Technical field
[0001]
　The present invention relates to a manufacturing method and a steel part of the steel part.
BACKGROUND
[0002]
　And industrial machinery mounted on a vehicle typified by a torque converter or the like is generally configured to include a plurality of steel parts having various shapes. And, many such steel parts are formed by pressing.
[0003]
　Such steel components, toughness increases as the C content of the steel sheet composing the steel part is lowered. Therefore, the steel parts high toughness is required, it may C content is lower steel plates are used. On the other hand, when the C content of the steel plate constituting the steel part is lowered, which lowers the fatigue strength and wear resistance of the steel part. In order to increase the fatigue strength and wear resistance without lowering the toughness, a method of curing the surface layer of the steel part is used. As a method of curing the surface layer of the steel parts, carburizing quenching is known.
[0004]
　However, when placed carburized a stamped steel plate, which may be deformed by the transformation strain occurring during quenching occurs. Therefore, in the carburization quenching, it decreases the shape fixability of the steel part. Patent Document 1, upon quenching the stamped steel plate, by cooling while constrained in a mold, a method of securing a good shape is disclosed, for using the C amount is high steel , toughness after quenching was not enough.
CITATION
Patent Document
[0005]
Patent Document 1: Japanese Patent Publication No. 10-96031
Summary of the Invention
Problems that the Invention is to Solve
[0006]
　An object of the present invention is to provide a manufacturing method and the steel part of the steel parts with a high toughness and wear resistance and fatigue strength and shape fixability.
Means for Solving the Problems
[0007]
　The present inventor has intensively studied and, as a result, have conceived to aspects of the invention described below.
[0008]
(1)
　in
　mass%,
　C: 0.0005
　~ 0.1%, Si: 0.01 ~ 2.0%, Mn: 0.05
　~ 3.0%, Al: 0.9% or
　less, P: than
　0.05%, S: 0.01% or
　less,
　Ti: 0.0 ~ 0.2%,
　Nb: 0.0 ~ 0.1%,
　Cr: 0 ~ 2%, Mo: 0.0 ~ 0
　% .2, B: 0.000 ~ 0.005%, and
　balance: Fe and impurities,
in having a chemical composition expressed,
　the area fraction of ferrite is 70% or more,
in a steel structure represented the steel sheet having been heated in the carburizing atmosphere, forming a carburized layer on the surface of the steel sheet,
　molding the steel sheet using a mold, performing quenching of the steel sheet while accommodated in the mold , together to transform the carburized layer to martensite, represented by ferrite and 50% or more in the area fraction of the inner side than the carburized layer of the steel sheet A step of the tissue
　manufacturing method of the steel parts, characterized in that it comprises a.
[0009]
(2)
　in the step of forming the carburized layer on the surface of the steel sheet, a manufacturing method of the steel part according to (1) to soaking the steel sheet at a temperature above 820 ° C..
[0010]
(3)
　in the step of forming the carburized layer on the surface of the steel sheet, characterized by soaking 1 hour or more the steel plate (1) or (2) The method of producing a steel part according to.
[0011]
(4)
　before the step of forming a carburized layer on the surface of the steel sheet,
　and further comprising the step of processing the steel sheet less than or 20% or more of equivalent strain 5% (1) (3 method for producing a steel part according to any one of).
[0012]
(5)
　by
　mass%,
　C: 0.0005
　~ 0.1%, Si: 0.01 ~ 2.0%, Mn: 0.05
　~ 3.0%, Al: 0.9% or
　less, P: than
　0.05%, S: 0.01% or
　less,
　Ti: 0.0 ~ 0.2%,
　Nb: 0.0 ~ 0.1%,
　Cr: 0 ~ 2%, Mo: 0.0 ~ 0
　% .2, B: 0.000 ~ 0.005%, and
　balance: Fe and impurities,
have in chemical composition represented,
the area fraction,
　the ferrite: 50% or more,
in the steel structure represented and the base material having,
　a carburized layer on the surface of the base material consists of martensitic
　steel part, characterized in that it comprises a.
[0013]
(6)
　carburized layer comprising the martensite containing C of from 0.5 to 0.9 percent by mass%,
　t / 4 the thickness of the steel part from the steel part of the surface of the case of the t in the range of ~ 3t / 4, steel part according to, characterized in that the ferrite grains of the total area fraction having 200μm or more particle size equivalent circle diameter is 5% or less (5).
[0014]
　Steel component according to the effective case depth is characterized by a 0.05 ~ 0.5 mm (5) or (6).
Effect of the invention
[0015]
　According to the manufacturing method of the steel part of the present invention can be produced steel part having a high toughness and wear resistance and fatigue strength and shape fixability. Further, the steel part of the present invention has a high toughness and wear resistance fatigue strength and shape fixability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016]
[1] Figure 1 was used in Examples is a plan view and a side view of a steel sheet after stamping.
[2] 2 was prepared in Example, a plan view and a side view of the steel part.
DESCRIPTION OF THE INVENTION
[0017]
　Hereinafter, detailed embodiments of the present invention. In the following description, a unit of content of each element contained in the steel sheet used in the steel part and its preparation "%", unless stated otherwise refers to "mass%".
[0018]
[First Embodiment]
[Production method of steel parts]
　method for producing a steel part according to the first embodiment, the step of forming a carburized layer of austenite on the surface of the steel sheet by heating the steel sheet in a carburizing atmosphere (hereinafter, referred to as "carburization step") and, by forming a steel plate by using a mold in a state in which austenite exists, by performing quenching of the steel sheet while accommodated in the mold, together to transform the austenite to martensite , the step of the steel structure of ferrite in an area fraction of the inner side than the carburized layer of the steel sheet is represented by 50% or more (hereinafter referred to as "hot stamp process") and a.
[0019]
[Chemical composition and steel structure of the steel sheet]
　First, a description will be given chemical composition of the steel sheet used in the production method of the steel component according to the present embodiment. Steel sheet, by mass%, C: 0.0005 ~ 0.1% , Si: 0.01 ~ 2.0%, Mn: 0.05 ~ 3.0%, Al: 0.9% or less, P: 0.05% or less, S: 0.01% or less, Ti: 0.0 ~ 0.2%, Nb: 0.0 ~ 0.1%, Cr: 0 ~ 2%, Mo: 0.0 ~ 0 .2%, B: 0.000 ~ 0.005 %, and the balance: has a chemical composition represented by Fe and impurities. As the impurity, those included in raw materials such as ores and scrap, intended to be included in the manufacturing process, is exemplified.
[0020]
(C: 0.0005 ~ 0.1%)
　C affects the toughness and tensile strength. If the C content is too high, toughness because martensite is cured resulting drops steel inside during quenching. On the other hand, too low content of C leads to increase in cost. Therefore, C content of the steel sheet is less from 0.0005 to 0.1%.
[0021]
(Si: 0.01 ~
　2.0%) Si increases the strength by solid solution strengthening. The Si content of 2.0 percent, excessively increases and Ac3 transformation point, the carburized layer becomes difficult to austenite is not obtained martensite upon quenching. Therefore, Si content is 2.0% or less. On the other hand, it leads to increased costs too reduces the Si content. Therefore, Si content is 0.01 to 2.0%.
[0022]
(Mn: 0.05 ~
　3.0%) Mn is an element that increases the strength by solid solution strengthening. Further, since it is an element improving the hardenability, the Mn content of 3.0 percent, even C content is 0.1% or less, the steel sheet internally when quenching becomes martensite toughness degrade. Therefore, Mn content is 3.0% or less. On the other hand, cost increases greatly when the Mn content is reduced to less than 0.05%. Therefore, Mn content is 0.05% or more.
[0023]
(Al: 0.9% or
　less) Al is not an essential element, is contained as an impurity, for example, steel sheet. It may also be optionally added because it is an element raising the strength by solid solution strengthening. Since the austenite grain size becomes hardenability to fines carburized layer of AlN formation is reduced, when not require solid solution strengthening, Al content is better as low as possible. Especially in the Al content of 0.9 percent, excessively increases and Ac3 transformation point, it becomes difficult to austenite the carburized layer. Therefore, Al content is 0.9% or less. However, costly in reducing the Al content, an attempt to reduce to less than 0.002%, the cost increases considerably. Therefore, Al content may be 0.002% or more.
[0024]
(P: 0.05% or less)
　P is not an essential element, is contained as an impurity, for example, steel sheet. From the viewpoint of the toughness of carburized layer after quenching, P content is better as low as possible. Especially in the P content of 0.05 percent, a significant decrease in toughness. Accordingly, P content is 0.05% or less. However, costly in reducing the P content, an attempt to reduce to less than 0.001%, the cost remarkably increases. Therefore, P content may be 0.001% or more.
[0025]
(S: 0.01% or less)
　S is not an essential element, is contained as an impurity, for example, steel sheet. Since forms MnS to lower the toughness of carburized layer after quenching and, S content is preferably as low as possible. Especially in S content is 0.01 greater than the significant reduction in toughness. Thus, S content is 0.01% or less. However, the reduction of the S content is costly, an attempt to reduce to less than 0.0005%, the cost remarkably increases. Therefore, S content may be 0.0005% or more.
[0026]
　Ti, Nb, Cr, Mo, B is not an essential element, a good optional elements be contained appropriately limit the predetermined amount on the steel sheet.
[0027]
(Ti: 0.0 ~
　0.2%) Ti contributes to refinement of ferrite grains, used to obtain a predetermined steel structure. Moreover, Ti increases the strength by precipitation strengthening. Therefore, Ti may be contained. However, Ti content is 0.2 percent, degrades the ductility of the steel sheet before the carburization process. Therefore, Ti content is 0.2% or less.
[0028]
(Nb: 0.0 ~
　0.1%) Nb contributes to refinement of ferrite grains, used to obtain a predetermined steel structure. Further, Nb increases the strength by precipitation strengthening. Therefore, Nb may be contained. However, Nb content is 0.1 percent, degrades the ductility of the steel sheet before the carburization process. Therefore, Nb content is 0.1% or less.
[0029]
(Cr: 0 ~ 2%) Cr
　is because it is a solid solution strengthening element, it may be appropriately added for strength increase. However, since it is an element improving the hardenability, the Cr content exceeds 2%, even C content is 0.1% or less, the steel sheet internally when quenching becomes martensite, the toughness make. Therefore, Cr content is 2% or less.
[0030]
(Mo: 0.0 ~
　0.2%) Mo is because it is an element improving the hardenability, the Mo content of 0.2 percent, C content be not more than 0.1% , steel internally when quenching becomes martensite degrades the toughness. Therefore, Mo content is 0.2% or less.
[0031]
(B: 0.000 ~ 0.005%)
　B enhances hardenability of the steel sheet, an element used for controlling the steel structure. Thus, B may be contained. However, B content is 0.005%, the effect is saturated. Therefore, B content is 0.005% or less.
[0032]
　Further, the steel sheet is preferably an area fraction of ferrite has a steel structure represented by 70% or more. This, bainite, martensite, the hard tissue of pearlite and the like there are many, formability of the steel sheet in order to degrade. Therefore, the area fraction of ferrite is preferably 70% or more.
[0033]
　Thickness t of the steel sheet is not particularly limited. It is preferable that the tensile strength of the steel sheet is less than 630 MPa, more preferably at most 590 MPa, and more preferably less 440 MPa. In this embodiment, the wear resistance and fatigue strength of steel parts to be produced, is ensured by the carburized layer of the surface layer portion. Therefore, by suppressing the strength of the steel sheet itself used in the manufacture of steel components, it is possible to increase the toughness of the steel parts produced. From the viewpoint of formability, strength is preferably low.
[0034]
　Further, the steel sheet used in the manufacture of steel parts are parts high toughness of the steel part is required, it is preferable that the non-plastic working in equivalent strain in the range of less than 5% or more 20%. This processed portion in equivalent strain in the range of 5% to less than 20% in the steel sheet, when the soaking in carburization step, crystal grains are coarsened, because the toughness of the steel parts produced is reduced is there. The portion where high toughness is not required, may be plastic working at strain corresponds to less than 5% to 20%.
[0035]
　Next, a method for manufacturing a steel component according to the present embodiment.
[0036]
[Carburization step]
　In this step, together with carrying out the carburizing the steel sheet having the above chemical composition and steel structure, the steel structure of the surface layer portion of the steel sheet to austenite. The "carburizing" in the present invention also includes carbonitriding. Thus, the carburized layer formed are also included carbonitrided layer. The carburization, for example, can be applied gas carburizing, vacuum carburizing.
[0037]
　Soaking temperature of the steel sheet in the carburization step (carburization temperature) is, for example, 820 ~ 1100 ° C.. Steel sheet after the carburizing is quenched with is directly pressed at mold. Therefore, in this carburizing step, austenitizing the steel structure of the surface layer portion which has entered and diffusion of carbon by carburization. For example, if the soaking temperature is 820 ° C. or higher, the steel structure of the surface layer portion of the steel sheet to austenite. Note that the soaking temperature is more preferably 840 ° C. or higher. On the other hand, the carburization temperature is 1100 ° C. than in some cases crystal grains are coarsened, toughness is reduced. Accordingly, the preferred upper limit of the soaking temperature is 1100 ° C.. The carbon potential of the carburizing atmosphere, C content of the surface layer portion is adjusted, for example, 0.9% or less than 0.5%.
[0038]
　Soaking time at the soaking temperature is, for example, 1.0 hours or more, or less 5.0 hours. If the soaking time is too short, the effective hardened layer depth is predetermined depth of the carburized layer, for example less likely than the 0.05 mm. Therefore, it is preferable that the soaking time is not less than 1.0 hours, more preferably at least 1.5 hours. On the other hand, if the soaking time is too long, too large effective case depth of the carburized layer, the predetermined depth, for example a depth of more than 0.5 mm. Thus, for example, it is preferable soaking time is less than 5.0 hours.
[0039]
　The temperature of austenitization of the steel structure of the steel sheet is started becomes lower as the C content increases. Therefore, the soaking temperature in this carburization step is lower than the temperature at which austenitization steel structure of the steel sheet is started, gradually becomes high C content from a surface of the steel sheet by intrusion of carbon, with it the temperature at which the austenite to start is getting low. In this case, in a portion where the temperature at which austenite begins is lower than the soaking temperature, austenite begins. Therefore, in this case, with gradually becomes thicker the thickness of the carburized layer with the passage of time, with an increase in the thickness of the carburized layer, the thickness of the austenitized portion also gradually thicker from the surface. In contrast, the soaking temperature in this carburization step is higher than the temperature at which austenitization steel structure of the steel sheet is started, the steel structure is an austenite of a steel sheet, and penetrates the carbon to austenitization portion go.
[0040]
Hot stamping Step]
　In this step, after the carburization process, while the presence of austenite in the steel sheet, for example, before the temperature of the steel sheet is 800 ° C. or less, hot stamping steel plate. It is to be noted that between carburizing process of hot stamping, not quenching the steel plate. At the end of the carburization step, the steel sheet is soaked at a temperature above 820 ° C., the steel structure of the surface layer portion of the steel sheet is in the austenite. Furthermore, C = amount of the surface layer portion of the steel sheet, for example, it has become 0.9% or less than 0.5%. Therefore, when implementing a hot stamping against the steel, austenite exists in the surface layer of the steel sheet becomes martensite. The inner than the carburized layer of the steel sheet, ferrite is steel structure represented by more than 50% area fraction. In the hot stamping process, for example, a mold cooling medium is circulated. Then, the steel sheet in a state in which austenite exists, quenching and heat extraction while press-molding using this mold.
[0041]
　As described above, in the manufacturing method of the steel part according to the present embodiment, lower steel plate C content, with respect to steel sheets for example C content of from 0.005 to 0.1 percent, the surface layer portion at a temperature of austenitizing after soaking, with the resulting austenite is present in the soaking, to implement the pressing. Therefore, possible C content of the base metal steel parts produced, low C content, for example, 0.005 to 0.1%. Further, the steel structure of the base material, the area fraction of ferrite can be 50% or more. Thus, it can be produced steel part having a high toughness. In addition, the carburization step and the hot stamping process, since the carburized layer formed of martensite in the surface layer of the steel part is formed, can be hard to compare surface portion therein (i.e. matrix). Thus, the carburized layer formed of martensite present in the surface layer of the steel part can be secured abrasion resistance and fatigue strength. Further, by forming a steel plate by using a mold, for hardening while videos steel sheet to the mold, can suppress deformation of the steel part in the hardening. Thus, it can be produced steel part having excellent shape fixability. More As, according to the method of manufacturing a steel component according to, it can be produced steel part having high toughness and wear resistance and fatigue strength and shape fixability.
[0042]
　Incidentally, the steel sheet, as long as that is not plastically processed in a considerable portion of the range of 5% to less than 20% by strain required to have high toughness, in the part, the ferrite grains become coarse by abnormal grain growth things (e.g., the particle size is equal to or greater than 200μm particles having a circle equivalent diameter) can be suppressed. Therefore, when the remaining steel components without ferrite grains austenitization present in the steel sheet, the portion where a high toughness is required, it is possible to suppress the deterioration of the toughness due to coarsening ferrite grains. In the case where ferrite grains are all austenite in soaking the steel sheet, since the ferrite grains produced from austenite does not coarsened, there is no decrease in toughness due to coarsening of the ferrite grains.
[0043]
[Steel Parts]
　By the above production method, a steel component according to an embodiment of the present invention is manufactured. Steel component according to the present embodiment includes a base material, and a carburized layer comprising a martensite on the surface of the base material.
[0044]
　C content of the base metal is 0.0005 to 0.1 percent, its chemical composition is the same as the chemical composition of the steel sheet. C content of the base metal, in the mid-thickness position of the steel parts, can be analyzed by EPMA. Steel structure of the base material, containing ferrite. In particular, the steel structure of the base material is preferably an area fraction of ferrite is 50% or more. Portion other than the ferrite of the steel structure of the base material is, for example, pearlite and bainite.
[0045]
　In the base material, the total area fraction of ferrite grains having a particle size of more than 200μm particles having a circle equivalent diameter (hereinafter, referred to as coarse ferrite ratio) is preferably 5% or less. Thus, the steel part manufactured by the manufacturing method of this embodiment, coarse ferrite grains by abnormal grain growth (ferrite grains having a particle size of more than 200μm equivalent circle diameter) that is less preferred.
[0046]
　Incidentally, coarse ferrite ratio of the base material is measured by the following method. Here, we define the thickness of the steel part and t (mm). From any position ranging from the surface of the steel part of t / 4 ~ 3t / 4, obtaining a sample. The surface (observation surface) of the sample is etched with nital. Any 5 field in the observation plane corroded (the field area = 2mm × 2mm), was observed with 100 times optical microscope to produce a photographic image. In each field of the photographic images obtained, to identify the ferrite grains. Determining a circle equivalent diameter of each ferrite grain was identified. Circle equivalent diameter ([mu] m) are each ferrite grain is defined as the diameter of a circle when converted to a circle having the same area. After determining the equivalent circle diameter of each ferrite grain, equivalent circle diameter to identify the more coarse ferrite grains 200 [mu] m. Seeking total area of ​​coarse ferrite grains in each field of view is divided by the total area of ​​all the measured field of view, determining the coarse ferrite ratio (area%). Specific ferrite, the calculation of the equivalent circle diameter, and coarse ferrite ratio, it can be performed using a general-purpose image processing applications.
[0047]
　Vickers hardness of the base material (HV) is preferably 250 or less. This is because the toughness is too high hardness is deteriorated. The lower limit of the Vickers hardness of the base material (HV) is preferably 80, more preferably 150. This is the strength of the mechanical component is determined depending on a hardness of the base material rather than a surface layer. Vickers hardness of the base material (HV) is obtained by the following method. Selecting any three points in the plate thickness center of the steel part. At a selected location, implement the Vickers hardness test according to JISZ 2244 (2009). At this time, the test force and 2.942N (0.3kgf). The average value of the obtained hardness is defined as the Vickers hardness of the base material (HV).
[0048]
　C content of the carburized layer is, for example, 0.5% or more and 0.9% or less. C content of the carburized layer can be analyzed by EPMA (electron probe microanalyzer). Steel structure of the carburized layer is made of martensite.
[0049]
　The lower limit of the Vickers hardness (HV) of the carburized layer is preferably 500, more preferably 600. This is to ensure wear resistance, fatigue strength. Vickers hardness of the carburized layer (HV) can be obtained by the following method. In the carburized layer portion of 0.05mm depth position from the surface of the steel component, select any three points. At a selected location, implement the Vickers hardness test according to JISZ 2244 (2009). At this time, the test force and 2.942N (0.3kgf). The average value of the obtained hardness is defined as the Vickers hardness of the carburized layer (HV).
[0050]
　As described above, C content of the base metal of the steel part according to the present embodiment is, for example, 0.0005 to 0.1%. Further, in the base material, the area fraction of ferrite is 50% or more, coarse ferrite ratio is 5% or less, for example. Therefore, since the base material has a high toughness, the steel part having such a base material, has a high toughness. Further, the surface of the base of the steel parts, carburized layer is present. The carburized layer consists martensite, has excellent hardness. Therefore, the steel part has a wear resistance and fatigue strength excellent. The steel part according to the present embodiment, while forming the steel sheet by using a mold, is prepared by quenching while videos steel sheet to the mold. Therefore, the steel component according to the present embodiment has an excellent shape fixability. Thus, the steel component according to the present embodiment has a high toughness and wear resistance fatigue strength and shape fixability.
[0051]
[Second Embodiment]
　Next, a method for manufacturing a steel component according to a second embodiment of the present invention. Method for producing a steel part according to the second embodiment is different from the first embodiment, at the stage before the carburization step, the step of introducing a distortion processing the steel sheet (hereinafter, "strain introduction step" further comprising a referred to as). The other steps (carburization step, hot stamping step) is the same as the first embodiment.
[0052]
[Strain introduction step]
　in strain introduction step of the steel sheet having the above chemical composition and the steel structure, with respect to portions where high toughness while being molded to the steel product is required, less than 5% or 20% or more equivalent strain in carrying out the process. Note that the method of plastic working is not intended to be limited, for example, it may be a press working, or rolling.
[0053]
　Plastic working portion as equivalent strain is less than 5% or more 20% of the steel sheet, is heated in the carburizing step, the abnormal grain growth of crystal grains occurs. On the other hand, plastic working portion in strain equivalent at least 5% or less than 20%, be heated in the carburizing step, the abnormal grain growth of crystal grains is less likely to occur. Therefore, in the case of processing the steel plate in the previous stage than carburization step (prefabricated), to the portion of high toughness while being molded to the steel product is required, less than 5% or equivalent more than 20% processed in strain. Thus, abnormal grain growth of ferrite grains is suppressed present in the steel sheet, it is possible to suppress the deterioration of the toughness of the produced steel part.
Example
[0054]
　Next, a description will be given of an embodiment of the present invention. Conditions in examples are conditions employed for confirming the workability and effects of the present invention, the present invention is not limited to these conditions Example. The present invention without departing from the spirit and intended as long to achieve the object, may employ various conditions.
[0055]
　In this embodiment, using a steel species having the chemical compositions shown in Table 1. Steel type A ~ D is in the range C content of the present invention, steel type E is outside the scope of the present invention. Tensile strength, at room temperature, was measured by a tensile test in the air. Any steels also ferrite fraction was 70% or more.
[0056]
[Table 1]

[0057]
　The steel sheet of the steel types of plate thickness 3.2mm stamped to, to produce a disc-shaped steel plates 10 shown in FIG. As shown in FIG. 1, the steel plate 10 is a circular plate having a through hole 20 in the center. The outer diameter of the steel plate 10 is 200 mm, an inner diameter of 50 mm.
[0058]
　Then, using the manufacturing method of test No. 1 to 15 shown in Table 2, were produced steel part 30 from the steel plate 10. Figure 2 shows a steel part 30 produced. Steel part 30 has a bottom 50 on the periphery, has a convex portion 40 in the central portion. The outer diameter of the convex portion 40 is 120 mm, the height from the bottom 50 of the protrusion 40 is 3 mm.
[0059]
[Table 2]

[0060]
　Table 2 in the "Step" is described the manufacturing process of each test number. "A" in the "step" column, then soaking to carburizing the steel plate 10 in a carburizing atmosphere, then the steel by quenching with press molding using a mold in a state where there is austenite steel 10 indicates that the method of manufacturing the component 30. "B" in the "step" column, the steel plate 10 by pressing and molding into the shape shown in FIG. 2, a molded steel sheet 10 is soaking to carburizing in a carburizing atmosphere, followed by oil quenched indicates a process of producing a steel part 30. That is, in the step "A", the convex portion 40 of the steel component 30, after soaking (carburization step) in a carburizing atmosphere, is one that is molded with quenching. On the other hand, in step "B", the convex portion 40 of the steel part 30 is for soaking in a carburizing atmosphere than (carburizing step) and subsequent quenching is formed by pressing in the preceding stage.
[0061]
　Column "equivalent strain rate in the pre-processing step" indicates a maximum value of equivalent strain was introduced into the steel sheet in advance processing step. Prefabrication process, at the stage before than soaking under carburizing atmosphere, a process that introduces distortion in the steel plate 10. In this pre-processing step, before punching in a disc shape, by cold rolling, introducing strain to the steel sheet 10, simulating the press-forming. "Soaking temperature" column, soaking temperature under carburizing atmosphere (carburization temperature, the unit is ° C.) shows a. "Soaking time" column indicates the soaking time of the steel plate 10 under carburizing atmosphere (hr).
[0062]
[Evaluation Test]
[microstructure observation test]
　carburized layer of the steel parts 30 of the respective test numbers were produced and steel structure of the base material were observed in the following manner. The steel part 30 is cut in the radial direction, the sample was filled resin includes a cutting surface of the bottom portion 50 was polished cut surface was polished was cut surface to nital corrosion. Of the cut surface was corroded, and the carburized layer of 0.05mm depth position from the surface in the surface layer, and a base material of the plate thickness center part, and observed at 100 to 500 times the optical microscope, the steel structure It was identified. Further, the surface layer part and mid-thickness portion, identified the C content to implement the EPMA.
[0063]
[Vickers hardness test]
　Among the steel part of each test number, Vickers hardness of the carburized layer (0.05 mm depth from the surface layer portion) and the base metal (thickness central part) to (HV), determined by the method described above It was. Note that Vickers hardness (HV) was determined using the bottom 50.
[0064]
[Coarse ferrite Rate
　of the steel member 30 of each test number, at the bottom 50, a sample was taken for microstructure observation from the surface at t / 4 ~ 3t / 4 position. Using samples taken to determine the coarse ferrite ratio by the methods described above.
[0065]
[Shape fixability]
　column "shape fixability" shows the evaluation results of the shape fixability of the steel parts 30. The symbol "" indicates that high shape fixability, "×" indicates that a lower shape fixability. Shape fixability was evaluated by the flatness of the projections 40 and the bottom 50 of the steel parts 30. Flatness was determined from the shape of the convex portion 40 and the bottom 50 of each test number was measured using a three-dimensional shape measuring instrument. Even one of the flatness of the projections 40 and the bottom 50 when it is 0.2mm or less was evaluated to be high shape fixability (indicated by "○" mark in Table 2). On the other hand, if any of the flatness of the protrusions 40 and bottom 50 exceeds 0.2 mm, it was evaluated to be low shape fixability (in Table 2 indicated by "×" symbols).
[0066]
[Toughness]
　column "toughness" indicates the evaluation results of the toughness of the steel parts 30. "◎" indicates that the toughness is high, "○" indicates that the toughness is slightly lower, "×" indicates that the toughness is low. However, evaluation of the "○" is also included in the examples. Toughness was evaluated based on the results of Charpy impact test.
[0067]
　Charpy impact test specimens were taken from the bottom 50 of the steel component 30 which is manufactured by using the collected test piece in conformity with JIS Z2242 (2005), was performed Charpy impact test at -20 ° C. and room temperature. After the test, the penetration of the cracks in the specimen is determined whether or not confirmed. If the cracks in the both -20 ° C. and room temperature does not penetrate, it is determined that the toughness is high (indicated by "◎" mark in Table 2). On the other hand, in both -20 ° C. and room temperature is also, (indicated by "×" mark in Table 2) to cracking in the test piece is to specimen bisected through after the test, the toughness was determined to be low. Further, in the -20 ° C. through (indicated by "○" mark in Table 2) If no penetration, toughness is determined slightly lower than normal temperature.
[0068]
[Test results]
　Next, a description will be given test result.
[0069]
　Test Nos. 1 to 5 and 9 to 11, C content of the steel sheet 10 and the manufacturing process, both of which are examples within the scope of the present invention. In particular, test numbers 1, 2, 4, and 5, and the equivalent strain rate and soaking temperature and soaking time in the pre-processing step is an example in the more preferred range. Steel part 30 of these test numbers 1, 2, 4, and 5, both the carburized layer formed of martensite consists of a base material containing ferrite, is 0.5 ~ C content of the carburized layer 0 in the range of .9%, C content of the base metal was less than 0.1%. Further, coarse ferrite ratio of the base material of the steel parts 30 of these test numbers 1, 2, 4, and 5, was 5% or less. Moreover, the Vickers hardness of the base material (HV) is lower than the carburized layer existing in the surface layer portion, and was 250 or less. The penetration of the cracks in the test piece in the Charpy impact test is not observed, showing excellent toughness. Further, the hardness of the carburized layer is 500 or more, had a high hardness. Furthermore, since carrying out the hot stamp process after the carburization step, the steel part 30 of these test numbers 1, 2, 4, and 5 had excellent shape fixability.
[0070]
　Test No. 1, 2, 4, and 5, a test numbers 9 and is an example of the soaking time is less than 1 hour, the test numbers 10 and an example below the soaking temperature is 820 ° C., soaking time is less than 1 hour in and when compared with test No. 11 is an example of less than the soaking temperature is 820 ° C., high carbon concentration in the surface layer portion, thick effective hardened layer, a high surface portion hardness. Further, in Test No. 10 and 11, low carbon concentration in the surface, but has a lower area ratio of martensite for sufficient austenitization, in Test No. 1, 2, 4, and 5, the area ratio of martensite in the surface layer portion There was obtained 100% of the organization.
[0071]
　Coarse ferrite ratio of the steel parts 30 of these test numbers 1, 2, 4, and 5, compared to the pre-processing steps equivalent strain rate of 5% or less than the steel part of the test No. 3 is an example not more than 20% by to, is low.
[0072]
　Test Nos. 6-8, step of the manufacturing method is an example that does not satisfy the requirements of the present invention. It is not good shape fixability because they are not hot stamp. Also, Test No. 7, equivalent strain rate in the pre-processing step, an example which does not satisfy the conditions is less than 5% or 20% or more. In Test No. 7, coarse ferrite is generated in the plate thickness center part, toughness is degraded.
[0073]
　Test No. 12 to 15, C content of the steel plate 10 is above 0.1%, an example which does not satisfy the requirements of the present invention. High C content of mid-thickness portion, since the ferrite fraction of 50% or less, have poor toughness.
[0074]
　Test No. 13, together with the C content of the steel plate 10 is greater than 0.1%, a soaking time is less than 1 hour, satisfies the C content and the soaking time and the requirement of the present invention in the steel sheet 10 it is an example that does not.
[0075]
　Test No. 14, together with the C content of the steel plate 10 is greater than 0.1%, a soaking temperature of less than 820 ° C., the C content and the soaking temperature of the steel plate 10 does not meet the requirements of the present invention it is an example.
[0076]
　Test No. 15 is more than 0.1% of C content of the steel sheet 10, a soaking temperature of less than 820 ° C., a soaking time is less than 1 hour, the C content of the steel plate 10 and the soaking temperature and soaking time are examples that do not satisfy the requirements of the present invention.
[0077]
　It has been described an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the embodiments described above, it can be implemented by changing the above-described embodiments without departing from the scope and spirit thereof as appropriate.
Industrial Applicability
[0078]
　The present invention is, for example, are available in industry in which press forming of the steel component.

The scope of the claims
[Requested item 1]
　By
　mass%,
　C: 0.0005
　~ 0.1%, Si: 0.01 ~ 2.0%, Mn: 0.05
　~ 3.0%, Al: 0.9% or
　less, P: 0.05 % or
　less, S: 0.01% or
　less,
　Ti: 0.0 ~
　0.2%, Nb: 0.0 ~ 0.1%,
　Cr: 0 ~ 2%, Mo: 0.0 ~ 0.2% ,
　B: 0.000 ~ 0.005%, and
　balance: Fe and impurities,
having in chemical composition represented,
　the area fraction of ferrite is 70% or more,
in a steel sheet having a steel structure represented by heating in a carburizing atmosphere, forming a carburized layer on the surface of the steel sheet,
　by performing molding the steel sheet using a die, quenching of the steel sheet remains housed in the mold, carburized layer to the causes is transformed into martensite, the steel structure of ferrite is represented by more than 50% area fraction inside than the carburized layer of the steel sheet A step,
　method for producing a steel part, characterized in that it comprises a.
[Requested item 2]
　Wherein in the step of forming a carburized layer on the surface of the steel sheet, a manufacturing method of a steel component according to claim 1, characterized in that the soaking of the steel sheet at 820 ° C. or higher.
[Requested item 3]
　Wherein in the step of forming a carburized layer on the surface of the steel sheet, a manufacturing method of a steel component according to claim 1 or 2, characterized in that soaking least 1 hour the steel sheet.
[Requested item 4]
　Before the step of forming a carburized layer on the surface of the steel sheet,
　any one of claims 1 to 3, further comprising the step of processing said steel plate at a strain corresponding more than 5% or less than 20% method for producing a steel part according to claim.
[Requested item 5]
　By
　mass%,
　C: 0.0005
　~ 0.1%, Si: 0.01 ~ 2.0%, Mn: 0.05
　~ 3.0%, Al: 0.9% or
　less, P: 0.05 % or
　less, S: 0.01% or
　less,
　Ti: 0.0 ~
　0.2%, Nb: 0.0 ~ 0.1%,
　Cr: 0 ~ 2%, Mo: 0.0 ~ 0.2% ,
　B: 0.000 ~ 0.005%, and
　balance: Fe and impurities,
having in chemical composition represented,
the area fraction,
　the ferrite: 50% or more,
the mother having in steel structure represented wood and,
　a carburized layer on the surface of the base material consists of martensitic
　steel part, characterized in that it comprises a.
[Requested item 6]
　The carburized layer formed of martensite, containing C of 0.5 to 0.9% by mass%,
　t / 4 from the steel part of the surface in the case where the thickness of the steel part was t ~ 3t / in 4 of the range, a steel component according to claim 5, wherein the ferrite grains of the total area fraction having a particle size of more than 200μm particles having a circle equivalent diameter is not more than 5%.
[Requested item 7]
　Steel component according to claim 5 or 6, wherein the effective hardened layer depth is 0.05 ~ 0.5 mm.