JP4470522B2 - Low toughness free-cutting non-tempered steel - Google Patents

Description

  The present invention relates to a non-tempered steel having a low toughness and excellent machinability and suitable for a connecting rod manufactured by fracture separation.

  When manufacturing a part that is used in a state where two or more individual parts are in close contact with each other at a predetermined abutting surface, the part that has been processed into a final shape is broken and separated at the position corresponding to the abutting surface. A manufacturing method is known (see, for example, Patent Document 1). For example, when manufacturing connecting rods (connecting rods; hereinafter referred to as connecting rods), they are forged integrally in the final shape and finished with machining or other processing as required, and then assembled to the crankshaft. The end is broken and separated. A notch is formed in the large end portion during forging or after forging. For example, a predetermined load is applied from the inner peripheral side to the outer peripheral side to cause the notch to break.

  According to such a manufacturing method, as compared with the case where it is divided into two individual parts by machining, the cutting margin becomes unnecessary, and the material resulting from this is eliminated, and the cutting separation surface is cut. This eliminates the need for finishing by polishing, polishing, etc., thereby reducing the required time and manufacturing cost. In addition, since fine and non-uniform irregularities are formed on the fracture surface, the flat and smooth joint surface that has been cut and polished without using a knock pin for alignment is abutted. Therefore, there is an advantage that high roundness of the inner peripheral surface of the large end portion can be ensured while reducing manufacturing costs.

  For example, in Europe, 0.6 to 0.75 (% by weight) C (carbon), 0.2 to 0.5 (% by weight) Mn (manganese), 0.04 to 0.12 (by weight) %) S (sulfur) (where Mn / S> 3), the balance being Fe (iron) and less than 1.2 (% by weight) of inevitable impurities (for example, Patent Document 2) reference). This steel consists of approximately 100% of pearlite single structure and corresponds to XC70 specified by SAE 1070.

However, although the above steel is excellent in fracture separation, it has low fatigue strength and yield strength and poor machinability, so that it has not been applied to automobile parts in Japan. Therefore, in Japan, in addition to this, non-tempered steel in which a small amount of V (vanadium) is added to a medium carbon steel for connecting rods, P (phosphorus), Ti (titanium), etc. are further added to improve toughness. Steel having improved break separation property by lowering is used.
JP-A-11-286746 US Pat. No. 5,135,587

  By the way, for example, in connecting rod applications for automobile parts, machinability is also an important characteristic in addition to fracture separability. The above-mentioned non-tempered steel added with V, P, and Ti has insufficient machinability, so at least one of carbosulfide inclusions of Ti and Zr (zirconium) is actively added to the base material. Precipitated non-tempered steel has been proposed. The steel described in the said patent document 1 is the example, and this non-tempered steel has the turning ability outstanding also, for example compared with the sulfur addition type non-tempered steel. However, since this non-tempered steel has insufficient drill machinability, it is unsuitable for connecting rods in which large-end bolt hole machining is an important machining step. In addition, it is generally known that machinability is improved by adding Pb (lead), but since it is not environmentally preferable, a non-tempered steel that does not contain Pb and has equivalent or better properties is desired. ing.

  The present invention has been made against the background of the above circumstances, and an object of the present invention is to provide a non-tempered steel that is excellent in machinability and excellent in break separation without containing lead. .

In order to achieve such an object, the gist of the low toughness free-cutting non-heat treated steel of the present invention is that (a) 0.15 to 0.5 ( mass %) C (carbon), 0.4 to 2 ( mass) %) Si (silicon), 0.4 to 1.6 ( mass %) Mn (manganese), 0.03 to 0.35 ( mass %) S (sulfur), and 0.02 to 0.15 ( mass %) P (phosphorus) 0.01 to 0.5 ( mass %) Cu (copper), 0.01 to 0.5 ( mass %) Ni (nickel), 0.01 to 1 ( mass %) Cr (chromium), 0.04 to 0.5 ( mass %) V (vanadium), 0.05 to 0.152 ( mass %) Ti (titanium), 0 to 0.143 (mass%) Zr (zirconium), 0.005 to 0.05 ( mass %) sol-Al (acid-soluble aluminum) and), and N (nitrogen) of 0.008 to 0.03 (wt%), and Ca (calcium) of 0.0005 to 0.01 (wt%), see contains the balance Fe and unavoidable impurities, the components according to (b) weight% When the content ratio is expressed by each element symbol, the following formula 1 is satisfied . (Ti + 0.52Zr) / S <1.9 ... (Formula 1)

In this way, because it is configured as described above, even if it does not contain Pb, it is excellent in machinability after hot forging, in particular, excellent in turning and drill machinability and excellent in break separation. A tough free-cutting non-heat treated steel is obtained. Such non-tempered steel is separated by fracture from the notch, for example, by providing a notch of a predetermined shape at the large end and applying a predetermined load from the inner periphery to the outer periphery. It is suitable for connecting rods manufactured by In addition, coexistence of (Mn, Ca) S and Ti carbosulfide can improve both turning and drill machinability while maintaining high ductility while keeping toughness sufficiently low. Ti, Zr and S have high affinity, so if the amount of S is not large enough relative to the total amount of Ti and Zr, the amount of (Mn, Ca) S produced cannot be increased sufficiently However, according to the present invention, both the turning property and the drill machinability can be further improved by satisfying the above-mentioned formula 1 and coexisting Ti carbon sulfide and the like with (Mn, Ca) S.

  Incidentally, in steel cutting, free cutting components in steel are melted or softened by heat generated by friction and function as a lubricant, and this reduces the frictional resistance between the cutting edge of the cutting part and the work material. Reduced. In the case of turning, the frictional heat generation is very large, so that even if the free-cutting component has a relatively high melting point, the frictional resistance is reduced by melting or softening. However, in the case of connecting rod bolt holes, the frictional heat generation is small even if the rotational speed is the same as that of turning. Therefore, if the free-cutting component has a high melting point, it does not melt or soften and the frictional resistance is not reduced. The conventional non-tempered steel on which Ti carbosulfide has been deposited is superior in turning compared to sulfur free-cutting steel, but the drill machinability is inferior to that of Ti free-cutting steel. This is probably because the melting point is higher than that of MnS (manganese (II) sulfide), a free-cutting component of sulfur steel.

  On the other hand, if Mn, S, and Ti are contained in the proportions as described above, MnS and Ti carbosulfide can coexist, that is, both are dispersed and interposed in the base material (matrix). Therefore, while keeping the toughness sufficiently low and maintaining high fracture separation, the turning performance is improved by Ti carbon sulfide, while the drillability is improved by MnS. Non-tempered steel with high machinability can be obtained. Since MnS has a low hardness of about Hv150, for example, the drill hardly wears even when not softened.

Among the above components, C is an element effective for securing the strength of the forged product by dissolving in iron. In order to acquire such an effect, it is necessary to contain 0.15 ( mass %) or more. Further, a part of C is combined with Ti (and Zr if included) and S to form Ti carbon sulfide, thereby improving turning properties. However, if the content is excessive, the hardness increases and the machinability decreases, so it is necessary to be 0.5 ( mass %) or less.

In addition, Si has deoxidizing and desulfurizing effects when steel is melted, and improves the strength of ferrite, which is a soft phase, which is the main cause of plastic deformation during fracture separation by dissolving in ferrite. By doing so, deformation at the time of break separation is suppressed, and adhesion between fractured surfaces is improved. In order to obtain such an effect, it is necessary to be contained in an amount of 0.4 ( mass %) or more. However, if the content is excessive, the hardness increases and the machinability decreases, so that it is 2 ( mass %) or less. It is necessary to be.

In addition, Mn is an element effective for securing the strength of the forged product, and combines with S and Ca to form MnS and (Mn, Ca) S in which Ca is dissolved (Mn, Ca) S (hereinafter, these are represented by the latter). Generate and improve drill machinability. In order to obtain such an effect, it is necessary to contain 0.4 ( mass %) or more, but when the content is excessive, the pearlite lamella interval is reduced to improve the ductility, and the break separation property is deteriorated. In addition, bainite is generated after forging, the hardness is remarkably increased, and the machinability is lowered, so that it is necessary to be 1.6 (% by mass ) or less.

Similarly to Mn, Cr is an element effective for securing the strength of a forged product. In order to obtain such an effect, the content must be 0.01 (% by mass ) or more. However, if the content is excessive, the pearlite lamella spacing is reduced and ductility is improved, rupture separation is deteriorated and bainite is generated after forging to significantly increase the hardness and reduce machinability. It is necessary to be ( mass %) or less.

In addition, P is an element that adds toughness by segregating at the grain boundaries, and in general steel materials, the content is as low as possible, but in the present invention that requires break separation, It is effective as an element that suppresses deformation and improves the adhesion between fractured surfaces. In order to obtain such an effect, the content must be 0.02 ( mass %) or more. However, if the content is excessive, the effect is saturated while hot workability is inhibited (that is, cracking is likely to occur), so it is necessary to set the content within the range of 0.15 ( mass %). .

S is an element that partially combines with Mn and Ca to generate (Mn, Ca) S and improve drill machinability. The balance is combined with Ti (and Zr if included) and C to form a carbon sulfide and improve the turning performance. In order to obtain such an effect, it is necessary that the content is 0.03 ( mass %) or more, and when it is excessive, hot workability is deteriorated, so 0.35 ( mass %) or less is necessary. .

In addition, Cu and Ni are inevitable impurities, and it is difficult to make the content lower than 0.01 ( mass %) and it is economically disadvantageous, but in order to increase the strength of forgings like Mn and Cr. Since it is an effective element, it may be contained. For this reason, it is necessary to be 0.01 ( mass %) or more. On the other hand, even if it is excessive, it is economically disadvantageous, and bainite is generated to significantly reduce the machinability, so that it is necessary to be 0.5 (% by mass ) or less.

V is an element that combines with C and N to form fine carbonitrides and increases the strength after forging. In order to acquire this effect, it is necessary to contain 0.04 ( mass %) or more. When the content is increased, the effect is saturated, but the hardness is increased and the machinability is lowered, so that it is necessary to be 0.5 (% by mass ) or less.

Ti, like V, is an element that produces fine carbonitrides such as C and N and increases the strength after forging. Further, this fine carbonitride has the effect of greatly reducing the toughness of the steel and suppressing the deformation at the time of fracture separation, thereby improving the adhesion between the fracture surfaces. In addition, Ti has the effect of partly added to combine with C and S to form Ti carbosulfide inclusions and improve turning properties, but in order to obtain these effects, 0.05 ( mass %) Or more is required. On the other hand, if the amount is excessive, these effects are saturated and disadvantageous economically, and the hot workability is hindered. Therefore, the content must be 0.65 ( mass %) or less.

In addition, sol-Al forms N and nitride in steel and is finely dispersed to suppress crystal grain growth during hot forging. To obtain this effect, 0.005 ( mass %) It is necessary to be included above. On the other hand, if the content is excessive, the effect is saturated and economically disadvantageous, so 0.05 ( mass %) or less is necessary.

N forms Ti and nitride, and functions to suppress crystal grain growth during hot forging. Further, N is an element that forms carbonitrides with Ti and V, and strengthens ferrite by fine precipitation of these. In order to obtain these effects, a content of 0.008 ( mass %) or more is necessary. On the other hand, if the amount is excessive, the effect is saturated and casting defects are caused. Therefore, the content must be 0.03 (% by mass ) or less.

Ca is an element that further improves drill machinability by forming a solid solution in MnS to form (Mn, Ca) S. In order to obtain this effect, a content of 0.0005 ( mass %) or more is necessary. Further, if it exceeds 0.01 ( mass %), a high melting point compound CaS (calcium sulfide) is generated, which causes obstacles such as blocking the nozzle in the casting process, so it is necessary to set it to 0.01 ( mass %) or less. .

Here, preferably, the low toughness free-cutting non-heat treated steel satisfies the following formula 1 when the content ratio of each component by mass % is expressed by each element symbol. Ti and Zr have similar functions as components of the non-tempered steel of the present invention, and since the atomic weight of Ti is 47.9 and the atomic weight of Zr is 91.2, Zr is 0.52 (= 47.9 / 91.2 in weight ratio). Equivalent to double mass Ti. Therefore, a part of Ti carbon sulfide can be replaced with Zr carbon sulfide, and these are hereinafter collectively referred to as “Ti carbon sulfide”. However, the Zr content may be zero. According to the present invention, (Mn, Ca) S and Ti carbon sulfide are allowed to coexist, so that the toughness and the drill machinability are maintained while keeping the ductility sufficiently low and maintaining high fracture separation. Both can be increased, but the affinity between Ti and Zr and S is high, so if the amount of S is not sufficiently large relative to the total amount of Ti and Zr, the amount of (Mn, Ca) S produced is sufficiently large. Can not do it. Therefore, in order to further improve both the turning property and the drill machinability by coexisting Ti carbon sulfide and the like and (Mn, Ca) S, it is desirable to satisfy the following formula 1.
(Ti + 0.52Zr) / S <1.9 ... (Formula 1)

Preferably, the low toughness free-cutting non-heat treated steel satisfies the following formula 2 when the content of each component by mass % is represented by the respective element symbol. The following formula 2 is a carbon equivalent formula for obtaining a particularly high strength in non-heat treated steel. When the value expressed by Equation 2 is less than 0.65, the hardness and strength after forging remain relatively low and the deformation at break separation becomes relatively large, so that high dimensional accuracy and shape accuracy can be obtained. It becomes difficult. On the other hand, if it exceeds 0.96, the hardness after forging becomes a relatively high value, the machinability is lowered, and the machining cost is increased.
0.65 ≦ C + 0.07Si + 0.16Mn + 0.61P + 0.19Cu + 0.17Ni + 0.2Cr + V ≦ 0.96 (Formula 2)

Further, preferably, the low toughness free-cutting non-heat treated steel satisfies the following formula 3 when the content of each component by mass % is expressed by each element symbol. Equation 3 below defines the pearlite area ratio. In order to obtain high assembly accuracy by suppressing side slip at the assembly joint surface after fracture separation, it is necessary that irregularities of an appropriate size be formed on the fractured surface after fracture separation. The non-tempered steel of the present invention has a ferrite pearlite structure, but when such a steel breaks brittlely, not only the ferrite pearlite structure boundary, but also the crack progress direction changes at the pearlite block boundary. It is known to do. For this reason, if the pearlite has an appropriate amount of pearlite, that is, a pearlite block size, the crack progresses with a certain degree of variation, and a fracture surface having moderately large unevenness can be obtained. A particularly preferred fracture surface is obtained when the pearlite area ratio is 15 (%) or more, but the pearlite area ratio varies depending on the content of C, Si, Mn, Cu, Ni, Cr, When 3 is satisfied, it becomes 15 (%) or more.
134C-3.6Si + 24Mn + 22Cu + 32Ni + 30Cr-12Ti + 41N ≧ 31 (Formula 3)

Also, preferably, the low ductility of the steel machinability improving microalloyed steel, Se of 0.005 to 0.4 (mass%) (selenium), Te of 0.05 to 0.1 (wt%) (tellurium), and 0.3 (mass% ) It contains at least one component selected from the following Pb (lead). Since these are general elements effective for improving the machinability, when it is desired to further improve the machinability in a forged product, one or more of them are selected as necessary. An appropriate amount may be included.

Among the elements described above, Pb is dispersed in the steel as metal grains, and the cutting resistance is reduced by the notch effect, and the friction between the tool and the chips is reduced by melting by cutting heat, and the machinability is further improved. However, when the content is excessive, since the lower the hot forgeability of the steel, it is desirable to keep 0.3 (mass%) or less. In addition, it is desirable not to contain Pb when the adverse effect on the environment becomes a problem.

Further, Se, Te is to suppress sulfide extension during hot working. As a result, the sulfide has a spherical or spindle shape with a large notch effect, cutting resistance is reduced, and machinability is improved. In order to obtain this effect , Se is preferably 0.005 ( mass %) or more, and Te is 0.05 ( mass %) or more. However, if the content is excessive, the hot workability of the steel is reduced and cracks frequently occur. Therefore , Se is preferably 0.4 ( mass %) or less and Te is preferably 0.1 ( mass %) or less.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the following embodiments, the drawings are appropriately simplified or modified, and the dimensional ratios, shapes, and the like of the respective parts are not necessarily drawn accurately.

  FIG. 1 is a plan view showing the whole connecting rod 10 to which the non-heat treated steel of one embodiment of the present invention is applied in its assembled state. The connecting rod 10 has a small end portion 16 having a small diameter hole 14 at one end located on the left side in FIG. 1 of the main body portion 12 and a large diameter hole 18 having a larger diameter at the other end located on the right side in FIG. Each has a large end 20. In the assembled state, a piston pin is fitted into the small end portion 16 through a metal or the like as necessary, and a crank shaft is fitted into the large end portion 20 through a metal or the like. Omitted.

  The large end portion 20 is composed of a main body side portion 22 and a cap 24 which are divided into two parts by a plane passing through the center of the large diameter hole 18. A semi-cylindrical surface is formed on each of the inner peripheral surfaces, and in the assembled state shown in the figure, the cap 24 is fastened and fixed to the main body side portion 22 by the bolts 26, 26, whereby the large-diameter hole is formed. 18 is formed. The roundness of the large-diameter hole 18 in this assembled state is an extremely small value of, for example, 20 (μm) or less.

  Further, the non-heat treated steel constituting the connecting rod 10 is composed of, for example, Fe as a main component, 0.15-0.5 (wt%) C, 0.4-2 (wt%) Si, 0.4-1.6 (wt%) Mn. 0.03-0.35 (wt%) S, 0.02-0.15 (wt%) P, 0.01-0.5 (wt%) Cu, 0.01-0.5 (wt%) Ni, 0.01-1 (wt%) Cr , 0.04-0.5 (wt%) V, 0.05-0.65 (wt%) Ti, 0.005-0.05 (wt%) sol-Al, 0.008-0.03 (wt%) N, and 0.0005-0.01 (wt%) ) Of Ca, for example, having a composition as in Examples 1 to 14 described later. In addition to these elements, non-tempered steel includes impurities such as Mo (molybdenum) of 0.05 (wt%) or less and O (oxygen) of 0.005 (wt%) or less, which are usually contained in steel. Yes. This non-heat treated steel is composed of a ferrite-pearlite structure, and has a pearlite area ratio Vfθ of 15 (%) or more, for example, about 22 to 83 (%). Further, it has relatively low toughness and ductility, high turning ability and drill machinability, and is suitable for a connecting rod in which the cap 24 of the large end portion 20 is divided and formed by fracture separation as will be described later. It is. This non-tempered steel has a moderate hardness with a Rockwell hardness HRC of about 20 to 35, a sufficient strength of 600 (MPa) or more with 0.2% proof stress (JIS Z 2241), Has high drilling efficiency, machinability, turning ability, and turning life.

  In manufacturing the connecting rod 10, for example, first, molten steel melted with a predetermined composition is ingoted, and this is hot-forged to produce a forging material of about 50 (mm) square, for example. Next, the forging material is heated and held at a temperature of, for example, about 1200 (° C.) for about 60 minutes, and then an integrally molded product in which the main body portion 12 and the like and the cap 24 are integrated is manufactured by hot forging. Allow to cool. The ferrite pearlite structure is formed in this cooling (air cooling) process. After turning the small end portion 16 and the large end portion 20 and finishing the inner peripheral surfaces of the small diameter hole 14 and the large diameter hole 18 in this integrally molded product and performing bolt hole processing, the large end portion 20 is As shown in FIG. 2, the connecting rod 10 is obtained by dividing into a main body side portion 22 and a cap 24 by breaking and separating. The non-heat treated steel constituting the connecting rod 10 of the present embodiment does not contain general Pb as a free-cutting element, but has the above-mentioned high drilling efficiency and turning ability as described above. Processing or the like can be performed as efficiently as or more than conventional Pb-added non-heat treated steel.

  In FIG. 2, the integrally molded product 28 is placed on the fracture processing table 30, and a pair of split molds whose planar shape forms a substantially semicircle in the large-diameter hole 18 of the large end 20. 32a and 32b are fitted. The split molds 32a and 32b are formed on inclined surfaces whose opposing end surfaces 34a and 34b are inclined at the same angle in opposite directions with respect to the axial direction of the large-diameter hole 18, respectively. Therefore, in the state where those arc-shaped outer peripheral surfaces are in close contact with the inner peripheral surface of the large-diameter hole 18, a gap is formed between them so that the interval becomes smaller toward the fracture processing table 30. Is fitted with a wedge 36 having a pair of side surfaces whose inclination angles coincide with the end surfaces 34a and 34b. A recess 38 is provided on the upper surface of the breakage processing table 30, and the integrally molded product 28 is placed on the breakage processing table 30 so that the wedge 36 is positioned on the recess 38. In such a state, when the wedge 36 is pressed toward the recess 38 as shown by an arrow P in FIG. 2 with a hydraulic press device (not shown) at room temperature, for example, the large end 20 is broken and the cap 24 is divided.

  3 and 4 are plan views respectively showing the form of the large end portion 20 before and after the fracture separation step. Prior to breaking and separating, the large end portion 20 is formed with notches 40 and 40 at two locations in the radial direction of the inner peripheral surface of the large diameter hole 18 as shown in FIG. These notches 40, 40 are formed, for example, by machining so as to have a tip R0.1 (mm) over the entire length of the inner peripheral surface in the axial direction, and the depth dimension is, for example, about 1 (mm). is there.

  When the integrally molded product 28 provided with the notches 40 and 40 is arranged as shown in FIG. 2 and the wedge 36 is pressed, the split molds 32a and 32b are pressed in the direction of being separated from each other by the side surfaces of the wedge 36. Therefore, the inner peripheral surface of the large-diameter hole 18 is pressed to the outer peripheral side by the outer peripheral surfaces of the split molds 32a and 32b. As a result, a tensile stress in the circumferential direction is generated at the inner peripheral surface of the large-diameter hole 18 at the large end portion 20, but the stress is concentrated at the notches 40, 40. A crack progresses in the radial direction, and the large end portion 20 is broken. That is, as shown in FIG. 4, the cap 24 is divided and formed by breaking and separating.

  At this time, since the connecting rod 10 is made of the non-heat treated steel having relatively low toughness and ductility as described above, the large end portion 20 has almost no crack in the above-described fracture separation process with almost no deformation. It progresses in the radial direction and breaks brittlely. In addition to having such brittleness, this non-heat treated steel is made of a ferrite / pearlite structure with a pearlite area ratio Vfθ of 15 (%) or more as described above. The main body side portion 22 of the large end 20 and the fractured surfaces 42a and 42b of the cap 24 are flat as a whole, but are fine uneven surfaces having a surface roughness Ra of, for example, 15 (μm) or more. Moreover, as a result of being formed by brittle fracture, these fractured surfaces 42a and 42b have irregular shapes that are inverted from each other.

  Therefore, when the cap 24 is fastened and fixed to the main body side portion 22 with the bolts 26 and 26 as shown in FIG. 5, the concavity and convexity of the fracture surfaces 42a and 42b are closely fitted to each other at the relative positions before the fracture. Therefore, it is possible to suppress the occurrence of lateral displacement at the fracture surfaces 42a and 42b during assembly. Accordingly, the shape of the large end portion 20 before fracture shown in FIG. 3 is restored in combination with almost no deformation at the time of fracture, so that the roundness of the large-diameter hole 18 is the accuracy before fracture. It is kept in 4 and 5, the irregularities of the fracture surfaces 42a and 42b are exaggerated.

  That is, according to the non-heat treated steel of this example, since it has the above-described composition, it has excellent machinability after hot forging even if it does not contain Pb, in particular, turning and drill machinability, and fracture separation. The notch 40, 40 is provided at the large end 20 and pressed by the split mold 32 from the inner periphery to the outer periphery, so that the notch 40, 40 is used as a starting point to produce a break. It is suitable for the connecting rod 10.

  Here, the non-heat treated steel of this example is manufactured with various compositions, and the results of comparison with comparative examples and conventional materials will be described. FIG. 6 shows the component composition of the evaluated non-heat treated steel, and FIG. 7 shows the results of the respective characteristic evaluations. The steel materials of these examples and comparative examples were each agglomerated after being melted with the component composition shown in the figure, and hot forged into a 50 (mm) square forged material, which was 1200 (° C). After heating and holding for 60 minutes, it was manufactured by hot forging into a 65 × 65 × 20 (mm) plate shape, leaving it on the floor at an appropriate interval, and allowing it to cool to room temperature. In these examples and the like, as in the case of the connecting rod 10, in addition to the components shown in FIG. In addition, the conventional materials are steel that is 0.1% (% by weight) of S added to non-heat treated steel for connecting rods S40VC, which is generally used in Japan, and non-heat treated steel for connecting rods to which a fracture separation process is applied in Europe. Steel XC70S. The characteristic evaluation was performed by cutting out, for example, a test piece 50 as shown in FIG. 8 simulating the large end portion 20 of the connecting rod 10 from the above plate material.

  In addition, in said evaluation result, hardness is the value which measured the hardness of the center part of each forged product with the Rockwell hardness meter. Further, the pearlite area ratio Vfθ was obtained by an image analyzer using an optical microscope texture photograph taken at 100 times. In addition, the fracture separation characteristics are such that the notches 54 and 54 having a depth of 0.5 (mm) are provided on the inner peripheral surface of the central hole 52 of the test piece 50 by, for example, a laser, and the fracture separation is performed at room temperature. Evaluated by change. As in the manufacturing process of the connecting rod 10, the fracture separation process is performed by inserting the split molds 32 and 32 (see FIG. 2) into the central hole 52 of the test piece 50, and the wedges 32 and 32 at the center. 36 (see FIG. 2) was pushed by a hydraulic press. Further, the surface roughness Ra of the fracture surfaces 56 and 56 after the fracture was measured with a surface roughness meter, and the degree of unevenness of the fracture surfaces 56 and 56 was quantified. In FIG. 8, 58 and 58 are bolt holes.

  Further, in addition to the test piece 50, a JIS No. 4 tensile test piece (parallel portion φ8 (mm), reduced size) was cut out from the plate material, and the strength was evaluated.

  Furthermore, after producing a round bar with a diameter of 90 (mm) by hot forging from the ingot-made material as described above, after normalizing, from this round bar, a test piece for drill efficiency test (20 (mm ) × 30 (mm) square) and a test material for turning life test (diameter 80 (mm)) were cut out and subjected to a machinability test.

  The test conditions of the drill efficiency test are as shown in Table 1 below. The drilling efficiency is the measured value of S40VC with 0.1 (wt%) of S added, measuring the cutting speed (m / min) at which the accumulated machining depth until the drill reaches the end of its life under these conditions is 1000 (mm). A comparison was made with relative values when 100 was 100.

[Table 1]
Tool SKH51
Drill diameter 5 (mm)
Feed 0.1 (mm / rev)
Hole depth 10 (mm)
Cutting oil None

Further, the turning life test VB100 was performed under the conditions shown in Table 2 below using an NC lathe. The turning time until the average flank wear width on the side flank of the tool reached 100 (μm) was defined as the turning life, and this was used as an index for turning performance evaluation. The higher the value, the better the turning ability.
[Table 2]
Tool P10 Carbide insert Cutting speed 200 (mm / min)
Feed 0.2 (mm / rev)
Depth of cut 2 (mm)
Cutting oil None

As a result of the above evaluation, as is apparent from the composition and characteristics shown in FIGS. 6 and 7, C is 0.15 to 0.48 (wt%), Si is 0.42 to 1.95 (wt%), and Mn is 0.43 to 1.58 (wt%), S 0.051 to 0.345 (wt%), P 0.02 to 0.15 (wt%), Cu 0.01 to 0.50 (wt%), Ni 0.01 to 0.45 (wt%), Cr 0.02 to 0.95 (wt%), V is 0.04 to 0.48 (wt%), Ti is 0.051 to 0.152 (wt%) , Zr is 0 to 0.143 (wt%) , sol-Al is 0.006 to 0.044 (wt%), Examples 1 to 9, 11, and 12 in which N is in the range of 0.010 to 0.025 (wt%) and Ca is in the range of 0.0015 to 0.0087 (wt%), since these elements are all in the preferred range, Each of them has high strength, and also has an excellent fracture separation property that the irregularities of the fracture surface 56 are large despite the small roundness change after fracture separation. In addition, Examples 10, 13, and 14 are descriptions as reference examples.

Moreover, in any of Examples 1-9 , 11, and 12 , the Ti, Zr, and S contents satisfy the following formula 1, so that the drill has high drilling efficiency, that is, high drill machinability. Ti and 0.52Zr are equivalent. When the S content relative to the total amount of these is less than the amount satisfying Equation 1, most of S is fixed as Ti and Zr carbon sulfide, and MnS or (Mn, Ca ) The amount of S produced is insufficient. Since MnS or (Mn, Ca) S is an inclusion that enhances the drill machinability by being interposed in the base material, Examples 1 to 9, 11 in which this sufficiently exists by satisfying Equation 1 , 12 , high drill machinability is obtained.
(Ti + 0.52Zr) / S <1.9 ... (Formula 1)

Moreover, the steel materials of Examples 1 to 9, 11, and 12 have a carbon equivalent (indicated as “Equation 2” in FIG. 7) in the range of 0.65 to 0.95, and the following Equation 2 (non-heat treated steel carbon equivalent equation) ), The hardness after hot forging becomes appropriate for connecting rod applications and has sufficient strength.
0.65 ≦ C + 0.07Si + 0.16Mn + 0.61P + 0.19Cu + 0.17Ni + 0.2Cr + V ≦ 0.96 (Formula 2)

Moreover, since the steel materials of Examples 1 to 9, 11, and 12 satisfy the following expression 3 within the range of 43 to 104, the value expressed as “expression 3” in FIG. Since the pearlite amount, that is, an appropriate pearlite block size, and the pearlite area ratio Vfθ is 22 (%) or more, cracks extending from the notches 54 and 54 are the boundaries of the ferrite pearlite structure and the boundaries of the pearlite block. By changing the advancing direction at, the crack progresses with appropriate variation. Therefore, the fracture surface 56 formed by this crack growth has appropriate irregularities of Ra of 17.4 to 28.2 (μm), that is, 15 (μm) or more as shown in FIG.
134C-3.6Si + 24Mn + 22Cu + 32Ni + 30Cr-12Ti + 41N ≧ 31 (Formula 3)

In addition, Examples 11 and 12 have higher characteristics because they contain Pb, Se, Te, etc. in addition to the basic additive components. For example, in Example 11, since 0.14 (% by weight) of Pb is contained, this is dispersed in the steel as metal particles, the cutting resistance is reduced by the notch effect, and the tool and chips are melted by cutting heat. Therefore, the machinability is further improved.

  Further, in Example 12, since 0.059 (wt%) of Se and 0.06 (wt%) of Te are contained, the extension of sulfide during hot working is suppressed by these, and as a result, the sulfide in the steel material is cut. A ball field or spindle shape with a large notch effect is obtained, cutting resistance is reduced, and machinability is further enhanced.

  In contrast to these examples, Comparative Example A has a low C content, and the value of Formula 2 is also not as low as 0.61, so the hardness is insufficient and sufficient strength cannot be obtained. Moreover, since the hardness is low, the roundness after fracture separation is as large as 43 (μm), and the pearlite area ratio is as small as 13 (%), so that the reassembly property is poor.

  In Comparative Example B, since the C content is too high and the value of Formula 2 is 0.98, which is not satisfactory, the hardness is high and the machinability is poor.

  Moreover, since the comparative example C has too high Si content, hardness is high and machinability is bad. Moreover, since the value of Formula 2 is too high, the machinability is further reduced.

  Moreover, since the comparative example D had high S content, the crack generate | occur | produced at the time of hot forging.

  In Comparative Example E, the Mn content was excessive and the value of Formula 2 was too high, so that bainite was generated after forging, the hardness was very high, and machinability was poor.

  In Comparative Example F, cracks occurred during forging because the P content was too high.

  In Comparative Example G, the Cr content was excessive and the value of Formula 2 was too high, so that bainite was generated after forging, the hardness became very high, and the machinability deteriorated.

  In Comparative Example H, since the value of Equation 3 was as low as 29, the pearlite area ratio was small, the fracture surface roughness was small, and the reassembly property was insufficient.

  In Comparative Example I, the V content was too high, resulting in high hardness and poor machinability.

  In addition, since Comparative Example J does not contain Ti, the strength is lower than that of the present Example (for example, Example 14) having the same hardness, and the toughness of the steel is not decreased and the fracture separability is reduced. (Ie roundness) was insufficient.

  In Comparative Example K, since the Ti content is excessive with respect to the S content, since the value of Formula 1 is high, the drill machinability, which is important for connecting rod processing, is greatly reduced.

  Moreover, since the comparative example L had little sol-Al content, the crystal grain after forge became coarse and the intensity | strength (0.2% yield strength) became low. Further, since the total amount of Ti and Zr is excessive with respect to the S content, the value of Formula 1 is high as in Comparative Example K, so that the drill machinability important for connecting rod processing also remains at a very low value. .

  In Comparative Example M, a casting defect occurred because the N content was too high.

  Further, since Comparative Example N does not contain Ca, both the drill machinability and the turning property are insufficient as compared with Comparative Example 1 having substantially the same composition.

  Note that the two conventional materials shown at the bottom of FIGS. 6 and 7 do not contain Ti, so in S40VC, the roundness is poor and the turning life is relatively low. In addition, XC70S is slightly low in strength and has insufficient drilling efficiency and turning life.

  As mentioned above, although this invention was demonstrated in detail with reference to drawings, this invention can be implemented also in another aspect, A various change can be added in the range which does not deviate from the main point.

It is a top view which abbreviate | omits a crankshaft and shows the assembly | attachment state of the connecting rod manufactured with the non-heat-treated steel of one Example of this invention. It is sectional drawing which shows the implementation state of the process of fracture-separating a big end part in the manufacture process of the connecting rod of FIG. It is a top view which shows the large end part before fracture | rupture isolation | separation. It is a top view which exaggerates the unevenness | corrugation of a fracture surface, and shows the large end part after a fracture | rupture isolation | separation. It is a top view which exaggerates the unevenness | corrugation of the torn surface, and the assembly state which made the torn surface contact | adhere mutually. It is a figure which shows the example of a component composition of the non-tempered steel of this invention compared with a comparative example and a conventional material. It is a figure which shows the characteristic evaluation result of the non-heat-treated steel of the component composition example of FIG. It is a top view which shows the board | plate material for characteristic evaluation.

Explanation of symbols

10: connecting rod, 20: large end, 24: cap, 40: notch, 42: fracture surface

Claims (4)

0.15 to 0.5 ( mass %) C (carbon), 0.4 to 2 ( mass %) Si (silicon), 0.4 to 1.6 ( mass %) Mn (manganese), 0.03 to 0.35 ( mass %) S (sulfur), 0.02 to 0.15 ( mass %) P (phosphorus), 0.01 to 0.5 ( mass %) Cu (copper), 0.01 to 0.5 ( mass %) Ni (nickel), 0.01 to 1 ( mass %) Cr (chromium), 0.04 to 0.5 ( mass %) V (vanadium), 0.05 to 0.152 ( mass %) Ti (titanium), 0 to 0.143 (mass%) Zr ( Zirconium), 0.005 to 0.05 ( mass %) sol-Al (acid-soluble aluminum), 0.008 to 0.03 ( mass %) N (nitrogen), 0.0005 to 0.01 ( mass %) Ca (calcium) , look including the balance Fe and unavoidable impurities, when representing the content of each component by weight percent in each element symbol, low toughness and ductility machinable microalloyed steel and satisfies the following formula 1.
(Ti + 0.52Zr) / S <1.9 ... (Formula 1) The low toughness free-cutting non-heat treated steel according to claim 1 , which satisfies the following formula 2 when the content of each component by mass % is expressed by each element symbol.
0.65 ≦ C + 0.07Si + 0.16Mn + 0.61P + 0.19Cu + 0.17Ni + 0.2Cr + V ≦ 0.96 (Formula 2) The low toughness free-cutting non-heat treated steel according to claim 1 or 2 , which satisfies the following formula 3 when the content of each component by mass % is expressed by each element symbol.
134C-3.6Si + 24Mn + 22Cu + 32Ni + 30Cr-12Ti + 41N ≧ 31 (Formula 3) Contains at least one component selected from 0.005 to 0.4 ( mass %) Se (selenium), 0.05 to 0.1 ( mass %) Te (tellurium), and 0.3 ( mass %) or less Pb (lead). The low toughness free-cutting tempered steel according to any one of claims 1 to 3 .

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