0001]The present invention is related to hot-rolled steel.
　The present application, on October 19, 2015, claiming priority based on Japanese Patent Application No. 2015-205436, filed in Japan, the contents of which are incorporated here.
Background technique
[0002]
　Engine parts for parts and foot around the vehicle, the steel was molded by hot forging, obtained by performing any heat treatment such as quenching tempering the (hereinafter referred to as tempering). 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]
　Connecting rod (hereinafter referred to as connecting rod) as a case for automobile engine parts and the like. 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 transferring power dynamic for 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 separation. 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 fracture separated fracture each other, 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 machining steps can be significantly reduced, the economic efficiency during component manufacturing can be greatly improved.
[0006]
　As steel subjected to 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 is composed of ductility and toughness lower pearlite structure. C70S6 Since plastic deformation of fracture near at break is small, excellent fracture splitting property. 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 deformation is increased, the problems on the part performance such roundness decreases.
[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, uneven as the increase of by fracture splitting surface (fracture 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 addition of embrittling elements such as Si or P in a large amount, a technique for improving the fracture splitting property by lowering the ductility and toughness of the material itself have been described. Patent Document 3 and Patent Document 4, by utilizing the precipitation strengthening of second phase particles to lower the ductility and toughness of the ferrite, thereby discloses a technique for improving the fracture splitting property. Further, Patent Documents 5-7, a technique for improving the fracture splitting property 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 surfaces to each other together. When chipping fracture 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 chipping and cracking becomes higher frequency of occurrence at break, and increased fracture uneven, simultaneously achievable steels both the chipping and cracks prevention 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, chipped, there is a tendency that the frequency of occurrence of cracks is high. This is when the pulling direction of the unevenness of the fracture surface is formed, cracking or the Ki develop into the fracture direction is because the recess is also formed. Fitted to fracture each other when applying the stress to fracture in order to conclude a fracture together, cracking or Taki progress in the fracture direction is recessed a stress concentration portion, wherein fine destruction It believed to occur. 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, there is the relationship contradictory to the prevention of chipping and cracks, achievement of both could not be solved by the current method.
[0012]
　In addition to the above, when the connecting rod manufacturing, cutting of drilling due drills are also important. By improving the machinability, because the work is more efficient, productivity can be produced that a significant economic effect of improving. That is, in the fracture split connecting rod, it is necessary to improve the machinability without impairing mechanical properties.
CITATION
Patent Document
[0013]
Patent Document 1: Japanese Patent No. 3637375 discloses
Patent Document 2: Japanese Japanese Patent No. 3756307
Patent Document 3: Japanese Patent No. 3355132 discloses
Patent Document 4: Japanese Japanese Patent No. 3988661
Patent Document 5: Japanese Patent No. 4314851 discloses
Patent Document 6: Japanese Patent No. 3671688 discloses
Patent Document 7: Japanese Patent No. 4268194 discloses
Patent Document 8: Japanese Patent No. 5522321 Publication
Summary of the Invention
Problems that the Invention is to Solve
[0014]
　In view of the above circumstances, to reduce the fracture near deformation amount during fracture splitting, and, while increasing the compatibility fitted to increase the unevenness of the fracture surface, to suppress chipping occurrence of fracture, further and to provide an excellent hot-rolled steel and steel parts machinability.
Means for Solving the Problems
[0015]
　The gist of the present invention is as follows.
[0016]
(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: 0.25mass% or less, V: 0.20 ~ 0.40mass%, Zr: 0.0002 ~ 0 .0050mass%, N: 0.0060 ~ 0.0150mass %, Bi: 0.0001 ~ 0.0050mass%, Ti: 0 ~ 0.050mass%, Nb: 0 ~ 0.030mass%, Mg: 0 ~ 0. 0050Mass%, and REM: 0 contained ~ 0.0010Mass%, the balance being iron and impurities, more than 90 area% of the metal structure is composed of ferrite and pearlite, the rolling direction Measured in a cross section parallel, a 1 ~ 5 [mu] m is equivalent circle diameter, and the average number density of the Mn sulfide aspect ratio is stretched along the rolling direction is 10 super 30 or less 50 to 200 / Mm 2 a is wood.
(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 0.0003 to 1 or two or more of 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: 0.25mass% or less, V: 0.20 ~ 0.40mass%, Zr: 0.0002 ~ 0. 0050mass%, N: 0.0060 ~ 0.0150mass %, Bi: 0.0001 ~ 0.0050mass%, Ti: 0 ~ 0.050mass%, Nb: 0 ~ 0.030mass%, Mg: 0 ~ 0.0050mass %, and REM: 0 contained ~ 0.0010Mass%, the balance being iron and impurities, more than 90 area% of the metal structure is composed of ferrite and pearlite, in the rolling direction Rights Is measured in a cross section, a circle equivalent diameter of 1 ~ 5 [mu] m, and an average number density of 50 to 200 pieces / mm of the Mn sulfide aspect ratio is stretched along the rolling direction is 10 super 30 or less 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 the tensile stress has a height difference of more than 80μm in the direction parallel, the step angle is 45 ° or less with respect to the direction parallel to the tensile stress, the fracture surface 2.0 points per 10mm in is formed at an average number density of above, wherein is 98% or more brittle fracture fracture surface in the area ratio of fracture, the observed in the cross section parallel to the rolling direction, with respect to the direction parallel to the tensile stress angle is 45 degrees greater, it is formed over a length greater than 80 [mu] m, the average number density of to crack or recesses progress inside the partially said steel part, 10 mm per 3.0 places in the fracture surface It is less than There.
(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: 0.0003 of ~ 0.0010mass% may contain one or two or more.
Effect of the invention
[0017]
　Hot rolled steel and steel parts according to one embodiment of the present invention, upon fracture splitting, small plastic deformation of fracture near and chipping occurrence of fracture is reduced. Therefore, when was the fitting of a fracture, without causing positional deviation, the accuracy can be well fitted, can be realized accuracy of the steel parts, the yield improved simultaneously. Further, hot rolled steel and steel parts according to one embodiment of the present invention contains the Bi ultratrace, high machinability. Further, by using a steel material and steel part according to one embodiment of the present invention, 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. Hot rolled steel according to one embodiment of the present invention is suitable for applications of the steel part obtained by molding by hot forging, in particular, suitable for use for use again fitted to fracture each other after fracture splitting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018]
[FIG. 1A] is a plan view showing a fracture isolation test piece for evaluation.
Is a side view of the FIG. 1B] fracture splitting of a test piece for evaluation.
It is a schematic diagram of the crack growth of the steel part according to FIG. 2 embodiment.
DESCRIPTION OF THE INVENTION
[0019]
　The following describes exemplary hot-rolled steel and steel parts in the form of the present invention.
[0020]
　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.
[0021]
　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.
[0022]
　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. 2, 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.
[0023]
　Furthermore, the present inventors have found that the addition of 0.0001 ~ 0.0050 mass% of Bi in the steel was found to improve machinability.
　Machinability and is an index indicating ease of machining. For example, chips short steel occurring when it is machined, the machinability is judged good. If chips are long, since chips hinders cutting, the shorter chips becomes higher efficiency of cutting.
[0024]
　Also in the prior art, Bi is was known to improve the machinability. For example about 0.1mass% or more Bi by acting as a fracture origin in chip formation regions, to reduce the cutting resistance, the effect of shortening the chip. However, such a mechanism, when the Bi content is less than about 0.1%, Bi is hardly acts as a fracture origin, since more number of fracture origin is reduced, and that it does not express enough . Meanwhile, Bi since also the elements aggravates the hot forging of steel, especially if there is no reason, it is necessary to reduce as much as possible the content. Therefore, according to the prior art, in the production of steel machinability between not required heat forging, Bi amount must low as possible, in the case of improving the machinability by utilizing Bi It had been a must to about 0.1% or more of Bi content. Further, to enhance both the hot forgeability and machinability, example attempted to achieve by using Bi no.
[0025]
　However, the present inventors have, 0.0001 ~ 0.0050 mass% of ultra-trace amounts of Bi has been found to improve the machinability of the steel. Bi itself ultratrace is not believed to act as starting points of fracture in chip formation region. However, the present inventors have found that Bi ultratrace crystal interface of the steel, and the interface between the matrix and inclusions (hereinafter referred to as "interface") segregated to. When Bi at the interface is segregated, bonding strength between the bonding strength or the matrix phase and inclusions crystal between is reduced. Therefore, when Bi at the interface is segregated, the interface lowers the cutting resistance working as fracture origin, it is to shorten the chip. Based on the present inventors' knowledge Machinable improved mechanism for the ultratrace Bi, Bi amount of the hot rolled steel of the present embodiment is much less 0.0001 ~ than the amount normally used as free-cutting agent It has been a 0.0050mass%. Thus, the hot rolled steel of the present embodiment, the machinability is improved without impairing the hot forgeability.
[0026]
　Hot rolled steel and chemical components of the steel component according to an embodiment of the present invention obtained based on the above findings, the form of Mn sulfide, and will be described embodiments of the fracture surface obtained by fracture split. 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.
[0027]
　Hot rolled steel of the present embodiment, as chemical components, a steel containing C, Si, Mn, P, S, Cr, V, Zr, N and Bi 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 further as chemical components, Ti, Nb, Mg, may optionally contain one or two or more of the REM.
[0028]
　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.
[0029]
C: 0.35 ~ 0.45Mass% C
　is effective to secure the hot-rolled steel and the tensile strength of the steel part of the present embodiment, and, better to reduce the amount of plastic deformation fracture near at break It has the effect of realizing the fracture isolation. 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, 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, the amount of plastic deformation fracture near the hot rolled steel is increased, fitting property decreases the fracture. A preferable lower limit of the C concentration is 0.36Mass%, or 0.37mass%. Preferred upper limit of the C content is 0.44mass%, 0.42mass%, or 0.40%.
[0030]
Si: 0.6 ~ 1.0
　mass% Si is to strengthen the ferrite by solid solution strengthening, thereby reducing 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 near at break, improve the fracture separation of the hot-rolled steel and steel parts. 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 fracture occurs 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%.
[0031]
Mn: 0.60 ~
　0.90Mass% Mn strengthens the ferrite by solid solution strengthening, thereby reducing 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 near at break, improve the fracture separation of the hot-rolled steel and steel parts. Further, Mn forms a Mn sulfide combines with S. When fracture splitting a steel part obtained from hot rolled steel of the present embodiment, crack has along the Mn sulfide elongated in the rolling direction is propagated. Therefore, Mn has the effect of preventing increase the unevenness of the fracture surface, the positional deviation in fitting the fracture surfaces. However, if the Mn content is insufficient, there is a case where the precipitation temperature of Mn sulfide is below crystallization temperature range is proper range, the deposition temperature range. The crystallization is that the other materials from the liquid phase to produce separation, precipitation and is that the other material is generated separate from the solid phase. Mn sulfide caused by crystallization tends to be coarse than Mn sulfide caused by precipitation. If the precipitation temperature of Mn sulfide is precipitated temperature range, the density of the Mn sulfides of propagating cracks in becomes excessive, the crack is divided into tensile direction, step growth of the pulling direction is inhibited there is a case that. On the other hand, if you want to over-containing Mn, the frequency of chipping at the time of broken ferrite it 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%.
[0032]
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. Reduction in ductility and toughness is to reduce the amount of plastic deformation fracture near at break, improve the fracture separation of the hot-rolled steel and steel parts. However, P causes excessive embrittlement of the grain boundaries, the lack of fracture more likely to occur. Therefore, a method of reducing the ductility and toughness by utilizing the addition of P should not be actively utilized in view of preventing chipping. Considering the above, the range of the P content is set to 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%.
[0033]
S: 0.06 ~ 0.10 mass%
　S combines with Mn to form the Mn sulfides. When to fracture dividing the obtained steel part 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 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 excessively contained, increasing the amount of plastic deformation near the fracture during fracture splitting, if fracture splitting resistance is lowered. Furthermore, S Excess may facilitate the chipping of fracture. 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%.
[0034]
Cr: 0.25 mass% or less
　Cr is to strengthen 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 near at break, improve the fracture separation of the hot-rolled steel and steel parts. To obtain the effect, it is preferable to set the lower limit of the Cr content to 0.02 mass%. However, if excessively contained Cr, lamellar spacing of pearlite is reduced, rather higher ductility and toughness of the pearlite. Therefore, plastic deformation of fracture 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 0.05mass%, 0.06mass%, 0.08mass% , or 0.10 mass%. Preferred upper limit of the Cr content is 0.23mass%, 0.20mass%, or 0.18mass%.
[0035]
V: 0.20 ~ 0.40%
　V, by forming the main carbides or carbonitrides during cooling after hot forging, to strengthen the ferrite, the ductility and toughness of the hot-rolled steel and steel parts decrease. Reduction in ductility and toughness, the amount of plastic deformation fracture near at break of the hot rolled steel and steel parts is reduced, to improve the fracture separation of a steel part made of hot-rolled steel. Also, V is an effect of increasing the yield ratio of the hot-rolled steel by precipitation strengthening of carbide or carbonitride. In order to obtain these effects, it is necessary that the lower limit of the V content to 0.20 mass%. The lower limit of the V content is preferably 0.23mass%, or 0.25 mass%. On the other hand, even if excessively contained V the effect because saturation, the upper limit of the V content is 0.40%. Preferably the upper limit of the V content is 0.38%, or 0.35mass%.
[0036]
Zr: 0.0002 ~ 0.0050
　mass% 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 near the hot rolled steel and steel components is reduced, an effect of increasing the fracture splitting property. In order to obtain these effects, it is necessary that the lower limit of Zr content in 0.0002mass%. However, since Zr has be contained excessively, the effect is saturated, and 0.0050 mass% to the upper limit of the Zr content. 0.0005 mass% the lower limit of the Zr content, or it is preferable to 0.0010mass%. Preferred upper limit of the Zr content is 0.0045mass%, 0.0040mass%, 0.0030mass% , or 0.0029mass%.
[0037]
N: 0.0060 ~ 0.0150Mass%
　N is by forming a hot predominantly V nitride during cooling after forging or V carbonitride act as transformation nuclei for ferrite and promotes the 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%, 0.0080mass% , or 0.0085mass%. Preferred upper limit of the N content is 0.0140mass%, 0.0130mass%, or 0.0120mass%.
[0038]
Bi: 0.0001 ~ 0.0050
　mass% is Bi segregated at the interface between the inclusions and grain boundaries, and the matrix phase, by reducing the bond strength at the interface, the effect of lowering the deformation resistance during cutting . According to the prior art, considered as a Bi itself to act as a fracture origin during cutting deformation, thereby to improve the machinability, it is necessary to approximately more than 0.1mass% of Bi content It is to have. However, the present inventors have found that rather than using Bi as fracture origin during cutting deformation, the use as an element embrittle the surface, is also a 0.0050 mass% or less of Bi improves the machinability It was found that.
　Although the lower limit of the Bi content for expressing the above-described effects were 0.0001% as the preferred range for sufficiently effective, the Bi content may be more than 0.0015%. However, if the Bi amount is 0.0050 mass% greater, there is a case where hot forging of the hot rolled steel and steel parts is deteriorated. Also, if the Bi amount is 0.0050 mass% greater, the interface is excessively brittle and may lack fracture is likely to occur. Therefore, the upper limit of the Bi content was 0.0050 mass%. Bi content is less 0.0045Mass%, less 0.0040mass%, 0.0035mass% or less, or it is preferably not more than 0.0030mass%.
[0039]
　Hot rolling steel material according to the present embodiment, in order to even more pronounced the effect of the invention, further, Ti: 0.050 mass% or less, Nb: 0.030 mass% or less, Mg: 0.0050 mass% or less, and REM : can be selected and contained one or more of the following 0.0010mass%. However, Ti, Nb, Mg, and even if the REM is not contained, the hot rolled steel and steel parts according to the present embodiment can solve the problem. Therefore, the lower limit of Ti, Nb, Mg, and REM are 0mass%.
[0040]
Ti: 0 ~ 0.050 mass% Ti
　is formed mainly carbides or carbonitrides during cooling after hot forging, to strengthen the ferrite by precipitation strengthening, thereby ductility and toughness of the hot-rolled steel and steel parts reduce the. Reduction in ductility and toughness is to reduce the amount of plastic deformation fracture near at break, thereby improving the fracture splitting property. However, its effect is saturated if excessively containing Ti. If the inclusion of Ti in order to obtain the effect described above, it is preferable to 0.050 mass% 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%.
[0041]
Nb: 0 ~ 0.030 mass% Nb
　is formed mainly carbides or carbonitrides during cooling after hot forging, to strengthen the ferrite by precipitation strengthening, thereby ductility of the hot rolled steel and steel parts and lowering the toughness. Reduction in ductility and toughness is to reduce the amount of plastic deformation fracture 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, it is preferable to 0.030 mass% the upper limit of Nb content. In order to sufficiently exhibit the effect of Nb is preferably set 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 0.0030mass%, 0.028mass%, or 0.025%.
[0042]
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 near enhancing fracture splitting of the hot-rolled steel and steel parts. However, since Mg has its effect is contained excessively is saturated, it is preferable to 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%, 0.0025mass%, or 0.0015%.
[0043]
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 near enhancing fracture splitting 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%. The upper limit of a more preferable content of REM is 0.0009mass%, or 0.0008mass%. Incidentally, the term "REM" refers to a total of 17 elements consisting of Sc, Y and lanthanoid, The "content of REM" means the total content of these 17 elements. When using a lanthanoid as REM, the industrial, REM is added in the form of misch metal.
[0044]
　The remainder of the chemical composition of the hot rolled steel and steel parts according to the present embodiment is iron 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 the present embodiment, within a range not to impair the effect may include elements other than the above components. For example 0 ~ 0.01 mass% of Te, 0 ~ 0.01 mass% of Zn, and 0 ~ Sn etc. 0.01 mass%, because does not impair the effect of the hot-rolled steel and steel parts according to the present embodiment, it may be contained.
[0045]
Metal structure 90 constituted area% or more from the ferrite and pearlite
　hot in this embodiment rolled steel and steel parts metal structure, 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.
[0046]
　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%, so long as more than 90% by area of ​​the metal structure is composed of ferrite and pearlite, 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.
[0047]
The average number density of measured in a cross section parallel to the rolling direction, the circle equivalent diameter of 1 ~ 5 [mu] m, and Mn sulfide aspect ratio is stretched along the rolling direction is 10 super 30 the following: 50 to 200 pieces / 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 to form a suitable uneven shape fracture surface obtained by fracture tensile hot-rolled steel and steel parts.
[0048]
　The hot-rolled steel and steel parts according to the present embodiment, the equivalent circle diameter of 1 ~ 5 [mu] m, and Mn sulfide aspect ratio is lengthening is 10 super 30 below the rolling direction as the long axis side 1mm 2 distributed at 200 or less per 50 or more. 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 together. Mn sulfide satisfying circle equivalent diameter defined and aspect ratio defined above is capable of optimizing the pulling direction size of the irregularities. The number of Mn sulfide satisfying circle equivalent diameter defined and aspect ratio defined above is 50 to 200 pieces / mm 2 when it is, it is possible to optimize the number of irregularities.
[0049]
　Mn sulfide aspect ratio of 10 or less, and the equivalent circle diameter of Mn sulfide of less than 1μm can not be sufficiently increased tensile direction size of the unevenness of the fracture surface, fitting the fracture surfaces between polymerizable of not contributing to the improvement. Mn sulfide aspect ratio exceeds 30, and the circle equivalent diameter is 5μm greater than Mn sulfide, although remarkable irregularities of fracture, because it increases the frequency of cracking and chipping, fitting the fracture surfaces between polymerizable the cause impaired. Therefore, the number density of Mn sulfide which do not satisfy the circle equivalent diameter defined and aspect ratio defined above is preferably small. However, number density of Mn sulfide satisfying circle equivalent diameter defined and aspect ratio defined above is in the range described above, content of the product source and comprising Mn and S in and Mn sulfide is within the above range If, Mn and S in the chemical components are consumed by the Mn sulfides satisfying the circle equivalent diameter defined and aspect ratio defined above, generation of Mn sulfide which do not satisfy the circle equivalent diameter defined and aspect ratio defined above is sufficiently It is suppressed. Therefore, the number density of Mn sulfide which do not satisfy the circle equivalent diameter defined and aspect ratio defined above is not particularly limited.
[0050]
　If the average number density of Mn sulfide satisfying circle equivalent diameter defined and aspect ratio defined above is less than the lower limit, the number of irregularities in the fracture is reduced, fitting the fracture surfaces after fracture splitting polymerizable is not enough to become. Further, when the average number density of Mn sulfide satisfying circle equivalent diameter defined and aspect ratio defined above is less than the lower limit, the number density of Mn sulfide which do not satisfy the circle equivalent diameter defined and aspect ratio defined above is increased to, may impair fracture splitting or fitting property. On the other hand, the lower limit of the average number density of Mn sulfide satisfying circle equivalent diameter defined and aspect ratio defined above, 80 / mm 2 , 95 pieces / mm 2 , or 110 amino / mm 2 may be.
　Further, if the average number density of Mn sulfide satisfying circle equivalent diameter defined and aspect ratio defined above is greater than the upper limit, cracks in fracture, chipping occurs, again fitting the fracture polymerizable is impaired It is. Further, if the average number density of Mn sulfide satisfying circle equivalent diameter defined and aspect ratio defined above is greater than the upper limit, cracks in the tensile direction becomes tends to be divided, step growth in the tensile direction is inhibited there is a case to be. On the other hand, the upper limit of the average number density of Mn sulfide satisfying circle equivalent diameter defined and aspect ratio defined above, 195 / mm 2 , 180 pieces / mm 2 , or 160 / mm 2 may be.
[0051]
　Hot rolled steel and steel parts, the measurement method of the average number density of Mn sulfide satisfying circle equivalent diameter defined and aspect ratio defined above 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, it is possible to determine the number density of Mn sulfide satisfying circle equivalent diameter defined and aspect ratio of above defined in a region where the picture was taken. Incidentally, some lengthening been Mn sulfide, some distributed aggregated in rows in the rolling direction are divided. However, it is arranged along the rolling direction (extension direction), and two Mn sulfide distance between the end portion is 10μm or less is considered one elongated Mn sulfide. Such two Mn sulfides in that propagate crack occurring when to break tensile hot-rolled steel or steel components in the pulling direction, because considered to exhibit the same behavior as the one Mn sulfide it is.
　Furthermore, repeated at least 10 times the analysis and photography, by averaging the number density obtained by this, the average number density of Mn sulfide satisfying circle equivalent diameter defined and aspect ratio defined above is obtained.
[0052]
　Next, a method for manufacturing a hot rolled steel of 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.
[0053]
　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, circle equivalent diameter of Mn sulfide becomes outside the scope, the Mn sulfide excessively stretched in bloom rolling step after addition, the circle equivalent diameter defined and aspect ratio of above Mn sulfide increases which does not satisfy the regulations.
[0054]
　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 according to the present embodiment in the. Incidentally, the rolling reduction of area at the time of the billet to the round bar shape is preferably 80% or more. Thus, it is possible to extend the Mn sulfides in the steel. 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.
[0055]
　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, cooling (cooling by standing the steel in the atmosphere), or air blast cooling (cooling sends wind to the steel ) by cooling to room temperature. 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.
[0056]
　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 has a height difference of more than 80μm toward a 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 2.0 points per 10mm to fracture. That is, if the drawn arbitrary line (for example line segment that passes through fracture center) to fracture the hot-rolled steel and steel parts of the present embodiment is formed by breaking tensile by parallel tensile stress in the rolling direction, the number density of the above steps on that line is an average 2.0 or / 10 mm or more. Further, 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, parallel to the tensile stress an angle of 45 ° than with respect to the direction, is formed over a length greater than 80 [mu] m, the average number density of to crack or recesses progress inside the partially steel parts, 3.0 points per 10mm in fracture It is limited to less than. Further, also, the 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, 98 areas in the brittle fracture fracture surface area ratio of the fracture % greater than or equal to.
[0057]
　The reason why defined for the nature of the fracture surface. The fracture surface together formed by tensile fitted, when stress is applied in the horizontal direction to fracture, the stress by the unevenness of the fracture, 90 ° in the horizontal direction and two in the normal direction (in-plane inclination direction, and are to fracture a three-dimensionally in the vertical direction) dispersion. 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.
[0058]
　Chipping generation amount, cracking or the Ki extending in a direction perpendicular to the pulling direction in the fracture (fracture direction) is correlated with the presence of the recess. 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, the fracture surface direction of the feeder 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 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 3.0 points per 10 mm.
[0059]
　In particular, since Mn sulfide form 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 is to control the dispersion state in the form of Mn sulfide is 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 . Therefore, in the hot-rolled steel and steel parts according to the present embodiment, a significant fracture uneven feasible experimentally to the extent that does not cause chipping of the fracture surface at break was defined as described above.
[0060]
　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.
[0061]
　Evaluation method of fracture surface shape is as follows.
　Area ratio of the brittle fracture surface occupies the fracture 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, together per the fracture assembled and bolted to a torque of 20 N · m, then repeat operations 10 times that by loosening the bolts release the fracture, the total weight of the debris to fall off by this was measured and the total weight determined by regarding the chipping occurrence of fracture.
　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 to measure the number density of the tensile direction step and fracture direction crack may can be formed anywhere as long as the test piece is parallel to the pulling direction, for convenience, fracture in the cutting plane it is preferable to form a cutting surface so the surface becomes as large as possible. 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.
[0062]
　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.
[0063]
　Hot rolled steel and steel parts of the present embodiment, upon fracture splitting, small plastic deformation of fracture near and chipping occurrence of fracture is reduced. Therefore, when was the fitting of a fracture, without causing positional deviation, the accuracy can be well fitted, can be realized accuracy of the steel parts, the yield improved simultaneously. Further, by containing Bi ultratrace in the steel, thereby improving the machinability. 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.
Example
[0064]
　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
[0065]
　Table 1 and having the composition shown in Table 2, were melted in a converter steel was prepared bloom by continuous casting, the bloom, the billet 162mm angle through the slabbing process, further by hot rolling diameter has a round rod shape of 56mm. During melting of the steel than steel O1 is degassing processes before or within 15 minutes after the start of the degassing process is performed molten steel using a RH, and adding Zr to the molten steel. During the melting of steel O1 it is, was not added to the Zr. When a round bar shape 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 a less than the detection limit value (impurity levels). 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 Table 2 is a numerical value outside the range of the present invention.
[0066]
　Included in the steel obtained by the method described above, measured in a cross section parallel to the rolling direction, is 1 ~ 5 [mu] m is equivalent circle diameter, and an aspect ratio which is stretched along the rolling direction is 10 super 30 or less the average number density of certain Mn sulfide (Mn sulfide number density) was calculated by the following method. First, the steel was cut parallel to the rolling direction was polished cut surface. Then, the enlarged photograph of the cut surface of the steel were taken by an optical microscope or an electron microscope. Magnification at this time was 100 times. Then, a photo by image analysis to determine the Mn sulfide number density in the area where the picture was taken. Incidentally, are arranged along the rolling direction, and two Mn sulfide distance between the end portion is 10μm or less, it was regarded as one elongated Mn sulfide. Furthermore, repeating the analysis and photography 10 times, by averaging the number density obtained by this to determine the Mn sulfide number density of steel.
[0067]
　Further, the total amount of ferrite and pearlite is included in the structure of the steel obtained by the method described above, the steel was cut, polished and etched cutting plane, taken an optical micrograph of the cut surface, the photographic image It was determined by analysis. Incidentally, repeated four times above steps the average value of the total weight of the obtained ferrite and pearlite were considered the total amount of ferrite and pearlite in the steel. As a result of the measurement, that more than 90% by area of ​​the metal structure of all the present invention steels is composed of ferrite and pearlite it was confirmed.
[0068]
　Next, in order to examine the fracture separation of a forged connecting rod considerable specimen was prepared by hot forging. Specifically, the diameter in the above steps 56 mm, a is a material steel bar length 100mm steel, after heating to 1150 ~ 1280 ° C., and a thickness of 20mm and forged perpendicular to the length direction of the steel bar. The forged steel was cooled to room temperature by air cooling (cooling by allowing to stand in the air) or air blast cooling (cooling sends wind to the test piece). 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. 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.
[0069]
　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. 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.
[0070]
　Method for measuring the area ratio of the brittle fracture surface occupied in fracture (brittle fracture area ratio) were as follows. First, the falling weight is fractured steel height 100 mm, were 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 deformation amount at the time of fracture splitting (deformation amount) was 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 (chipping generation amount) were as follows. After the deformation amount measurement described above, assembling and bolting a torque of 20 N · m by butt-jointing the fracture, then repeated work 10 times that by loosening the bolts release the fracture surface. Thereby measuring the total weight of the debris to fall off, the total weight was chipping occurrence of fracture.
[0071]
　The fracture separation of good steel, fracture morphology of the fracture surface is brittle, and small amount of deformation of the vicinity of fracture by fracture splitting a and chipping generation amount is small steel. The present inventors are the area ratio of the brittle fracture surface is 98% or more, the amount of deformation of the fracture near is not less 100μm or less, and the missing steel generation amount is equal to or less than 1.0 mg, good fracture splitting property it is determined that the steel having a.
[0072]
　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-out 裂数) is a fracture surface which is limited to less than 3.0 points per 10mm are formed of steel, were considered fracture splitting resistance good steel. 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) (tensile direction step number), the sample fracture surface is formed at least 2.0 points per 10 mm, it was regarded as fitting property higher sample.
[0073]
　The number density of the tensile direction step and fracture direction Crack fracture was determined by the following method. First, cutting the specimen in a tensile direction, and can be observed from the direction perpendicular to fracture shape pulling direction. By observing the fracture surface section 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.
[0074]
　As shown in Table 3, preparation No. Both Inventive Examples 1 to 29 have met the acceptance criteria described above, excellent fracture splitting property, fitting property was found to be good at the same time. In addition, production No. For 1-29, among Mn sulfides in the steel, the circle equivalent diameter defined and Mn sulfide satisfying the aspect ratio defined in the present invention, 1 mm 2 were distributed per 50 or more.
[0075]
　On the other hand, as shown in Table 2, production No. 30-46 is, C, Si, Mn, P, S, Cr, V, Zr, N, and the content of one or more of Bi is out of the scope of the present invention. These for the following reasons, as shown in Table 4, it does not meet the requirements of the present invention.
[0076]
　Production No. Each 30,32,36,41,44 C, Si, P, V , the content of N is less than the lower limit of the scope of the present invention, plastic deformation at fracture separation is good fracture splitting of the conditions more than a 100μm.
　Production No. Each 31, 33, 35, 37 C, Si, Mn, P content is above the upper limit of the range of the present invention, chipping generation amount at break of more than 1.0 mg.
　Manufacturing No.34 content of Mn is less than the lower limit range of the present invention, since the generation temperature of Mn sulfide is lower than the appropriate temperature range, crystallization of Mn sulfide is reduced, Mn sulfide the amount of precipitation has increased. As a result, the number density of the size of the Mn sulfides circle equivalent diameter 1 ~ 5 [mu] m exceeds the upper limit, cracks in the tensile direction becomes tends to be divided, since the step growth of the pulling direction is inhibited , production No. Pulling direction step number of the fracture surface of 34 did not meet the requirements of the present invention.
　Production No. 38 is a condition content exceeds the upper limit of the range of the present invention, together with chipping generation amount at break exceeds 1.0 mg, the amount of plastic deformation during fracture splitting is excellent fracture splitting of S 100 [mu] m more than.
　Production No. 39 is less than the lower limit content of the scope of the present invention the S, the volume fraction of Mn sulfide is insufficient elongation degree is, uneven places the number of fracture is less than the requirements of the present invention.
　Production No. 40 content of Cr exceeds the upper limit of the range of the present invention, more than 100μm plastic deformation at break separation is good fracture splitting of the condition.
　Production No. 42 Zr was not present, the distribution of Mn sulfide is dispersed in rough, with uneven portions the number of fracture is less than the requirements of the present invention, the amount of plastic deformation good fracture splitting property at break isolation more than 100μm is a condition.
　Production No. 43 N content exceeds the upper limit of the range of the present invention, the steel manufacturing stage, i.e., the flaw is frequently cast and hot rolling step, the unsuitable as a material to be applied to the steel part.
　Production No. 45 the content of Bi is less than the lower limit of the scope of the present invention. Production No. Fracture splitting of the 45 were within the adequate range, but poor machinability, which will be described later.
　Production No. 46 the content of Bi is above the upper limit of the range of the present invention, the steel manufacturing stage, i.e., the flaw is frequently cast and hot rolling steps, it was unsuitable as a material to be applied to the steel part.
[0077]
　With respect to machinability, investigated by after hot forging the hot rolled steel with a diameter of 56mm as described above to a diameter 25 mm, was cut to a length 500 mm, by using a NC lathe, to turning under the following conditions It was evaluated on the basis of the chip control.
　Chip control was evaluated by the following method. The chips discharged by 10 seconds in machinability tests were recovered. Examining the length of the recovered chips, it was selected 10 chips in order of longer. And the total weight of the 10 pieces of chips that have been selected and defined as "chips weight". Results chips led long when the total number of chips is less than 10, and the recovered value 10 times the 1 per average weight of Chip is defined as a "chip weight". For example, a seven total chip, if the total weight is 12.0 g, chip weight was calculated 12.0 g × 10 cells / 7 = 17.1 g, a.
　Chip used in the machinability evaluation was as follows.
Preform material: Carbide P20 or Grade
Coating: None
　Moreover, turning conditions, follows a was the
peripheral speed: 150 meters / min
Feed: 0.2 mm / rev
cut: 0.4 mm
Lubrication: water-soluble cutting oil used
　sample chips weight of each mark is 15g or less is determined to be high chip control, labeled "GOOD" in tables 3 and 4. Article chips weighing more than 15g is evaluated to be low chip control, labeled "BAD" in Tables 3 and 4. The added even chips weight one for steel within the scope of the present invention the Bi whereas at 15g or less, produced without the addition of Bi No.30 ~ 33 and No. 45 chips weight is more than 15g, it was inferior in machinability.
[0078]
[Table 1]

[0079]
[Table 2]

[0080]
[table 3]

[0081]
[Table 4]

Example 2
[0082]
　Steel as set forth in Table 1 No. Steel having the same chemical composition as the A No. A-2 ~ steel No. The A-4, prepared under the conditions described in Table 5 was measured mean number density of Mn sulfide satisfying circle equivalent diameter defined and aspect ratio provisions of the present invention contained in these steels (Mn sulfide number density) . "Time to Zr-on" in Table 5 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, preparation No. It was the same as those of 1-46.
[0083]
[table 5]

[0084]
　As shown in Table 1, steel No. A is any of the chemical components and production conditions so was within the range of the present invention, Mn sulfide number density falls within the scope of the present invention. On the other hand, as shown in Table 5, Steel No. A-2 ~ steel No. Although A-4 chemical composition is within the scope of the present invention, since the production conditions were outside the scope of the present invention, Mn sulfide number density is below the range of the present invention.
　Steel No. A-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 ensured, the steel No. Mn sulfide number density is estimated to have insufficient in A-2.
　Steel No. A-3 is an example whole cross sectional reduction ratio of the hot rolling is less than 80%, the steel No. A-4 is, reduction rate in the temperature range of 1000 ° C. or less Mn sulfide is at a temperature range enabling easy stretch is an example less than 50%. Since Mn sulfide is not sufficiently stretched during hot rolling, the steel No. A-3 and steel No. Mn sulfide number density is estimated to have insufficient in A-4.

claims

[Claim 1]Chemical composition C:0.35 ~ 0.45Mass%, Si: 0.6 ~ 1.0 mass%, Mn: 0.60 ~ 0.90Mass%, P: 0.010 ~ 0.035Mass%, S: 0 ~ 0.10 mass% .06, Cr: 0.25 mass% or less, V: 0.20 ~ 0.40%, Zr: 0.0002 ~ 0.0050% or less, N: 0.0060 ~ 0.0150Mass%, Bi: 0.0001 ~ 0.0050 mass%, Ti: 0 ~ 0.050 mass%, Nb: 0 ~ 0.030 mass%, Mg: 0 ~ 0.0050 mass%, and REM: 0 ~ 0.0010mass% contain , the balance being iron 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, the circle equivalent diameter of 1 5μm in and, and the average number density of Mn sulfide aspect ratio is stretched along the rolling direction is 10 super 30 or less 50 to 200 / mm 2 is hot-rolled steel, characterized in that.

[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%
of containing one or more among
hot-rolled steel according to claim 1, characterized in that.
[Claim 3]
　Chemical composition C:
0.35 ~ 0.45Mass%, Si: 0.6 ~ 1.0 mass%, Mn: 0.60 ~ 0.90Mass%, P: 0.010 ~ 0.035Mass%, S: 0 ~ 0.10 mass% .06, Cr: 0.25 mass% or less, V: 0.20 ~ 0.40%, Zr: 0.0002 ~ 0.0050 mass% or less, N: 0.0060 ~ 0.0150Mass%, Bi: 0.0001 ~ 0.0050 mass%, Ti: 0 ~ 0.050 mass%, Nb: 0 ~ 0.030 mass%, Mg: 0 ~ 0.0050 mass%, and REM: 0 ~ 0.0010mass% contain , the balance being iron 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, the circle phase Diameter of 1 ~ 5 [mu] m, and an average number density of the Mn sulfide aspect ratio is stretched along the rolling direction is 10 super 30 or less 50 to 200 / mm 2 is steel, characterized in that 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 is 45 ° or less with respect to the direction parallel to the tensile stress, is formed at an average number density of more than 2.0 points per 10mm in the fracture surface,
　brittle in the fracture surface and a fracture fracture more than 98% in the area ratio,
　the observed in the cross section parallel to the rolling direction, the angle is 45 degrees than for parallel the direction to the tensile stress, over more than a length 80μm formed Te, the average number density of progression to crack or recess in the interior of a portion of the steel parts, in the fracture surface is less than 3.0 points per 10mm
of claim 3, characterized in that steel parts.
[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%
of one or containing two or more out
steel part according to claim 3 or 4, characterized in that.