Technical field
[0001]
　The present invention is related to hot-rolled steel is steel parts and the material.
　The present application, on March 9, 2015, claiming priority based on Japanese Patent Application No. 2015-045855, filed in Japan, the contents of which are incorporated here.
Background technique
[0002]
　Engine parts for parts and foot around the automobile can be obtained by performing the steel was formed by hot forging, heat treatment such as optionally quenching tempering the (temper). Referred to parts refining was performed with tempering parts, the parts that were not tempered called non-heat treated component. In either case, the mechanical properties required for components to be applied is ensured. Recently, part from the viewpoint of economic efficiency in the manufacturing process, is omitted refining, i.e., non-heat treated component is most popular.
[0003]
　A case for automotive engine parts, connecting rod (hereinafter referred to as connecting rod) are exemplified. This component, when converting the piston reciprocating motion to rotary motion by the crankshaft in the engine, a part for transmitting power. Connecting rod is composed of a cap portion and the rod portion are assembled to the crankshaft by the eccentric portion of the designated crankshaft and pin fastened sandwiched between the cap portion and the rod portion, the pin portion and the rotational sliding It transmits power by moving a mechanism.
[0004]
　In order to improve the consistency between the cap portion and the rod portion, when producing the conventional connecting rod, it is necessary to smooth the mating surfaces of the cap portion and the rod portion. Further, when the fastening sandwich the pin portion with the cap portion and the rod portion, it is necessary to align the cap portion and the rod portion, when producing the conventional connecting rod, the cap portion and the rod portion it is necessary to provide a mating surface irregularities for positioning between. The mating surfaces were smooth, and the machining process for providing irregularities to a mating surface increases the connecting rod manufacturing time and manufacturing cost. To omit the machining step for providing the irregularities, in recent years, fracture splitting type connecting rod is often employed.
[0005]
　The fracture splitting type connecting rod, by performing hot forging the steel, after which the cap portion and the rod portion is formed into a shape that integrates notch in a portion corresponding to the boundary between the cap portion and the rod portion put, is obtained by method of fracture splitting connecting rods. Mating surface of the cap portion and a rod portion obtained by this method was obtained by fracture separation, a fracture surface having unevenness. By fitting the irregularities between the brittle fracture fracture, it is possible to perform alignment when assembling the connecting rod to the crankshaft. Thus, in the manufacture of fracture splitting type connecting rod, also machined to enhance the integrity of the mating surfaces, it can be omitted also machined to provide the mating surface irregularities for positioning. Therefore, fracture splitting type connecting rod is a part of the processing steps can be significantly reduced, the economic efficiency during component manufacturing can be greatly improved.
[0006]
　As the material of the fracture splitting-type connecting rod, the are popular in Europe is the C70S6 of DIN standard. C70S6 is a high carbon microalloyed steel containing C of 0.7 mass%, in order to suppress the dimensional change during fracture splitting, the metal structure becomes ductile and toughness low pearlite structure. C70S6 has excellent fracture splitting property because the amount of plastic deformation fracture surface near at break is small. The fracture separation of the steel, which is an index for evaluating the fitting performance of the fracture surfaces between the obtained steel by breaking machining steel. Small amount of deformation of the fracture near a large brittle fracture area ratio of the fracture surface, the steel chipping generation amount at break processing is small, fracture splitting resistance is determined to be good. However, C70S6 the ferrite carbon microalloyed steel in a steel for conventional connecting rod - so tissue compared to pearlite structure is coarse, yield ratio (yield strength / tensile strength) is low, a high buckling the high-strength connecting rod strength is required can not be applied.
[0007]
　To increase the yield ratio of the steel, it is necessary to increase the carbon content of the steel lower SomosomoSae, the ferrite fraction of the steel. However, when increasing the ferrite fraction of the steel, and improves the ductility of the steel, since plastic deformation of fracture near the time of fracture splitting increases, the connecting rod sliding portion which is fastened to the pin portion of the crankshaft increased deformation is, roundness of a connecting rod sliding portion component performance issues may deteriorate.
[0008]
　In recent years, along with the engine output increases due to the spread of high-power diesel engines or turbo engine, preventing displacement of the cap portion and the rod portion of the connecting rod, i.e., there is a need for such fittability improved and the fastening force improved. Among them, for the fitting property improved, so as to increase the unevenness of were broken separating surface, it is effective to control the steel structure.
[0009]
　Suitable steel material fracture splitting type connecting rod of high strength, some non-heat treated steels have been proposed. Patent Document 1 and Patent Document 2, the embrittlement elements such as Si or P was added in large amounts in the steel, describes a technique for improving the fracture splitting of a steel by lowering the ductility and toughness of the steel there. Patent Document 3 and Patent Document 4, by utilizing the precipitation strengthening of second phase particles, techniques for improving the fracture splitting of a steel by reducing the ductility and toughness of the ferrite of the steel is described. Further, Patent Documents 5-7, a technique for improving the fracture splitting of a steel by controlling the form of Mn sulfides in the steel is described.
[0010]
　These techniques, while reducing the amount of deformation of the portion fractured separated and brittle material. Therefore, the steel obtained by these techniques, chipping occurs when allowed at break separation, or fitting the fracture faces together. When chipping of fracture surface occurs, occurs positional displacement of the fitting portion, a problem occurs such can not be accurately fitted. In particular, increasing the irregularity of the fracture surface, so the frequency is increased to chipping and cracking occurs at break, and increased unevenness of the fracture surface, simultaneously achievable steels both the prevention of chipping and cracks at break It has been demanded. Missing, the solution cracks prevention, and to reduce the segregation of V as described in Patent Document 8. Incidentally, V is a chemical component added for the purpose of strengthening.
[0011]
　However, also lack other segregation and V, are contributing cause cracks. In fact, if the unevenness of the fracture surface is excessively large, chipping tends to occur frequently crack increases. This is when the pulling direction of the unevenness of the fracture surface is formed, because cracks or recesses Ki develop into fracture direction is formed. Fitted to fracture each other when applying the stress to fracture in order to conclude a fracture, crack or recess Taki progress in fracture direction becomes the stress concentration portion, fine breakdown occurs here it is conceivable that. Meanwhile, in order to improve the fitting property of the fracture to each other, it is necessary to increase the unevenness of the fracture surface. As described above, the polymerizable improved fit due to huge fracture surface irregularities, and the prevention of chipping and cracks are related to contradictory, achievement of both could not be solved by the current method.
CITATION
Patent Literature
[0012]
Patent Document 1: Japanese Patent Publication No. 3637375
Patent Document 2: Japanese Patent Publication No. 3756307
Patent Document 3: Japanese Patent Publication No. 3355132
Patent Document 4: Japanese Patent Publication No. 3988661
Patent Document 5: Japanese JP Patent No. 4314851
Patent Document 6: Japanese Patent Publication No. 3671688
Patent Document 7: Japanese Patent Publication No. 4268194
Patent Document 8: Japanese Patent Publication No. 5,522,321
Summary of the Invention
Problems that the Invention is to Solve
[0013]
　In view of the above circumstances, to reduce the fracture surface vicinity amount of deformation at break separation, and increased greatly to mateability irregularities fracture surface, and between inhibited chipping occurrence of fracture surface heat and to provide a rolled steel and steel parts.
Means for Solving the Problems
[0014]
　The gist of the present invention is as follows.
[0015]
(1) hot-rolled steel according to one embodiment of the present invention, the chemical ingredients C: 0.35 ~ 0.45mass%, Si : 0.6 ~ 1.0mass%, Mn: 0.60 ~ 0.90mass %, P: 0.010 ~ 0.035mass% , S: 0.06 ~ 0.10mass%, Cr: 0.02 ~ 0.25mass% or less, V: 0.20 ~ 0.40mass%, Zr: 0 .0001 ~ 0.0050 mass% or less, N: 0.0060 ~ 0.0150mass% Ti : 0 ~ 0.050mass%, Nb: 0 ~ 0.030mass%, Mg: 0 ~ 0.0050mass%, and REM: 0 containing ~ 0.0010Mass%, the balance being Fe and impurities, more than 90 area% of the metal structure is composed of ferrite and pearlite, as measured in a cross section parallel to the rolling direction, before The average number density of the aspect ratio which is stretched along the rolling direction is 10 super 30 The following Mn sulfide 50 to 200 / mm 2 it is.
(2) hot-rolled steel according to the above (1), the chemical composition, Ti: 0.005 ~ 0.050mass%, Nb: 0.005 ~ 0.030mass%, Mg: 0.0005 ~ 0 .0050mass%, and REM: may contain one or more members selected from the group consisting of 0.0003 ~ 0.0010mass%.
(3) Steel component according to another aspect of the present invention, the chemical ingredients C: 0.35 ~ 0.45mass%, Si : 0.6 ~ 1.0mass%, Mn: 0.60 ~ 0.90mass% , P: 0.010 ~ 0.035mass%, S: 0.06 ~ 0.10mass%, Cr: 0.02 ~ 0.25mass% or less, V: 0.20 ~ 0.40mass%, Zr: 0. 0001 ~ 0.0050 mass% or less, N: 0.0060 ~ 0.0150mass% Ti : 0 ~ 0.050mass%, Nb: 0 ~ 0.030mass%, Mg: 0 ~ 0.0050mass%, and REM: 0 ~ containing 0.0010Mass%, the balance being Fe and impurities, more than 90 area% of the metal structure is composed of ferrite and pearlite, as measured in a cross section parallel to the rolling direction, Asupe The average number density of Ratio of 10 super 30 The following Mn sulfide 50 to 200 / mm 2 is.
(4) Steel component according to the above (3), when the steel component to form a fracture by breaking tensile by parallel tensile stress in the rolling direction was observed in parallel with the cross section in the rolling direction that has a height difference of more than 80μm in a direction parallel to the tensile stress, the step angle to the pull the direction parallel to the stress is less than 45 degrees, the fracture surface at two positions or more per 10mm is formed at an average number density, the observed in parallel the cross section in the rolling direction, the angle is 45 degrees than for parallel the direction to the tensile stress, are formed over more than the length 80 [mu] m, a part there average number density inside progress to crack or recess of the steel part, said limited to less than three per 10mm at fracture, brittle fracture fracture in the fracture surface may be 98 area% or more .
(5) Steel component according to the above (3) or (4), the chemical composition, Ti: 0.005 ~ 0.050mass%, Nb: 0.005 ~ 0.030mass%, Mg: 0.0005 ~ 0.0050 mass%, and REM: may contain one or more members selected from the group consisting of 0.0003 ~ 0.0010mass%.
Effect of the invention
[0016]
　Hot rolled steel and steel parts according to one embodiment of the present invention, upon fracture splitting, plastic deformation of the fracture surface vicinity is small and chipping occurrence of fracture surface is reduced. Therefore, when was the fitting of a fracture surface, without causing positional deviation, the accuracy can be well fitted, can be realized accuracy of the steel parts, the yield improved simultaneously. Further, by using a steel material and steel part of the present invention, it is possible to omit the step of shake off the chipping, it can reduce manufacturing costs, thereby, is very effective in improving the economic efficiency of the industry. Therefore, the steel of the present invention, suitable for use steel part obtained by molding by hot forging, the steel and the steel part of the present invention, on the intended use fitted again fracture faces after fracture splitting Suitable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017]
[FIG. 1A] is a plan view of the fracture splitting of a test piece for evaluation.
[FIG 1B] is a side view of a fracture separation of a test piece for evaluation.
FIG. 2 is a cross-sectional photograph of a fracture surface was observed unevenness condition of the fracture surface.
3 is a schematic diagram of the crack growth of the steel part according to the present embodiment.
DESCRIPTION OF THE INVENTION
[0018]
　The following describes exemplary hot-rolled steel and steel parts in the form of the present invention.
[0019]
　The present inventor has found that by controlling the shape of Mn sulfide present in the steel, an uneven fracture size in the vertical direction of the fracture surface and preferably the control obtained by fracture separation, found that the chipping quantity can be suppressed did.
[0020]
　According to the present inventors have made findings, fracture of the uneven shape is influenced by the elongation of about and distribution frequency of Mn sulfide. Elongation of Mn sulfide is excessive (i.e. Mn aspect ratio of sulfide is large), by fracture vertical size of the irregularities is significantly larger, Ya chipping at break during separation and fracture fitting crack occurs in the fracture, voids during fracture fitting occur fittability decreases. On the other hand, when the distribution frequency of lengthening been Mn sulfide increases, the number of irregularities in the fracture surface is improved compatibility fitting increases.
[0021]
　The present inventors have found that the above phenomenon is speculated that is caused by the following mechanism. Mn sulfides steel part, when the hot rolling of the hot rolled steel which is the material of the steel part is extended into the rolling direction. As shown in FIG. 3, when the fracture splitting the steel part 10 in a direction substantially perpendicular to the rolling direction, first progress crack 12 can vertically to the rolling direction from the fracture starting point 13. However, if the crack 12 reaches the Mn sulfide 11 which is extended into the rolling direction, advancement direction of the crack 12 is changed greatly, cracks 12 at the interface between the matrix phase of Mn sulfide and the steel part 10 It believed to progress substantially parallel to the rolling direction along. When crack 12 reaches the end of the Mn sulfide 12, extending direction of the crack 12 is changed to the stress direction, crack 12 progresses to the rolling direction and substantially perpendicular again. Crack 12 by proceeds fracture splitting while repeating and parallel progress in the rolling direction perpendicular progress in the rolling direction, is considered to uneven fracture is formed. The present inventors have found that the above reasons, it the number of irregularities is much the more Mn sulfide 11 was estimated that the larger the aspect ratio of Mn sulfide 11 size along the rolling direction of the irregularities becomes large.
[0022]
　The present invention, is obtained by the finding of the present inventors described above. Hereinafter, hot rolled steel and chemical components of the steel component according to an embodiment of the present invention, the form of Mn sulfides, and will be described embodiments of the fracture surface obtained by fracture split.
[0023]
　The chemical components of the hot-rolled steel does not change by hot working. Further, since the size of the Mn sulfides are very small compared to the size of the deformation imparted by the hot working, the form of Mn sulfides hot rolled steel is also hardly changed by hot working. Accordingly, the form of a hot-rolled steel according to the present embodiment the chemical components and the Mn sulfides steel part according to the present embodiment is obtained by hot working, hot rolling according to the present embodiment are described below it is the same as those of steel. Further, the form of the fracture surface obtained by fracture splitting, so determined by the form of the chemical composition and Mn sulfide, the form of the fracture surface obtained by fracture splitting, the hot-rolled steel and the embodiment according to the present embodiment the steel part according identical.
[0024]
　Hot rolled steel of the present embodiment, as chemical components, a steel containing C, Si, Mn, P, S, Cr, V, Zr and N with a predetermined content. Hot rolled steel of the present embodiment, by including the chemical components to be described below, the ductility is preferably controlled to improve the rate of brittle fracture fracture in the fracture surface obtained by the tensile stress (tensile fracture) and it can precipitate the Mn sulfide to increase the fracture vertical size of the unevenness of the fracture with. Thus, the hot rolled steel of the present embodiment has a fracture is high fitting property obtained when the fracture splitting. Moreover, the hot rolled steel of the present embodiment, optionally as chemical components, Ti, Nb, Mg, may contain one or more of the REM.
[0025]
　Hereinafter, we describe reasons for limiting the chemical components of the hot rolled steel and steel parts of the present embodiment. Hereinafter, unless otherwise specified, reasons for limiting the chemical components of the hot rolled steel is equal to the reasons for limiting the chemical components of the steel part.
[0026]
(C: 0.35 ~ 0.45Mass%)
　C, the effect of securing the hot rolled steel and tensile strength of the steel part of the present embodiment, and the plastic deformation of the fracture surface near at break decreased Te has the effect of realizing the excellent fracture splitting property. With the increase and C, by volume fraction of pearlite structure increases, tensile strength increases, and ductility and toughness is reduced. These effects in order to maximize, to set the C content in the steel to 0.35 ~ 0.45mass%. When C content exceeds the upper limit amount, pearlite fraction of the hot rolled steel becomes excessive, the frequency of occurrence of chipping at break increases. Further, when the C content is less than the lower limit amount, the amount of plastic deformation fracture surface near the hot rolled steel is increased, fitting property decreases the fracture surface. A preferable lower limit of the C content is 0.36Mass%, or 0.37mass%. Preferred upper limit of the C content is 0.44mass%, 0.42mass%, or 0.40%.
[0027]
(Si: 0.6 ~ 1.0
　mass%) Si causes the strengthening ferrite by solid solution strengthening, thereby reducing ductility and toughness of the hot-rolled steel and steel parts. The decrease in ductility and toughness, fracture plastic deformation of fracture surface near the time of separation is reduced, fracture splitting of the hot-rolled steel and the steel part is improved. To obtain this effect, it is necessary that the lower limit of the Si content to 0.6 mass%. On the other hand, when Si is contained excessively, since the frequency of chipping of the fracture surface is generated is increased, the upper limit of the Si content is made 1.0 mass%. A preferable lower limit of the Si content is 0.7 mass%. Preferred upper limit of the Si content is 0.9mass%.
[0028]
(Mn: 0.60 ~
　0.90Mass%) Mn causes the strengthening ferrite by solid solution strengthening, thereby reducing ductility and toughness of the hot-rolled steel and steel parts. The decrease in ductility and toughness, fracture plastic deformation of fracture surface near the time of separation is reduced, fracture splitting of the hot-rolled steel and the steel part is improved. Further, Mn forms a Mn sulfide combines with S. When to fracture split steel part obtained from hot rolled steel of the present embodiment, crack has along the Mn sulfide elongated in the rolling direction so propagated, the size of the rolling direction of the unevenness of the fracture surface increases. Therefore, Mn has the effect of preventing the positional deviation in fitting the fracture surfaces. On the other hand, Mn may be contained excessively, the frequency of chipping at the time of broken ferrite is too hard to generate increases. In view of these, Mn content is 0.60 ~ 0.90mass%. A preferable lower limit of the Mn content is 0.65mass%, 0.70mass%, or 0.75mass%. Preferred upper limit of the Mn content is 0.85mass%, 0.83mass%, or 0.80%.
[0029]
(P: 0.010 ~ 0.035Mass%)
　P lowers the ductility and toughness of ferrite and pearlite, thereby reducing ductility and toughness of the hot-rolled steel and steel parts. The decrease in ductility and toughness, fracture plastic deformation of fracture surface near the time of separation is reduced, fracture splitting of the hot-rolled steel and the steel part is improved. However, P is cause embrittlement of the grain boundaries, to facilitate generating a chipping of the fracture surface. Therefore, reducing the ductility and toughness by utilizing the P is not preferable from the viewpoint of prevention of chipping occurrence. In view of the foregoing, the range of P content is 0.010 ~ 0.035mass%. Preferred lower limit of the P content is 0.012mass%, 0.013mass%, or 0.015 mass%. Preferred upper limit of the P content is 0.030mass%, 0.028mass%, or 0.025%.
[0030]
(S: 0.06 ~ 0.10 mass%)
　S to form a Mn sulfide combines with Mn. When to fracture split steel part obtained from hot rolled steel of the present embodiment, since cracks have along the Mn sulfide elongated in the rolling direction is propagated, Mn sulfide fracture vertical unevenness of fracture surface increasing the size, the effect of preventing positional deviation in fitting the fracture surfaces. To obtain the effect, it is necessary that the lower limit of the S content to 0.06 mass%. On the other hand, when S is contained excessively, increasing the amount of plastic deformation fracture surface near at break divided, if fracture splitting resistance is lowered. Furthermore, S Excess may facilitate the chipping of the fracture surface. For the above reasons, and 0.06 ~ 0.10 mass% preferred range of S. Preferred lower limit of the S content is 0.07mass%. Preferred upper limit of the S content is 0.09mass%.
[0031]
(Cr: 0.02 ~ 0.25
　mass%) Cr strengthens a ferrite by solid solution strengthening as with Mn, reducing the ductility and toughness of the hot-rolled steel and steel parts. Reduction in ductility and toughness is to reduce the amount of plastic deformation fracture surface near at break, improve the fracture separation of the hot-rolled steel and steel parts. To obtain the effect, it is necessary that the lower limit of the Cr content to 0.02 mass%. However, if excessively contained Cr, lamellar spacing of pearlite is reduced, the ductility and toughness of the pearlite is increased. Therefore, plastic deformation of fracture surface near at break increases, fracture splitting is lowered. Further, if excessively contained Cr becomes bainite is easily generated, fracture splitting resistance may be lowered significantly. Therefore, if the inclusion of Cr, its content is less 0.25 mass%. Preferred lower limit of the Cr content is 0.05 mass%, 0.08 mass%, or 0.10 mass%. Preferred upper limit of the Cr content is 0.23mass%, 0.20mass%, or 0.18mass%.
[0032]
(V: 0.20 ~ 0.40%) V
　by forming a mainly carbides or carbonitrides during cooling after hot forging, to strengthen the ferrite, ductility of the hot rolled steel and steel parts and lowering the toughness. Reduction in ductility and toughness of the hot-rolled steel and steel parts, the plastic deformation of the fracture surface near at break of the hot rolled steel and steel part with a smaller fracture splitting of a steel component formed of hot rolled steel the to improve. Also, V is the precipitation strengthening of carbide or carbonitride, an effect of increasing the yield ratio of the hot-rolled steel, in order to obtain these effects, should be the lower limit of the V content to 0.20 mass% there is. On the other hand, even if excessively contained V the effect because saturation, the upper limit of the V content is 0.40%. Preferred lower limit of the V content is 0.23mass%, or 0.25 mass%. Preferred upper limit of the V content is 0.38%, or 0.35mass%.
[0033]
: (Zr 0.0001 ~
　0.0050mass%) is Zr forms oxides, the Zr oxides becomes crystal strapping or precipitation nuclei of Mn sulfide, uniformly finely dispersed the Mn sulfides. Mn sulfide this which is finely dispersed becomes a propagation path of cracks during fracture splitting, the plastic deformation of the fracture surface near the hot rolled steel and steel components is reduced, an effect of increasing the fracture splitting property. However, since Zr has be contained excessively, the effect is saturated, and 0.0050 mass% to the upper limit of the Zr content. To sufficiently achieve this effect and 0.0001Mass% the lower limit of the Zr content. Preferred lower limit of Zr content is 0.0005 mass%, or 0.0010mass%. Preferred upper limit of the Zr content is 0.0045mass%, 0.0040mass%, or 0.0030mass%.
[0034]
(N: 0.0060 ~ 0.0150Mass%)
　N, by acting as transformation nuclei for the ferrite to form a predominantly V nitride or V carbonitride during cooling after hot forging, promotes ferrite transformation . Thus, N is the, the effect of suppressing the generation of significantly impair bainite fracture separation of the resulting steel part from hot rolled steel. To obtain this effect, the lower limit of the N content and 0.0060mass%. Excessive containing N, hot ductility of the hot rolled steel and steel components is reduced, in some cases cracks or flaws during hot working is likely to occur. Therefore, the upper limit of the N content and 0.0150mass%. A preferable lower limit of the N content is 0.0065mass%, 0.0070mass%, or 0.0080mass%. Preferred upper limit of the N content is 0.0140mass%, 0.0130mass%, or 0.0120mass%.
[0035]
　Hot rolling steel material according to the present embodiment, in order to even more pronounced its effect, further, Ti: 0.050 mass% or less, and, Nb: 0.030 mass% or less, Mg: 0.0050 mass% or less, REM : can be selected and contained one or more members selected from the group consisting of 0.0010mass%. However, even if these elements are not contained, the hot rolled steel and steel parts according to the present embodiment it is possible to solve the problem, the lower limit of Ti, Nb, Mg, and REM are 0mass%.
[0036]
(Ti: 0 ~ 0.050 mass%)
　Ti, upon cooling after hot forging to form a mainly carbides or carbonitrides, the precipitation strengthening strengthening ferrite, thereby the hot-rolled steel and steel parts reducing the ductility and toughness. Reduction in ductility and toughness is to reduce the amount of plastic deformation fracture surface near at break, thereby improving the fracture splitting of the hot-rolled steel and steel parts. However, if excessively containing Ti, its effect is saturated. If the inclusion of Ti in order to obtain the effect described above, and 0.050 mass% to the upper limit of the Ti content. In order to sufficiently exhibit the effect of Ti is preferably set to 0.005 mass% the lower limit of the Ti content. More lower limit of the preferred Ti content is 0.015mass%, 0.018mass%, or 0.020 mass%. More upper limit of the preferred Ti content is 0.040mass%, 0.035mass%, or 0.030 mass%.
[0037]
(Nb: 0 ~ 0.030 mass%)
　Nb, upon cooling after hot forging to form a mainly carbides or carbonitrides, the precipitation strengthening strengthening ferrite, thereby the hot-rolled steel and steel parts reducing the ductility and toughness. Reduction in ductility and toughness is to reduce the amount of plastic deformation fracture surface near at break, thereby improving the fracture splitting of the hot-rolled steel and steel parts. However, if excessively containing Nb, the effect is saturated. If the inclusion of Nb in order to obtain the effect described above, and 0.030 mass% to the upper limit of Nb content. For Nb effect to be fully exhibited, it is preferable to 0.005 mass% the lower limit of the Nb content. More lower limit of the preferred Nb content is 0.010 mass%. More upper limit of the preferred Nb content is 0.030mass%, 0.028mass%, or 0.025%.
[0038]
(Mg: 0 ~ 0.0050
　mass%) Mg is to form the oxide becomes crystalline strapping or precipitation nuclei of Mn sulfide, thereby uniformly finely dispersed the Mn sulfides. The Mn sulfide becomes a propagation path of cracks during fracture splitting, to reduce the amount of plastic deformation fracture surface vicinity, increase the fracture separation of the hot-rolled steel and steel parts. However, since Mg is contained excessively to be its effect is saturated, and 0.0050 mass% to the upper limit of the Mg content. To sufficiently achieve this effect, it is preferable to 0.0005 mass% the lower limit of the Mg content. More lower limit of the preferred Mg content is 0.0006mass%. More upper limit of the preferred content of Mg 0.0045mass%, 0.0040mass%, 0.0035mass% , 0.0030mass%, or 0.0025 mass%.
[0039]
(REM: 0 ~
　0.0010Mass%) REM becomes a crystal strapping or precipitation nuclei of Mn sulfide to form oxysulfides, thereby uniformly finely dispersed the Mn sulfides. The Mn sulfide becomes a propagation path of cracks during fracture splitting, to reduce the amount of plastic deformation fracture surface vicinity, increase the fracture separation of the hot-rolled steel and steel parts. However, when REM is contained excessively, the steel manufacturing stage, problems will be caused, such as nozzle clogging in the casting process. Therefore, the 0.0010Mass% the upper limit of the REM content. It is preferable to 0.0003Mass% the lower limit of the REM content in order to sufficiently exhibit this effect. The lower limit of a more preferable content of REM is 0.0004mass%, or 0.0005 mass%. More upper limit of the preferred content of REM is 0.0009mass%, 0.0008mass%, or 0.0007mass%. Incidentally, the term "REM" is, Sc, refers to a total of 17 elements consisting of Y and lanthanoid, the "content of REM" means a total content of these 17 elements. When using a lanthanoid as REM, the industrial, REM is added in the form of misch metal.
[0040]
　The remainder of the chemical composition of the hot rolled steel and steel parts according to the present embodiment is Fe and impurities. The impurities, be those mixed from raw materials and production environment, such as ore and scrap refers to one which does not adversely affect the hot-rolled steel and properties of the steel component according to the present embodiment. Further, hot rolled steel and steel parts according to this embodiment, in addition to the above components, within a range that does not impair the effect from 0 to 0.01% Te 0 to 0.01% Zn, and 0 it can contain 0.01% Sn and the like.
[0041]
(Metal structure: composed of 90 or more area% of ferrite and pearlite)
　metal structure of the hot rolled steel and steel parts of the present embodiment is a so-called ferrite - are pearlite structure. It might contain bainite or the like metal structure, but bainite undesirably impair the fracture splitting property. The present inventors have found that a metal structure of the hot rolled steel and steel parts of the present embodiment, defined as to include a total of 90 area% or more of ferrite and pearlite. This provision amount bainite is limited to 10 area% or less, fracture splitting of the hot-rolled steel and the steel part is kept good. Hot rolled steel and steel components of a metal structure of the present embodiment, 92 area% in total, may contain 95 area%, or 98% or more by area of ferrite and pearlite.
[0042]
　Unless the total amount of ferrite and pearlite is within the above range, the proportion of both is not particularly limited. For example, even ferrite or pearlite 0 area%, good fracture splitting resistance is maintained. Further, as long as the total amount of ferrite and pearlite is within the above range, structure of the remainder of the metal structure is not particularly limited. The amount of ferrite and pearlite is included in the metal structure, the optical micrograph of polished and etched cross-section taken, obtained by this photographic image analysis.
[0043]
(Average number density of the aspect ratio which is stretched along the rolling direction is 10 super 30 The following Mn sulfide 50 to 200 / mm 2 )
　in the interior of the hot rolled steel and steel parts of the present embodiment, Mn sulfide is formed. Mn sulfides are lengthening along the rolling direction of the hot rolled steel. Lengthening been Mn sulfide is an essential inclusions for forming the uneven shape on the fracture surface obtained by fracture tensile hot-rolled steel and steel parts. For elongation of Mn sulfide in the production of steel in hot rolling, it is necessary to at least 80% or more reduction of area to steel bar from the billet.
[0044]
　The hot-rolled steel and steel parts according to the present embodiment, an aspect ratio of 10 than the rolling direction as a long shaft, 30 following lengthening been Mn sulfide 1 mm 2 per 50 or more, distributed in 200 or less to. Lengthening been Mn sulfide, the fracture surface formed by the tensile of the rolling direction, a tensile direction by forming irregularities, enhance the fitting property of the fracture faces. Aspect ratio 10 super 30 The following Mn sulfides, it is possible to optimize the pulling direction size of the irregularities. The number of aspect ratio of 10 super 30 The following Mn sulfide 50 to 200 / mm 2 when it is, it is possible to optimize the number of irregularities.
[0045]
　Mn sulfide aspect ratio of 10 or less can not be sufficiently increased tensile direction size of the unevenness of the fracture surface, it does not contribute to the improvement of fittability of the fracture faces. Mn sulfide aspect ratio exceeds 30, although remarkable irregularities of fracture surface, since the frequency of cracking or chipping increases, thereby impairing the fitting property of the fracture faces. Therefore, the number density of the aspect ratio of 10 or less or 30 than a is Mn sulfide is preferably small. However, number density of aspect ratio of 10 super 30 The following Mn sulfide is in the range described above, and when the content of the generated source become Mn and S of Mn sulfide is within the above range, the chemical composition Mn and S in is, the aspect ratio is consumed by 10 super 30 the following Mn sulfide generation of Mn sulfide aspect ratio of 10 or less, or greater than 30 is sufficiently suppressed. Therefore, the number density of the aspect ratio of 10 or less or 30 than a is Mn sulfide is not particularly limited.
[0046]
　If the aspect ratio is 10 less than the average number density lower limit of the super 30 the following Mn sulfide, the number of irregularities of the fracture surface is reduced, fitting property of the fracture surface after fracture separation is insufficient. Further, if the aspect ratio of 10 average number density of super 30 below Mn sulfide is less than the lower limit, the number density of less than the aspect ratio of 10 or 30 than the Mn sulfide is increased, impairing the fracture isolation there is a risk. On the other hand, when the average number density of aspect ratio of 10 super 30 The following Mn sulfide exceeds the upper limit, cracks in the fracture surface, chipping occurs, is impaired in this case also fitted a fracture surface compatibility.
[0047]
　Hot rolled steel and steel parts, the measurement method of the average number density of the aspect ratio which is stretched along the rolling direction is 10 super 30 The following Mn sulfide is as follows.
　First, the hot-rolled steel and steel part cut parallel to the rolling direction is polished cut surface. Since Mn sulfide extending along the rolling direction, when cutting the hot-rolled steel and steel parts, the extending direction of the Mn sulfides, it can be regarded as the rolling direction of the hot rolled steel and steel parts.
　Then, the enlarged photograph of the cut surface is taken by an optical microscope or an electron microscope. Although magnification at this time is not particularly limited, it is preferably about 100-fold. Since Mn sulfide are substantially uniformly distributed, area to be photographed is not particularly limited.
　Then, a photo by image analysis, the aspect ratio in the region where the photo was taken can be determined and the number density of 10 super 30 The following Mn sulfides. Incidentally, some lengthening been Mn sulfide, some distributed aggregated in rows in the rolling direction are divided. However, the two Mn sulfide interval is 10μm or less is considered one elongated Mn sulfide. Are arranged in the direction of elongation, is and two Mn sulfide interval is 10μm or less, in that propagate crack occurring when to break tensile hot-rolled steel or steel components in the pulling direction, one Mn it is considered to have the same effect as sulfides.
　Furthermore, repeated at least 10 times the analysis and photography, by averaging the number density obtained by this, the hot-rolled steel and the steel parts, the aspect ratio which is stretched along the rolling direction is 10 super 30 the average number density of less Mn sulfide is obtained.
[0048]
(Method of manufacturing a hot-rolled steel and steel part)
　Next, a method for manufacturing a hot-rolled steel and steel parts according to the present embodiment. Method for manufacturing a hot-rolled steel according to the present embodiment includes the steps of obtaining a bloom of steel having the same chemical composition as the hot-rolled steel according to the present embodiment smelted and continuously cast, slabbing like Bloom obtaining a hot working was in billets, and a step of obtaining a round bar billets hot rolling, Zr is added in the initial stage of the secondary refining in melting, the total of the hot rolling reduction of area is 80% or more, and reduction of area at 1000 ° C. the following hot rolling is characterized in that 50% or more. Method for producing a steel part according to the present embodiment, air or a process and hot forged hot rolled steel to a hot-rolled steel is heated to 1150 ~ 1280 ° C. hot forging according to the present embodiment to room temperature step of blast cooling or hot-rolled steel according to the present embodiment and the step of cold-forged, by cutting the cooled hot rolled steel, and a step of obtaining a steel part having a predetermined shape.
[0049]
　Details of the method for manufacturing the hot-rolled steel according to the present embodiment is as follows. First, a steel having the same chemical composition as the hot-rolled steel according to the present embodiment, were melted in a converter furnace, to produce a bloom by continuous casting. During converter melting, adding Zr to the molten steel during secondary refining before or secondary refining. Was sufficiently flotation from molten steel coarse Zr oxide, a Zr oxide in order to finely disperse the resulting Mn sulfide in the molten steel as nuclei and, secondary refining early stage (e.g. RH (Ruhrstahl-Heraeus ) and the like before performing a degassing treatment to the molten steel with, or within 15 minutes after the start of processing a between by degassing RH) it is necessary to adding Zr in. The addition of Zr is, if done after the degassing treatment starts after 15 minutes using a RH etc., the time for the refinement of Mn sulfide with Zr oxides becomes insufficient, in Broome Mn sulfide is coarse. If Mn sulfide in bloom is coarsened after the Mn sulfides in the bloom rolling process is excessively stretched, Mn sulfide aspect ratio greater than 30 is increased, the aspect ratio of 10 super 30 following Mn sulfide the number density of the object is insufficient.
[0050]
　The resulting bloom, and billet through a further slabbing process or the like. The resulting billet is further round bars by hot rolling. Thus the production of hot-rolled steel of the present embodiment in the. For extension of Mn sulfide rolling reduction of area at the time of the billet to the round bar shape is preferably 80% or more. Furthermore, in order to significantly elongation of Mn sulfide, a relatively low temperature range a high temperature hardness against steel Mn sulfides, i.e., hot rolling at a temperature region where extension of Mn sulfide is facilitated It is necessary. Specifically, the rolling reduction of area at 1000 ° C. or less is required to be 50% or more. Thus, it is possible to extend the Mn sulfides in the steel. If these rolling conditions is not satisfied, Mn sulfide is not sufficiently stretched. Incidentally, hot rolled steel after hot rolling may be cooled to room temperature, may be further subjected to hot forging prior to cooling.
[0051]
　Details of the method of manufacturing the steel member according to the present embodiment is as follows. Heating the hot-rolled steel obtained by the method described above for example, 1150 ~ 1280 ° C. and hot forging, to room temperature by air cooling (cooling in the air) or air blast cooling (steel feed wind cooling) Cooling. By cutting the forged material after cooling to a predetermined shape steel parts. When forging a hot rolled steel is not limited to hot forging may be cold forging.
[0052]
　Hot rolled steel and steel parts of the present embodiment, the case of forming the fracture by breaking tensile by parallel tensile stress in the rolling direction is observed in a cross section parallel to the rolling direction, in a direction parallel to the tensile stress towards a height difference of more than 80μm, the step angle with respect to a direction parallel to the tensile stress is less than 45 degrees, is formed at an average number density of more than two locations per 10mm in fracture surface, cross section parallel to the rolling direction in the observed angle with respect to a direction parallel to the tensile stress is 45 degrees greater, are formed over a length greater than 80 [mu] m, a part of the average number density of cracks or recess Taki developed inside the steel part , is limited to less than three per 10mm at fracture, brittle fracture fracture in the fracture is 98 area% or more.
[0053]
　The reason for defining the nature of the fracture surface will be described below. Fitted to fracture each other formed by the tensile and stress the horizontal direction fracture surface, the slope of the stress by the unevenness of the fracture, 90 ° in the horizontal direction and two in the normal direction (in-plane direction, and it is three-dimensionally distributed in the fracture surface vertical direction). In this case, the applied stress is distributed larger the tensile direction size of the unevenness of the fracture surface. It has steps formed by the unevenness, the angle with respect to a direction parallel to the tensile stress is not more than 45 degrees, if it has a height difference of more than 80μm in a direction parallel to and tensile stress, this uneven There was determined to contribute to the fitting property. Further, as long as the lack of fracture does not occur, the larger the tensile direction size of the step of fracture, it is possible to prevent the positional deviation of the stress at applied more reliably.
[0054]
　Chipping generation amount, cracking or can fracture surface direction of the fracture surface is correlated with the presence of the concave portion of the fracture surface direction. That is, as the predetermined size or more fracture direction crack or fracture direction of the recess that is large, the amount of chipping increases. When fitting the fracture surfaces, fracture direction can cracking or by being broken finely acts recesses as stress concentrators, it believed chipping. The present inventors have found that in order to suppress chipping occurrence of fracture surface was found that it is necessary to minimize the portion of the fracture surface direction of crack or recess. More specifically, in order to sufficiently suppress the chipping generation amount is observed in a cross section parallel to the rolling direction, an angle with respect to a direction parallel to the tensile stress is 45 degrees greater than over more than the length 80μm it is formed, a part of the present invention have found that it should limit the average number density of cracks or recess Taki developed inside the steel part to less than three per 10 mm.
[0055]
　In particular, since Mn forms sulfide and dispersed state greatly affects the fracture shape, in order to maximize the unevenness of the fracture surface in a range that does not cause chipping controls the dispersion state in the form of Mn sulfide This is very important. More specifically, by extending the Mn sulfide which is a route of crack propagation within the proper range, and that in a large amount is dispersed, contributing to increasing the pulling direction size of the unevenness of the fracture surface . Therefore, in this embodiment, the fracture surface irregularities can be realized experimentally in a range not to generate chipping of fracture surface at break was defined as described above.
[0056]
　Further, hot rolled steel and steel parts according to the present embodiment, the chemical composition is preferably controlled, more than 90 area% of the metal structure is a ferrite and pearlite, further Mn sulfide dispersed therein having a predetermined form because it is, 98% or more by area of ​​the fracture surface obtained by dividing by hot rolled steel and tensile stress parallel to the steel parts in the rolling direction according to the present embodiment is a brittle fracture surface. The deformation is ductile fracture occurs, ductile fracture impairs compatibility fitting the fracture surfaces. If more than 98 area% of the fracture surface is a brittle fracture, fracture of the fitting property is maintained preferably.
[0057]
　Evaluation method of fracture surface shape is as follows.
　Area ratio of the brittle fracture surface occupying the fracture surface by analyzing the photos according to the procedure of a normal fracture surface analysis, cleavage fracture, and the like pseudo cleavage fracture or intergranular cracking brittle fracture occurs region defining a area of the brittle fracture surface area is determined by calculating the percentage of the area of the entire fracture surface.
　Deformation amount by fracture splitting is bolted combined per a hot rolled steel or steel part after breaking, the breaking direction of the inner diameter, measures the difference between the vertical direction of the inner diameter of the breaking direction, breaking this difference It determined by regarding the amount of deformation due to division.
　Chipping occurrence of fracture surface are assembled and bolted to a torque of 20 N · m by butt-jointing the fracture surface, then repeat operations 10 times releasing the fracture surface by loosening bolts, total weight of debris was eliminated by this was measured and calculated by regarding chipping occurrence of fracture surface of the total weight.
　Rolling direction is observed in a cross section parallel to the tensile has a height difference of more than 80μm in a direction parallel to the stress, the angle with respect to a direction parallel to the tensile stress is less than 45 degrees difference in level (pull direction step) number density, and observed in a cross section parallel to the rolling direction, an angle with respect to a direction parallel to the tensile stress is 45 degrees greater, it is formed over more than the length 80 [mu] m, progress partially inside the steel part number density of to crack or recesses (fracture direction crack) is evaluated by the following methods. First, the fracture surface is cut parallel to the tensile direction the formed hot-rolled steel or steel components, which can be observed from the direction perpendicular to fracture shape pulling direction. By filling resin fracture surfaces prior to cutting, it may be fractured surface shape is maintained during cutting. By observing the fracture surface shape at the cut surface of the above, it is possible to observe the tensile direction of the irregularities, and the fracture surface direction of the unevenness. Incidentally, the cut surface is a tensile can be formed anywhere as long as the test piece is parallel to the direction, for convenience, it is preferable to form a cutting surface so fracture becomes large as possible in the cutting plane . Observation was performed at any five or more visual fields in a cross section, when observing the number density per 10mm tensile direction step and fracture direction Crack in each field is measured to determine the average value thereof. Thus, the number density of the pulling direction step and fracture direction cracks are obtained.
[0058]
　Method of breaking the hot-rolled steel and steel parts according to the present embodiment is not particularly limited, it is preferable to break with the parallel tensile stress in the rolling direction. Since hot Mn sulfides rolled steel and steel parts according to the present embodiment is extending parallel to the rolling direction, to form a substantially vertical fracture in the rolling direction by adding parallel tensile stress in the rolling direction Accordingly, roughening effect of Mn sulfide is maximized. Further, in order to improve the fracture splitting property, the position of forming the fracture surface, it is preferable to subjecting the notching before adding the tensile stress. The method of notching is not particularly limited, for example, by broaching or laser machining may be performed notching.
Example
[0059]
　Detailed below examples of the present invention. It should be noted that these examples are intended to illustrate the technical significance and effects of the present invention and are not intended to limit the scope of the present invention.
Example 1
[0060]
　Table 1-1 and having the chemical compositions shown in Table 1-2, to produce a bloom by steel 1-28 and steel 101-115 were melted in a converter to continuous casting, the bloom, slabbing step a billet of 162mm square through further diameter by hot rolling and round bars of 56 mm. Steel 1-28, steels 101-112, steel 114, and when Steels 115, within 15 minutes before or degassing after start performing degassing treatment to the molten steel using a RH, adding Zr to the molten steel did. The steel 113 was not added Zr. When the round bar billets hot rolling, the total area reduction rate is 90%, the reduction rate in the temperature range of 1000 ° C. or less was 80%. Incidentally, in the table "-" sign with the content of the element according to the location where the symbol is described indicates that the impurity level. The heating temperature and heating time of blooming before Bloom, 1270 ° C., respectively, and a 140 min, heating temperature and heating time of the billet before hot rolling was respectively 1240 ° C., and 90min. Underlined values ​​in Tables 1-1 and 1-2 are numerical values ​​outside the scope of the present invention.
[0061]
　Contained in Examples 1 to 28 and Comparative Examples 101 to 115 obtained by the method described above, measured in a cross section parallel to the rolling direction, an aspect ratio, which is stretched along the rolling direction is 10 super 30 following Mn the average number density of sulfides (Mn sulfide number density) was calculated by the following method. First, Examples 1 to 28 and Comparative Examples 101 to 115 cut parallel to the rolling direction was polished cut surface. Then, the enlarged photograph of the cut surface of Examples 1 to 28 and Comparative Examples 101 to 115 were taken by an optical microscope or an electron microscope. Magnification at this time was 100 times. Then, a photo by image analysis, the aspect ratio in the region where the photo was taken was determined the number density of 10 super 30 The following Mn sulfides. Incidentally, the two Mn sulfide interval is 10μm or less, was regarded as one elongated Mn sulfide. Furthermore, repeating the analysis and photography 10 times, by averaging the number density obtained by this, in Examples 1 to 28 and Comparative Examples 101 to 115 aspect ratio is stretched along the rolling direction is 10 to obtain an average number density of the super 30 the following Mn sulfides. Table 1-1 and the "Mn sulfide number density" in Table 1-2, are measured in a cross section parallel to the rolling direction, an aspect ratio, which is stretched along the rolling direction is 10 super 30 The following Mn sulfide it is the average number density of.
[0062]
　Next, in order to examine the fracture separation properties, and the forged connecting rod corresponding specimens 1-28 and 101-115 created in hot forging. Specifically, the diameter in the above steps 56 mm, steel 1-28 and steel 101-115, which is a raw material steel bar length 100 mm, after heating to 1150 ~ 1280 ° C., forged perpendicular to the length direction of the steel bar and a thickness of 20mm Te. Then, the steel 1-27 and steel 101-115 was cooled to room temperature by air cooling (cooling in the air), the steel 28 was cooled to room temperature by air blast cooling (to test piece feeding wind cooling). Forged material after cooling, and JIS4 No. tensile specimen, the fracture separation of a test piece for evaluation of the connecting rod big end corresponding shape machined. JIS4 No. Tensile test piece at 30mm position from the side face of the forged material were taken along the longitudinal direction of the forging. The fracture isolation test piece for evaluation, as shown in FIG. 1, the central portion of the plate shape of 80 mm × 80 mm and a thickness of 18 mm, are those with a hole with a diameter of 50mm, on the inner surface of the hole with a diameter of 50mm , a position two locations ± 90 degrees to the length direction of the steel bar which is a material before forging, 1 mm and the depth was subjected to V-notch processing of 45-degree tip curvature 0.5 mm. Further, the through-holes of 8mm diameter as bolt holes, the center line is opened so as to be located at a position of 8mm from the side of the notch machining side.
[0063]
　The test apparatus of fracture splitting assessment is composed of a split mold and the falling weight tester. Split type in a shape divided into two parts along the center line of the cylinder of diameter 46.5mm were molded onto rectangular steel, one is fixed, one moves on rails. Wedge holes are machined on the mating surfaces of two half-cylinder. At break tests, fitting holes 50mm diameter test piece cylinder of the split mold having a diameter of 46.5 mm, is placed to put a wedge on the falling weight. Falling weight is the mass 200 kg, is a mechanism to fall along the guide. Dropping the falling weight, wedge is driven, the test piece is fractured tensile two. Tensile Tensile stress applied to the test piece during the fracture is parallel to the hot rolling direction. In addition, as the test piece is not released from split at break, specimens are fixed around so as to be pressed against the split molds.
[0064]
　Method for measuring the area ratio of the brittle fracture surface occupying the fracture surface (brittle fracture area ratio) were as follows. First, the falling weight is broken specimens at a height 100 mm, it was taken optical micrograph of the fracture surface. By analyzing the photos according to the procedure of a normal fracture surface analysis, defining a region where cleavage fracture, pseudo cleavage fracture or composed brittle fracture surface like intergranular cracking occurs, the area of the brittle fracture region It was calculated percentage of the area of the entire fracture surface.
　Method of measuring the amount of deformation at break separation were as follows. Bolted combined per the test piece after breaking, the breaking direction of the inside diameter was measured difference between the vertical direction of the inner diameter of the fracture direction. The difference was the amount of deformation due to fracture split.
　Method of measuring the chipping occurrence of fracture surface were as follows. After the deformation amount measurement described above, assembling and bolting a torque of 20 N · m by butt-jointing the fracture surface, the work then loosening the bolts release the fracture surface was repeated 10 times. Thereby measuring the total weight of the debris to fall off, and the total weight and chipping occurrence of fracture surface.
[0065]
　The fracture separation of good steel, fracture morphology of the fracture surface is brittle, a and fracture separation according to the amount of deformation is small steel fracture vicinity. The present inventors are the area ratio of the brittle fracture surface is 98% or more, and is not less 100μm or less deformation of the fracture near the further chipping sample generation amount is less than 1.0 mg, fracture separation properties It was regarded as a good sample. Moreover, observed in a cross section parallel to the rolling direction, an angle with respect to a direction parallel to the tensile stress is 45 degrees greater, are formed over more than the length 80 [mu] m, a part of Taki developed inside the steel part the average number density of cracks or depressions (fracture direction crack) is a sample restricted fracture surface was formed in less than three per 10 mm, it was regarded fracture splitting property good sample.
[0066]
　To enhance the fitting property of fracture between the pulling direction of the size of the unevenness of the fracture surface (i.e., the size of the step formed by irregularities) is large, it is necessary to and irregularities are present at a high frequency is there. The present inventors have observed in a cross section parallel to the rolling direction, have a height difference of more than 80μm in a direction parallel to the tensile stress, the angle with respect to a direction parallel to the tensile stress is less than 45 degrees step number density of (tensile direction step) is a sample fracture surface is formed at two or more places per 10 mm, it was regarded as fitting property higher sample.
[0067]
　The number density of the tensile direction step and fracture direction Crack fracture surface was measured by the following method. First, it cuts in parallel with the specimen tensile direction, and can be observed from the direction perpendicular to fracture shape pulling direction. By observing the fracture surface shape at the cut surface of the above it was observed pulling direction of the irregularities, and the fracture surface direction of the unevenness. Cut surface was formed to include the center of the fracture surface. Observation was performed at any five field in a cross section. During observation, the number density per 10mm tensile direction step and fracture direction Crack in each field was measured to determine the average value thereof. The cross-sectional observation photograph of cases used in the evaluation of unevenness condition of fracture surface shown above is shown in FIG. 2.
[0068]
　The results are shown in Table 2-1 and Table 2-2.
　In Examples 1-28, the aspect ratio which is stretched along the rolling direction is an average number density of 10 super 30 The following Mn sulfides, 1 mm 2 was per 50 or more. Furthermore, the chemical composition of Examples 1 to 28 were within the specified range of the present invention. Thus, none of the Examples 1 to 28, excellent fracture splitting property, fitting property was good at the same time. That rupture, Examples 1-28, after air cooling or air blast cooling after hot forging, when performing the fracture splitting, chipping occurrence of and fracture surface plastic deformation is small fracture surface vicinity is small, excellent It had separability. Plastic deformation of the fracture surface is small, the further chipping, characterized in that generation is small, the Examples 1 to 28, can be fitted accurately fracture surface without positional deviation at the time of mating the fracture surface occurs, parts improve the yield of production. Moreover, this feature, Examples 1 to 28 can be omitted a step of shake off the chipping, leading to reduction in manufacturing cost, this is a large extremely effective industrially.
[0069]
　On the other hand, Comparative Examples 101 to 115, C, Si, Mn, P, S, Cr, V, Zr, and the content of any of the N is out of the scope of the present invention. These for the following reasons, it does not meet the requirements of the present invention.
[0070]
　Comparative Example 101,103,107,112, and 115, C, respectively, Si, P, V, and since the content of N is less than the lower limit of the scope of the present invention, plastic deformation at break separation 100μm the beyond. Accordingly, Comparative Examples 101,103,107,112, and 115, it is determined that no good fracture splitting property.
　Comparative Examples 102, 104, 106, and 108, C, respectively, Si, Mn, and so the content of P exceeds the upper limit of the range of the present invention, chipping occurs at the time of fracture splitting exceeds 1.0mg . Accordingly, Comparative Examples 102, 104, 106, and 108, it is determined that no good fracture splitting property.
　Comparative Example 105, since the Mn content is less than the lower limit of the scope of the present invention, the average number density 1mm aspect ratio that is stretched along the rolling direction is 10 super 30 The following Mn sulfide 2 50 per a and, i.e., the number and extension degree of Mn sulfide is insufficient below. Accordingly, Comparative Example 105, uneven places the number of fracture is insufficient, it is determined that no satisfactory fitting property.
　Comparative Example 109, since the content of S exceeds the upper limit of the range of the present invention, the average number density 1mm aspect ratio that is stretched along the rolling direction is 10 super 30 The following Mn sulfide 2 per 200 number exceeded. Accordingly, Comparative Example 109, chipping occurs with more than 1.0mg at break, the amount of plastic deformation during fracture splitting exceeds 100 [mu] m, is determined to have no good fracture splitting property.
　Comparative Example 110, since the content of S is less than the lower limit of the scope of the present invention, the average number density 1mm aspect ratio that is stretched along the rolling direction is 10 super 30 The following Mn sulfide 2 per 50 becomes less than number, the number and extension degree i.e. Mn sulfide was insufficient. Accordingly, Comparative Example 110, uneven places the number of fracture is insufficient, it is determined that no satisfactory fitting property.
　Comparative Example 111, since the content of Cr exceeds the upper limit of the range of the present invention, the amount of plastic deformation during fracture splitting exceeds 100 [mu] m, is determined to have no good fracture splitting property.
　Comparative Example 113, Zr was not present, the aspect ratio is 10 more than, 1 mm in a 30 following MnS steel 2 is less than 50 per distribution of Mn sulfide is dispersed in the crude, the fracture uneven portion number does not satisfy the requirements of the present invention, it exceeded 100μm plastic deformation at break separation is good fracture splitting of the condition.
　Comparative Example 114, the content of N has exceeded the upper limit of the range of the present invention, the steel manufacturing stage, i.e., flaws to the multiple casting and hot rolling step. Therefore, it is determined that Comparative Example 114 is inappropriate example as fracture split members, evaluation of fracture splitting resistance was not performed.
　Comparative Example 115, since the content of N is less than the lower limit of the scope of the present invention, ferrite transformation is not promoted, broken plastic deformation during separation exceeds 100 [mu] m, determined to have no good fracture separability It has been.
[0071]
[Table 1-1]

[0072]
[Table 1-2]

[0073]
[table 2-1]

[0074]
[Table 2-2]

Example 2
[0075]
　Steel 1-2 to 1-4 having the same chemical composition as the steel 1 shown in Table 1-1, were prepared under the conditions described in Table 3, the aspect ratio of 10 super 30 The following Mn sulfide contained in these steels the number density of the measurement. "Time to Zr-on" in Table 3 is the time in minutes from degassing start to Zr on, "total reduction rate" is a whole cross sectional reduction ratio in hot rolling (%), "reduction of area at 1000 ° C. or less" is in hot rolling, a whole cross sectional reduction rate within a period temperature of the steel is 1000 ° C. or less (%), "Mn sulfide number density", the aspect ratio There number density of 10 super 30 the following Mn sulfide (pieces / mm 2 is). Manufacturing conditions that are not listed in Table 3 were the same as those of Examples 1 to 28 and Comparative Examples 101 to 115.
[0076]
[table 3]

[0077]
　As shown in Table 1-1, the steel 1, chemical composition and so any production conditions were also within the scope of the present invention, the number density (Mn sulfides number of aspect ratio 10 super 30 The following Mn sulfide density) falls within the scope of the present invention. On the other hand, as shown in Table 3, although the steel 1-2 to 1-4 chemical composition is within the scope of the present invention, since the production conditions were outside the scope of the present invention, an aspect ratio of 10 super 30 number density of less Mn sulfide is below the range of the present invention.
　Steel 1-2 is an example in which Zr is added after 15 minutes ultrasonic elapsed since the degassing start. Since Zr oxide is time to sufficiently miniaturize the Mn sulfide was not secured, it is estimated that the number density of the aspect ratio of 10 super 30 The following Mn sulfides in the steel 1-2 is insufficient.
　Steel 1-3 is an example whole cross sectional reduction ratio of the hot rolling is less than 80%, the steel 1-4, Mn sulfides in the temperature range below 1000 ° C. is a temperature region enabling easy stretch reduction of area is an example of less than 50%. Since Mn sulfide is not sufficiently stretched during hot rolling, it is estimated that the number density of the aspect ratio of 10 super 30 The following Mn sulfides in the steel 1-3 and 1-4 is insufficient.
Industrial Applicability
[0078]
　Hot rolled steel and steel parts of the present embodiment, upon fracture splitting, plastic deformation of the fracture surface vicinity is small and chipping occurrence of fracture surface is reduced. Therefore, when was the fitting of a fracture surface, without causing positional deviation, the accuracy can be well fitted, can be realized accuracy of the steel parts, the yield improved simultaneously. Further, by using a hot-rolled steel and steel parts of the present embodiment, it is possible to omit the step of shake off the chipping, can reduce the manufacturing cost, thereby a large effect in improving the economic efficiency of the industrial there is.
DESCRIPTION OF SYMBOLS
[0079]
1 test piece
2 holes
3 V notch
4 through holes
10 steel part
11 Mn sulfide
12 Crack
-out 21 fracture direction crack
22 pull direction step

The scope of the claims
[Claim 1]
　Ga chemical composition of C:
0.35 ~ 0.45 mass%, Si: 0.6 ~ 1.0 mass%, Mn: 0.60 ~ 0.90mass%, P: 0.035mass ~ 0.010%, S: 0 0.10 mass% ~ .06, of Cr: 0.02 ~ 0.25 mass% or less, V: 0.20 ~ 0.40mass%, Zr: from 0.0001 ~ 0.0050 mass% or less, N: 0.0060 ~ 0 % .0150mass of Ti: 0 ~ 0.050mass%, of Nb: 0 ~ 0.030mass%, of Mg: 0 ~ 0.0050 mass%, yoびおREM: 0 ~ 0.0010mass% wo shi containing, remnants ga Fe and impuritiesびkara ri na , 　metallic structureの90 area% ga fu S Factory LITEとpaーLITEとkara configuration Connecticut DomNode-, 　-pressure casting direction ni parallel na sectionalでmeasured Connecticut DomNode- ru, before note-pressure casting direction ni in ~ te extending Connecticut re taアSpectrapore Suites less than 30 over 10 ga Mn sulfideののga average number density of 50 to 200 / mm 2でthou ru koととwo special Zhi-pressure heat between suru steel casting.

[Claim 2]
　The chemical
composition,
Ti: 0.005 ~ 0.050
mass%, Nb: 0.005 ~ 0.030 mass%, Mg: 0.0005 ~ 0.0050 mass%, and
REM: 0.0003 ~ 0.0010mass%
from containing one or more selected from the group consisting of
hot-rolled steel according to claim 1, characterized in that.
[Claim 3]
　Ga chemical composition of C:
0.35 ~ 0.45 mass%, Si: 0.6 ~ 1.0 mass%, Mn: 0.60 ~ 0.90mass%, P: 0.035mass ~ 0.010%, S: 0 0.10 mass% ~ .06, of Cr: 0.02 ~ 0.25 mass% or less, V: 0.20 ~ 0.40mass%, Zr: from 0.0001 ~ 0.0050 mass% or less, N: 0.0060 ~ 0 % .0150mass of Ti: 0 ~ 0.050mass%, of Nb: 0 ~ 0.030mass%, of Mg: 0 ~ 0.0050 mass%, yoびおREM: 0 ~ 0.0010mass% wo shi containing, remnants ga Fe and impuritiesびkara ri na , 　metallic structureの90 area% ga fu S Factory LITEとpaーLITEとkara configuration Connecticut DomNode-, 　-pressure casting direction ni parallel na sectionalでmeasured Connecticut DomNode- ru, manufactured Spectrapore ku Suites ratio ga 10 super 30 the followingのMn sulfur Gaのaverage number density was 50 to 200 / mm 2でthou ru koととspecial Zhi wo suru steel parts.

[Claim 4]
　When the steel component to form a fracture by breaking tensile by parallel tensile stress in the rolling direction,
　the observed in the cross section parallel to the rolling direction, 80 [mu] m or more in a direction parallel to the tensile stress of having a height difference, the step angle with respect to the direction parallel to the tensile stress is less than 45 degrees, the fracture surface is formed at an average number density of more than two locations per 10mm to,
　the parallel to the rolling direction observed in cross-section, the angle with respect to the direction parallel to the tensile stress is 45 degrees greater, are formed over more than the length 80 [mu] m, partially inside Taki progress crack or recess of the steel part average number density, the limited to less than three per 10mm at fracture,
　brittle fracture fracture in the fracture surface is 98 area% or more
steel part according to claim 3, characterized in that.
[Claim 5]
　The chemical
composition,
Ti: 0.005 ~ 0.050
mass%, Nb: 0.005 ~ 0.030 mass%, Mg: 0.0005 ~ 0.0050 mass%, and
REM: 0.0003 ~ 0.0010mass%
from one or containing two or more selected from the group consisting of
steel part according to claim 3 or 4, characterized in that.