% or less, B: further containing one or more than 0.0005% 0.01% or less, carburizing steel sheet according to [1].
[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, hot finish rolling, after finishing at 800 ° C. or higher 920 ° C. lower than the temperature range, the temperature range from hot finish rolling end temperature to the cooling stop temperature 50 ° C. / s or higher 250 ° C. / s or less the average was cooled at a cooling rate, a hot-rolling process for winding at 700 ° C. temperature below the steel sheet obtained by the hot rolling process, or the hot cold rolling after the rolling process has been performed steel the at annealing atmosphere is controlled to less than 25% nitrogen concentration in the volume fraction, 1 ° C. / h at least 100 ° C. / h or less of the average heating rate, Ac defined by the following formula (1) 1 the following point heated to a temperature range, the Ac 1 first annealing step of holding 1h or 100h less in a temperature range below point The steel sheet having passed through the first annealing step, the at 1 ° 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 points than 790 ° C. below the temperature range , the Ac 1 and a second annealing step of holding at points exceeding 790 ° C. below the temperature range 1h or 100h less for steel sheet after annealing at the second annealing step, during annealing finished in the second annealing step comprising the cooling step of performing cooling of the average cooling rate 1 ° C. / h or higher 100 ° C. / h or less in a temperature range of up to 550 ° C. from the temperature, the method for producing a carburizing steel.
[5] between the hot rolling step and the first annealing step, the steel sheet obtained by the hot rolling process, in air, at a temperature of 40 ° C. or higher 70 ° C. or less, held 72h or 350h following further comprising, a manufacturing method of carburizing steel sheet according to [4] the retaining step.
[0014]
[Number 1]

[0015]
　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
[0016]
　According to the present invention described above, it is possible to provide hardenability, good carburizing steel sheet by formability and ductility.
DESCRIPTION OF THE INVENTION
[0017]
　The following describes in detail preferred embodiments of the present invention.
[0018]
(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 have found that a method for improving the ductility was investigated.
[0019]
　Ductility is the uniform elongation, a characteristic composed of a local elongation. Traditionally, one of the two aspects of the ductility as described above, have been proposed a technique to improve the predominantly uniform elongation. However, in order to mold the complex shaped parts is not only uniform elongation, local elongation is also important to improve simultaneously. The uniform elongation and local elongation, the microstructure control guidance for improvement are different. Therefore, the present inventors have for these two elongation simultaneously improvable tissue control method of intensive studies. As a result, in order to improve both the both the uniform elongation and local elongation, to reduce the number density of carbides, in addition, it is effective to refine the grain of the ferrite by the inclusion of Ti, with the finding of It led to obtain.
[0020]
　Including techniques proposed in the above Patent Documents 1 to 3, a conventional, in order to improve the uniform elongation for the purpose of improving the workability, because ferrite having a particle diameter of preferably larger, the grain refining effect the inclusion of high Ti is, it is actively have not been carried out. In the present invention, as described below, and performs a two-step annealing in producing a carburizing steel sheet according to the present invention. Here, as in the prior art, if the Ti as steel component was not included in a predetermined amount, will be grain coarsening is promoted by performing a two-step annealing, the deterioration of local elongation of the ductile was inevitable. However, the present inventors have studied intensively, it was possible to obtain information about the organization control method capable of both improving both the uniform elongation and local elongation. Below, for such knowledge, it will be described in detail.
[0021]
　First, in order to improve the uniform elongation, it is effective to suppress the generation of voids in the tensile deformation. The tensile deformation, voids from the interface between the hard tissue and soft tissue is likely to occur in the carburizing steel plate, at the interface between ferrite and carbides, generation of voids is promoted. Therefore, the present inventors have found that to reduce the total area of ​​the interface between ferrite and carbide by reducing the number density of carbides present in the steel sheet, the idea of ​​it is possible to suppress the generation of voids I came to get.
[0022]
　Based on this idea, the present inventors have made extensive studies, the heating condition of spheroidizing annealing by two steps, could be achieved to reduce the number density of carbides. Specifically, the present inventors have found that, in the spheroidizing annealing step, the steel sheet after the hot rolling step, Ac 1 and heated to a temperature below zone points, according Ac 1 1h or 100h less in a temperature range below point annealed in the first stage to hold, then the steel sheet after the annealing in the first stage, Ac 1 and heated to points than 790 ° C. or less, according Ac 1 held 1h or 100h following point than 790 ° C. below the temperature range by performing annealing in the second stage to have succeeded in reducing the number density of carbides.
[0023]
　As this mechanism, firstly, the heat retention of the first stage Ac 1 by carrying out the following points, and to promote the diffusion of carbon, the plate-like carbides generated at the hot rolling step is spheroidized. In this first step, the steel sheet structure is mainly are composed of ferrite and carbides, while the steel sheet structure, fine carbide and coarse carbides are mixed. Then, the heat retention of the second stage Ac 1 by performing at points greater than dissolve the fine carbide, thereby reducing the number density of carbides. The Ac 1 at a temperature range of points exceeding, for Ostwald growth of carbides occurs, by promoting the dissolution of fine carbide is considered possible to reduce the number density of carbides.
[0024]
　Next, in order to improve the local elongation, it is important to suppress the coupling of the void, the coupling suppressing voids, grain refining of ferrite mother phase is effective. The present inventors have found that when the grain boundary is increased by grain refinement, leading to obtaining the idea of ​​voids generated at the interface between the carbide and ferrite is not easily connected. The present inventors have made intensive studies based on this idea, by controlling the average crystal grain size of the ferrite to 10μm or less, it was found that the coupling effect of suppressing voids is obtained.
[0025]
　Accordingly, the present inventors have ferrite result of extensive further studied method for grain refinement, and by subjecting the steel sheet which contains a Ti of 0.010% or more to the hot rolling, the austenite before transformation fine It can be granulated, in addition, by winding by cooling the steel sheet at an average cooling rate of more than 50 ° C. / s immediately after hot finish rolling, while suppressing the grain growth of the austenite phase transformation to ferrite It found that it is possible to start the. Thus, ferrite nucleation sites is increased, it is possible to refine the ferrite grains.
[0026]
　As described above, the microstructure control from two perspectives, both both uniform elongation and local elongation can be improved, as a result, while maintaining the hardenability, obtain excellent carburizing steel sheet by ductile in particular it was successful. Such carburizing steel sheet, better results by ductility, better formability is exhibited.
[0027]
　Incidentally, improvement of the above-mentioned ductility (uniform elongation and local elongation) is more is high steel, the effect is high hardenability. For example, tensile strength 340 MPa class, such as 440MPa grade, tensile strength in the above high-strength steel sheet 340 MPa, the ductility is remarkably improved. Therefore, the tissue control as outlined above, while maintaining the hardenability, it is possible to improve the ductility. Such carburizing steel sheet, better results by ductility, better formability is exhibited.
[0028]
　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.
[0029]
(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, 1000 .mu.m 2 number of carbides per can is 100 or less, the ratio of the number of carbide aspect ratio of 2.0 or less is 10% of the total carbide or more, the average circle equivalent diameter of carbide, not more than 5.0 .mu.m, and the average crystal grain size of the ferrite is referred to is 10μm or less, and has a specific microstructure. Thus, carburizing steel sheet according to the present embodiment, while maintaining the hardenability, exhibits a more excellent ductility and formability.
[0030]

　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.
[0031]
[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. In 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 local elongation.
[0032]
　When the content of C is less than 0.02% does not improve the effect of local elongation as described above can 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, uniform elongation 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. Further, elongation uniform elongation and local, as well, considering the respective balance of hardenability, the content of C is more preferably 0.10% or less, and still more preferably, less than 0.10% .
[0033]
[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 S is 0.5% or more, Si solid-solved in carbides to stabilize the carbides, in the first stage of the annealing, the number density of inhibition to carbide dissolution of carbides There is not reduced, uniform elongation is impaired. 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%, more preferably less than 0.1%.
[0034]
[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 to stabilize the carbides, in the first stage of the annealing, the number density of inhibition to carbide dissolution of carbides There is not reduced, uniform elongation is impaired. Therefore, the carburizing steel sheet according to the present embodiment, 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%.
[0035]
[P: 0.1% or less]
　P (phosphorus) is segregated at the grain boundaries of the ferrite is an element which is conducive to brittle fracture degrade ductility. When the content of P exceeds 0.1%, the intergranular strength of the ferrite is significantly reduced, uniform elongation is deteriorated. 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%.
[0036]
[S: 0.1% or
　less] S (sulfur) is, the inclusions formed, is an element that degrades the ductility. When the content of S exceeds 0.1%, the uniform elongation is deteriorated 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 Der 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%.
[0037]
[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 uniform elongation 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.
[0038]
: [N 0.2% or less]
　In carburizing steel sheet according to the present embodiment, the content of N (nitrogen) is required to be 0.2% or less. When the content of N exceeds 0.2%, the local elongation is remarkably reduced in generation of 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 0.05% or less, further preferably 0.01% or less. 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%.
[0039]
[Ti: 0.010% or more 0.150% or
　less] Ti (titanium), at the hot rolling step, contributes to grain refinement of the ferrite by refining the old austenite grains, contribute to the improvement of local elongation it is an element. To obtain a fine effect of such ferrite, in a carburizing steel sheet according to the present embodiment, the content of Ti, 0.010% or more. The content of Ti is preferably 0.015% or more. On the other hand, considering the influence of the generation of carbides and nitrides, in order to obtain the effect of improving the local elongation, the content of Ti is 0.150% or less. The content of Ti is preferably not more than 0.075%.
[0040]
[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 an element which contributes to the further improvement of local elongation and refining grain. 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 a further improvement of local elongation, the content of Cr preferably set to 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 of local elongation, 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.
[0041]
[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 an element which contributes to the further improvement of local elongation and refining grain. 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 a further improvement of local elongation, the content of Mo preferably set to 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 of local elongation, the content of Mo is preferably 1.0% or less. The content of Mo is more preferably 0.8% or less.
[0042]
[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 an element which contributes to the further improvement of local elongation and refining grain. 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 of local elongation, 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 of local elongation, 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.
[0043]
[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 an element which contributes to the further improvement of local elongation and refining grain. 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 of local elongation, 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 of local elongation, the content of Cu is preferably 2.0% or less. The content of Cu is more preferably 0.80%, more preferably not more than 0.50%.
[0044]
[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 an element which contributes to the further improvement of local elongation and refining grain. 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 of local elongation, 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 of local elongation, the content of Co is preferably 2.0% or less. The content of Co is more preferably 0.80%.
[0045]
[Nb: 0.010% or more 0.150% or
　less] Nb (niobium) is an element contributing to further improvement of local elongation and refining crystal grains. 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 of local elongation, 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 of local elongation, 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, even more preferably 0.050% or less.
[0046]
[V: 0.0005% to 1.0% or
　less] V (vanadium) is an element contributing to further improvement of local elongation of crystal grains of ferrite are miniaturized. 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 of local elongation, the content of V 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 of local elongation, 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 0.050% or less.
[0047]
[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 uniform elongation. 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 of the uniform elongation, 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, for further improvement of the uniform elongation as described above 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 0.0030% or less.
[0048]
[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.
[0049]
[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.
[0050]
[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 0.005% or less.
[0051]
[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.
[0052]
　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 is applicable not hardenability and moldability only exerts an effect of excellent in ductility. Moreover, even contain a metal REM such as metal La and Ce, carburizing steel sheet according to the present embodiment exhibits excellent ductility.
[0053]
: [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.
[0054]
　Although the chemical components with the carburizing steel sheet according to the present embodiment has been described in detail.
[0055]

　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 for example 85 to 95%, the area ratio of carbides, for example in the range of 5-15% there are, and configured such that the total area ratio of the ferrite and carbides does not exceed 100%.
[0056]
　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).
[0057]
　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.
[0058]
　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, the ratio of the number of carbides, such as detailed below, to the various carbide containing iron-based carbides may be one that as a population, only iron-based carbides may be one that as a population . Iron-based carbides, for example, can be identified using the Diffraction analysis and EDS (Energy dispersive X-ray spectrometry ) to the sample.
[0059]
　As prior described, in order to improve the ductility of carburizing steel sheet reduces the number density of carbides, and further, it is important to refine the crystal grains of the ferrite by the inclusion of Ti.
[0060]
　Ductility, as described above, composed of a uniform elongation and local elongation. Traditionally, one of the two aspects of ductility, primarily a technique for improving the uniform elongation have been proposed, in order to mold the complex shaped parts is not only uniform elongation, the local elongation also be improved at the same time is important. The uniform elongation and local elongation, since the microstructure control guidance for improvement are different, the present inventors have for these two simultaneously improvable tissue control means elongation of intensive studies. As a result, it was possible to obtain the following findings.
[0061]
　First, in order to improve the uniform elongation, it is effective to suppress the generation of voids in the tensile deformation. The tensile deformation, voids from the interface between the hard tissue and soft tissue is likely to occur in the carburizing steel plate, generation of voids is promoted at the interface between ferrite and carbides. Therefore, the present inventors have made intensive studies, as a result, have found that by reducing the total area of ​​the interface between ferrite and carbide by reducing the number density of carbides, to suppress the generation of voids.
[0062]
　Next, the improvement of local elongation, it is important to suppress the coupling of the void, the coupling suppressing voids, grain refining of ferrite mother phase is effective. The present inventors have found that when the grain boundary is increased by grain refining, led to idea of the voids generated at the interface between the carbide and ferrite is not easily connected. The present inventors have made intensive studies based on this idea, by controlling the average crystal grain size of the ferrite to 10μm or less, it was found to inhibit the coupling of the void.
　Hereinafter, the reasons for limitation of the microstructure constituting the carburizing steel sheet according to the present embodiment will be described in detail.
[0063]
[1000 .mu.m 2 number of carbides per: 100 or less]
　carbides in the present embodiment, as described above, cementite (Fe 3 C) and ε carbide (Fe 2 ~ 3 mainly by C) an iron-based carbides such as constructed. Result of studies by the present inventors, 1000 .mu.m 2 if the number of carbides per is 100 or less, it has been found that it is possible to obtain a good uniform elongation. Accordingly, the carburizing steel sheet according to the present embodiment, 1000 .mu.m 2 number of carbides per shall be 100 or less. Here, as is clear from the measurement method described below, "1000 .mu.m in this embodiment 2 the number of carbides per" in thickness 1/4 position of carburizing steel, 1000 .mu.m 2 optionally having a width of which is the average number of carbide in the area. 1000 .mu.m 2 number of carbides per is preferably less 90. Incidentally, 1000 .mu.m 2 the lower limit of the number of carbides per is not particularly limited. However, in actual operation, 1000 .mu.m 2 because it is difficult to reduce the number of carbides per less than 5, 5 is substantially the lower.
[0064]
[The number ratio of the carbide aspect ratio of 2.0 or less of the total carbide: 10% or more]
　result of studies by the present inventors, of the total carbide, the ratio of the number of carbide aspect ratio of 2.0 or less There if 10% or more, it has been found that it is possible to obtain a good uniform elongation. When the number proportion of the carbide aspect ratio of 2.0 or less of the total carbides is less than 10%, the occurrence of cracks during tensile deformation is promoted, it is impossible to obtain a good uniform elongation. Therefore, in the carburizing steel sheet according to the present embodiment, the aspect ratio of the total carbides the number ratio of carbides is 2.0 or less, to 10% or more. Of all the carbides, the number ratio of the carbide aspect ratio of 2.0 or less, for the purpose of further improvement of the uniform elongation is preferably 20% 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 limited. However, it is difficult to more than 98% in actual operation, it is a substantial upper limit of 98%.
[0065]
: [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 tensile cracks occur during deformation, it is impossible to obtain a good uniform elongation. The smaller the average circle equivalent diameter of carbide, uniform elongation is good, the average circle equivalent diameter of carbide is preferably 1.0μm or less. The lower limit of the average circle equivalent diameter of carbide is not particularly limited. 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.
[0066]
: Average grain size of the ferrite 10μm or less]
　In the microstructure of carburizing steel sheet according to the present embodiment, the average crystal grain size of the ferrite is required to be 10μm or less. When the average crystal grain size of the ferrite is more than 10μm, the extension of cracks during tensile deformation is promoted, it is impossible to obtain a good local elongation. As the average crystal grain size of the ferrite is small, local elongation is good, the average crystal grain size of the ferrite is preferably 8.0μm or less. The lower limit of the average crystal grain size of the ferrite is not particularly limited. However, in actual operation, it is difficult to average crystal grain size of the ferrite and 0.1 [mu] m or less, 0.1 [mu] m is substantially the lower.
[0067]
　Subsequently, the number and the ratio of the number of carbides in the microstructure, the average circle equivalent diameter of carbide, as well as method for measuring the average crystal grain size of the ferrite will be explained 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 number density of carbides, aspect ratio, average equivalent circular diameter, and measuring the average crystal grain size of the ferrite. Polishing, for example, after polishing the measurement surface using silicon carbide paper size 1500 from size 600, using the particle size and the 6μm diamond powder dispersed in a diluent or pure water such as an alcohol from 1μm liquid to, it Shiagere to the mirror surface. Corrosion, the interface between the carbide and ferrite, or, if a method of corroding the ferrite grain boundaries preferentially, the present invention is not particularly limited, for example, 3% nitric acid - may be etched with an alcohol solution as a means to corrode the grain boundary carbide and the base steel, a non-aqueous solvent electrolyte by constant potential electrolysis etching method (Fumio Kurosawa et al., Japan Institute of Metals Journal, 43,1068, (1979)) by such a base steel it may be adopted a method to leave only the carbide is removed several micrometers.
[0068]
　The number density of the carbides, using a thermal field emission scanning electron microscope (e.g., JEOL Ltd. JSM-7001F), the sheet thickness 1/4 position of the sample, 2500 [mu] m 2 range of, 20 [mu] m in plate thickness direction, rolling direction shooting range of 50μm to, image analysis software (e.g., Media Cybernetics manufactured iMage-Pro Plus) is used to measure the number of carbides in the photographed field. The same analysis at 5 fields, the average value of the five fields, 1000 .mu.m 2 to the number of carbides per.
[0069]
　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, 2500 [mu] m 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.
[0070]
　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.
[0071]
　The average crystal grain size of the ferrite, using a thermal field emission scanning electron microscope (e.g., JEOL Ltd. JSM-7001F), the sheet thickness 1/4 position of the sample, 2500 [mu] m 2 shooting range, the resulting image calculated by applying the line segment method with respect.
[0072]
　Although the microstructure having the carburizing steel sheet according to the present embodiment has been described in detail.
[0073]

　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, more preferably not more than 5.8 mm.
[0074]
　Although the carburizing steel sheet according to the present embodiment has been described in detail.
[0075]
(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.
[0076]
　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 process, according to a specific heat treatment conditions, the first a first annealing step of performing annealing treatment of the stage, and (C) with respect to the steel sheet that has passed through the first annealing step, according to a specific heat treatment conditions, the second annealing step of performing annealing treatment of the second stage , including a cooling step of cooling with reference to (D) a steel sheet after annealing in the second annealing step, a predetermined cooling condition.
　Hereinafter, the above hot rolling process, a first annealing step, the second annealing step, and, the cooling step will be described in detail.
[0077]

　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.
[0078]
　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.
[0079]
　More specifically, using a steel having a chemical composition as described prior, subjected to hot rolling by heating such steel, after hot finish rolling was completed at a temperature range below 800 ° C. or higher 920 ° C., the temperature range from the temperature at the end of rolling finish hot to cooling stop temperature by cooling below the average cooling rate 50 ° C. / s or higher 250 ° C. / s, and coiling the hot rolled steel sheet at 700 ° C. below the temperature .
[0080]
[Hot finish rolling rolling temperature: 800 ° C. or higher 920 than ℃]
　In the hot rolling step according to the present embodiment, the rolling of the hot finish rolling, it is necessary to perform at 800 ° C. or more rolling temperature. Hot finish rolling at the rolling temperature (i.e., finish rolling temperature) when low temperature and becomes less than 800 ° C., since the ferrite transformation start temperature lowers, carbides precipitated ends up coarsening, the uniform elongation to degrade. Thus, in the hot rolling step according to the present embodiment, the finish rolling temperature to 800 ° C. or higher. Finish rolling temperature is preferably 830 ° C. or higher. On the other hand, if the finish rolling temperature becomes 920 ° C. or more, becomes remarkable coarsening of austenite grains as a result of the generation site of the ferrite is reduced, leading to coarsening of ferrite grains, local elongation is degraded. Therefore, in the hot rolling step according to the present embodiment, the finish rolling temperature lower than 920 ° C.. Finish rolling temperature is preferably below 900 ° C..
[0081]
Average cooling rate after hot finish rolling: 50 ° C. / s or higher 250 ° C. / s or less]
　In the hot rolling step according to the present embodiment, after completion of the hot finish rolling, steel sheet 50 ° C. / s or higher 250 ° C. / s to cool following average cooling rate. When the average cooling rate is less than 50 ° C. / s is excessively progressed grain growth of austenite, it can not be obtained grain refining effect of ferrite, leading to deterioration of local elongation. Average cooling rate after finish rolling hot is preferably 60 ° C. / s or higher, more preferably 100 ° C. / s or higher. On the other hand, if the average cooling rate exceeds 250 ° C. / s is suppressed transformation to ferrite, the carburizing steel, it is difficult to control the crystal grain size of the ferrite to 10μm or less. Average cooling rate after finish rolling hot is preferably less than or equal to 170 ° C. / s.

claims

[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
　Ti: 0.010% or more 0.150% or less
contained, and the balance consists of Fe and impurities,
　1000 .mu.m 2 per carbide number is, is 100 or less,
　the ratio of the number of carbide aspect ratio of 2.0 or less is 10% or more of the total carbide,
　the average circle equivalent diameter of carbide, not more than 5.0 .mu.m,
　the average crystal grain size of the ferrite is 10μm or less, carburizing steel.
[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
　V: 0.0005% or more and 1.0% or less
　B: 0.01% 0.0005% or more or less
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 and was completed in 800 ° C. or higher 920 temperature range below ° C., a temperature range from the temperature at the end of rolling finish hot to cooling stop temperature was cooled at an average cooling rate of below 50 ° C. / s or higher 250 ° C. / s Te, a 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 volume fraction of nitrogen concentration in the control was annealed atmosphere to less than 25% at a rate, 1 ° C. / h or higher 100 ° C. / h by the following average heating rate, Ac defined by the following formula (1) 1 was heated to a temperature range below point, the Ac 1 a first annealing step of holding 1h or 100h following points following temperature range,
　the second The steel sheet after the annealing step, said at 1 ° 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 points than 790 ° C. below the temperature range, the Ac 1 a second annealing step of holding at a point super 790 ° C. below the temperature range 1h or 100h following
　relative steel sheet after annealing at the second annealing step, the temperature at the annealing completion in the second annealing step 550 the average cooling rate to 100 ° C. / h or less 1 ° C. / h or more at a temperature range of up to ° C.
performing cooling and cooling process,
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]

[Requested item 5]
　Between the first annealing step and the hot rolling process, the steel sheet obtained by the hot rolling process, in air, at a temperature of 40 ° C. or higher 70 ° C. or less, a holding step of holding 72h or 350h following further comprising, a manufacturing method of carburizing steel sheet according to claim 4.