JP6652020B2 - High strength hot forged non-heat treated steel parts - Google Patents

Description

  The present invention relates to a high-strength hot forged non-heat treated steel part.

  For automotive engine parts and undercarriage parts, they are formed by hot forging and then heat-treated such as quenching and tempering (hereinafter, the parts subjected to heat treatment are referred to as tempered parts) or without heat treatment. (Hereinafter, parts that are not subjected to heat treatment are referred to as non-refined parts), and the mechanical properties required for the parts to be applied are secured. In recent years, from the viewpoint of economic efficiency in the manufacturing process, many components manufactured without the tempering, that is, non-tempered components have been widely used.

  A connecting rod (hereinafter referred to as a connecting rod) is an example of an automobile engine component. This component transmits power when converting the reciprocating motion of the piston into the rotational motion of the crankshaft in the engine. The connecting rod clamps an eccentric portion called a pin portion of a crankshaft by sandwiching it between a cap portion and a rod portion of the connecting rod, and transmits power by a mechanism in which the pin portion and the connecting portion of the connecting rod rotate and slide. In recent years, in order to increase the efficiency of the connection between the cap portion and the rod portion, a break-separation type connecting rod is often used in recent years.

  With a breakable connecting rod, after forming a steel material into a shape in which the cap portion and the rod portion are integrated by hot forging or the like, cut out a part corresponding to the boundary between the cap portion and the rod portion, It adopts a method of breaking and separating. In this method, since the fractured surfaces that are fractured and separated at the mating surface of the cap portion and the rod portion are fitted to each other, machining of the mating surface is not required, and a process for positioning can be omitted as necessary. . As a result, the number of processing steps for parts can be greatly reduced, and the economic efficiency in manufacturing parts can be greatly improved. In the fracture-separating connecting rod manufactured by such a method, the fracture form of the fracture surface is brittle, the deformation amount near the fracture surface due to fracture separation is small, and the amount of chipping generated by fracture separation is small, that is, Good break separation is required.

  DIN standard C70S6 is widely used in the United States and Europe as a steel material to be used for a fracture-separating connecting rod. This is a high-carbon non-heat treated steel containing 0.7% by mass of C, and has a pearlite structure with low ductility and toughness in order to suppress a dimensional change upon fracture separation. C70S6 has a small amount of plastic deformation in the vicinity of the fracture surface at the time of fracture and thus has excellent fracture separability, but has a coarser structure than the ferrite-pearlite structure of medium-carbon non-heat treated steel, which is the current steel for connecting rods. However, there is a problem that the yield ratio (= yield strength / tensile strength) is low and cannot be applied to a high-strength connecting rod that requires a high buckling strength.

  In order to increase the yield ratio of steel, it is necessary to reduce the amount of carbon and increase the ferrite fraction. However, when the ferrite fraction is increased, the ductility of the steel material is improved, the amount of plastic deformation at the time of fracture separation increases, and the deformation of the connecting rod sliding portion fastened to the crankshaft pin increases. A part performance problem such as a decrease in roundness of the part occurs.

  Some non-heat treated steels have been proposed as steel materials suitable for high-strength fracture-separating connecting rods. For example, Patent Literature 1 and Patent Literature 2 describe a technique in which a brittle element such as Si or P is added to a steel material in a large amount to reduce the ductility and toughness of the material itself, thereby improving fracture separation. ing. Patent Literature 3 and Patent Literature 4 describe a technique for improving the fracture separability of a steel material by reducing the ductility and toughness of ferrite by utilizing precipitation strengthening by second phase particles. Further, Patent Literatures 5 to 7 disclose techniques for improving the break-separability of steel by controlling the form of Mn sulfide.

  On the other hand, in recent years, with the increase in engine output due to the spread of high-output diesel engines or turbo engines, the need for high-strength connecting rods has increased. As one of means for increasing the strength, for example, in the techniques described in Patent Documents 1 to 7, a large amount of V is added, and precipitation strengthening of steel by fine VC has been used. Among the elements that form alloy carbides, V has a large amount of solid solution in steel and a large amount of precipitation strengthening by heating (around 1250 ° C.) before hot forging. However, the amount of solid solution of V in steel is limited, and it is difficult to further increase the strength by precipitation strengthening of VC.

  As described above, steel capable of manufacturing a fracture-separating type connecting rod having excellent strength that can respond to recent demands for increasing the strength of connecting rods has not been obtained at present.

Japanese Patent No. 3637375 Japanese Patent No. 3756307 Japanese Patent No. 3355132 Japanese Patent No. 3988661 Japanese Patent No. 4314851 Japanese Patent No. 367688 Japanese Patent No. 4268194

  In view of the above circumstances, an object of the present invention is to provide a high-strength hot-forged non-heat-treated steel part having excellent strength, excellent yield strength and yield ratio.

  In order to solve the above-mentioned problems, the present inventors have developed a high-strength hot-forged non-heat-treated steel part having excellent strength, excellent yield strength and yield ratio (hereinafter, referred to as “high-strength hot- Forged non-heat treated steel parts "may be simply described as" steel ". As a result, the following findings (a) to (d) were obtained.

  (A) When the C content is increased in order to increase the tensile strength of the ferrite-pearlite structure steel, the yield ratio decreases. This is because the increase in the yield strength is smaller than the increase in the tensile strength accompanying the increase in the C content. This is because a steel having a relatively low C content having a ferrite-pearlite structure mainly composed of ferrite causes a yield point phenomenon (discontinuous yielding) at the time of tensile fracture, whereas a steel having a ferrite-pearlite structure mainly composed of pearlite. This is because steel having a high target C content has a continuous yield in which the form of yield at the time of tensile fracture is a smooth transition from elastic deformation to plastic deformation.

  (B) The present inventors have repeatedly studied means for suppressing a decrease in the amount of ferrite while increasing the C content. As a result, they have found that by finely dispersing Mn sulfide, which is the core of ferrite transformation, ferrite transformation can be promoted during the production of steel and the amount of ferrite structure in the ferrite-pearlite structure can be increased. It has been found that by setting the area ratio of the ferrite structure in the ferrite / pearlite structure to 20% by area or more, a yield point phenomenon occurs even with a high carbon composition, and a high yield ratio can be obtained.

  (C) The present inventors have studied various means for finely dispersing Mn sulfide while keeping the Mn and S contents within a predetermined range and maintaining break separation properties. As a result, they have found that it is effective to contain one or more of a small amount selected from the group consisting of Sb, Sn, Te and Pb.

  (D) In addition to increasing the C content and increasing the area ratio of the ferrite structure in the ferrite-pearlite structure, the present inventors have improved the yield ratio by combining precipitation strengthening with VC, and further improved the yield ratio. It has been found that strength is improved.

  Based on the findings of (a) to (d) above, Mn sulfide is contained by containing a trace amount of one or more selected from the group consisting of Sb, Sn, Te and Pb in steel. By finely dispersing, if the area ratio of the ferrite structure in the ferrite / pearlite structure is set to 20 area% or more even though the C content is a high carbon composition of 0.50% or more, the yield ratio of the steel is improved. The present inventors have found that the yield strength can be improved, and have accomplished the present invention.

The gist of the invention is as follows.
(1) The high-strength hot-forged non-heat-treated steel part according to one embodiment of the present invention has a chemical component in unit mass% of C: 0.50 to 0.65%, Si: 0.60 to 1. 20%, Mn: 0.60 to 1.00%, P: 0.040 to 0.060%, S: 0.060 to 0.100%, Cr: 0.05 to 0.20%, V: 0 .25 to 0.40%, N: 0.0020 to 0.0080%, Sb: 0.0001 to 0.0050%, Sn: 0.0001 to 0.0050%, Te: 0.0001 ０．００0.0050% and Pb: one or more selected from the group consisting of 0.0001 to 0.0050%, the balance being Fe and unavoidable impurities, and the steel structure being ferrite-pearlite. Wherein the area ratio of the ferrite structure is at least 20 area%.
(2) In the present invention, the chemical component further contains one or two selected from the group consisting of Ti: 0.10% or less and Nb: 0.05% or less in unit mass%. Can be.
(3) In the present invention, further, the chemical component is selected from the group consisting of Ca: 0.005% or less, Zr: 0.005% or less, and Mg: 0.005% or less in unit mass%. Species or two or more species may be contained.

  According to the present invention, it is possible to provide a high-strength hot-forged non-heat-treated steel part such as a fracture-separation type high-strength connecting rod having excellent strength, excellent yield strength and yield ratio.

It is a disassembled perspective view which shows the fracture | rupture separation type connecting rod which is an example of the high-strength hot forging non-heat-treated steel part which concerns on this embodiment.

FIG. 1 is an exploded perspective view showing an example of a fracture-separable connecting rod made of a high-strength hot-forged non-heat-treated steel part according to the present invention.
As shown in FIG. 1, the fracture-separating connecting rod 1 of this example is composed of a semicircular upper half body 2 with a rod and a lower half half body 3 with a semicircular arc. ing.
A screw hole 5 having a screw groove for fixing to the lower half half 3 is formed on each of both ends of the semicircular arc portion 2A of the upper half half 2. At both ends of the semicircular arc portion 3A of the lower half body 3, insertion holes 6 for fixing to the upper half body 2 are formed.

  The semi-circular portion 2A of the upper half-body 2 and the semi-circular portion 3A of the lower half-body 3 are annularly aligned, and the connecting bolt 7 is inserted into the through hole 6 and the screw hole 5 at both ends. An annular big end portion 8 is formed by screwing. An annular small end portion 9 is formed on the upper end side of the rod portion 2B of the upper half body 2.

  A breakable connecting rod 1 having the structure shown in FIG. 1 is incorporated in an internal combustion engine for converting reciprocating motion of a piston of an internal combustion engine such as an automobile engine into rotary motion. The small end portion 9 is connected to a piston (not shown), and the big end portion 8 is connected to a connecting rod journal (not shown) of the internal combustion engine.

  The fracture-separating connecting rod 1 of the present embodiment is formed from a high-strength hot-forged non-heat-treated steel having a component, a structure, and a Mn sulfide dispersion state described below, and a semi-circular portion of the upper half body 2. 2A and the semicircular arc portion 3A of the lower half body 3 are formed by brittle fracture of a portion that was originally one annular component. As an example of a method of manufacturing the fracture-separating connecting rod 1, a notch is provided in a part of a hot forged product, the fracture is separated brittlely from the notch as a starting point, and the semicircular arc portion 2 </ b> A of the upper half body 2 is formed. And the butting surface 2a of the lower half body 3 and the butting surface 3a of the semicircular arc portion 3A of the lower half body 3. These butting surfaces 2a, 3a are originally formed by breaking and separating one member, so that butting can be performed with good positioning accuracy.

  The break-separation type connecting rod 1 having this structure does not require a new processing of the abutting surface or a positioning pin, thereby greatly simplifying the manufacturing process.

Hereinafter, the high-strength non-heat-treated steel for hot forging that constitutes the fracture-separable connecting rod 1 will be described.
As an example, the fracture-separating type connecting rod 1 has a chemical component in unit mass% of C: 0.50 to 0.65%, Si: 0.60 to 1.20%, and Mn: 0.60 to 1.00. %, P: 0.040 to 0.060%, S: 0.060 to 0.100%, Cr: 0.05 to 0.20%, V: 0.25 to 0.40%, N: 0. 0020-0.0080%, Sb: 0.0001-0.0050%, Sn: 0.0001-0.0050%, Te: 0.0001-0.0050%, and Pb: 0.0001- One or more selected from the group consisting of 0.0050%, the balance being Fe and unavoidable impurities, the steel structure is ferrite / pearlite, and the area ratio of the ferrite structure is 20 area%. It consists of the above steel. The steel of this composition is hot-forged and air-cooled to obtain a non-heat-treated steel, whereby a high-strength hot-forged non-heat-treated steel part having the above-described target composition is obtained.

<Steel component>
First, the reason for limiting the component composition of the high-strength hot-forged non-heat-treated steel part according to the present embodiment will be described.

(C: 0.50 to 0.65%)
C has the effect of ensuring the tensile strength of a high-strength hot forged non-heat treated steel part. To obtain the required strength, the lower limit of the C content needs to be 0.50%. When the C content is 0.50% or more, the amount of ferrite contained in the metal structure (steel structure) of the steel is usually less than 20 area%, and the yield ratio of the steel is low. However, since the steel according to the present embodiment contains Mn, S, Sb, Sn, Te and Pb within a predetermined range as described later, the Mn sulfide is finely dispersed, so that the C content is set to 0. The ferrite amount can be set to 20 area% or more while being set to 0.50% or more. A preferred lower limit of the C content is 0.52%, 0.55%, or 0.58%.

  However, when the C content exceeds 0.65%, the amount of ferrite in the steel is insufficient even if the Mn sulfide is finely dispersed as in the steel according to the present embodiment. Insufficient amount of ferrite causes a decrease in the ratio of yield strength to tensile strength (yield ratio). Therefore, the upper limit of the C content is set to 0.65%. A preferred upper limit of the C content is 0.63%, 0.60%, or 0.59%.

  In the present embodiment, when the range of the element content is described as 0.50 to 0.65%, the range includes the upper limit and the lower limit unless otherwise specified. Therefore, 0.50 to 0.65% means a range of 0.50% or more and 0.65% or less.

(Si: 0.60 to 1.20%)
Si strengthens ferrite by solid solution strengthening and reduces ductility and toughness of steel. The decrease in ductility and toughness of the steel reduces the amount of plastic deformation near the fracture surface at the time of fracture, and improves fracture separability. To obtain this effect, the lower limit of the Si content needs to be 0.60%. On the other hand, if Si is excessively contained, the frequency of occurrence of chipping of the fracture surface increases. Therefore, the upper limit of the Si content is set to 1.20%. A preferred lower limit of the Si content is 0.70%, 0.80%, or 0.85%. A preferred upper limit of the Si content is 1.00%, 0.95%, or 0.90%.

(Mn: 0.60-1.00%)
Mn strengthens ferrite by solid solution strengthening and reduces ductility and toughness of steel. The reduction in ductility and toughness of the steel reduces the amount of plastic deformation near the fracture surface at the time of fracture, and improves fracture separation. Mn combines with S to form Mn sulfide. The Mn sulfide serves as a nucleus of ferrite transformation in a cooling process after component forming by hot forging, and has an effect of increasing the amount of ferrite. On the other hand, when Mn is excessively contained, the ferrite becomes too hard, and the frequency of occurrence of chipping at break increases. In view of these, the range of the Mn content is 0.60 to 1.00%. A preferred lower limit of the Mn content is 0.70%, 0.80%, or 0.85%. A preferred upper limit of the Mn content is 0.95%, 0.92%, or 0.90%.

(P: 0.040-0.060%)
P reduces the ductility and toughness of ferrite and pearlite. The reduction in ductility and toughness has the effect of reducing the amount of plastic deformation in the vicinity of the fracture surface at the time of fracture of steel and improving the fracture separation. However, P not only produces the above-mentioned effects, but also remarkably produces the effect of causing embrittlement of crystal grain boundaries and easily causing chipping of a fractured surface. Taking the above into consideration, the range of the P content is 0.040 to 0.060%. A preferred lower limit of the P content is 0.042%, 0.045%, or 0.048%. A preferred upper limit of the P content is 0.055%, 0.053%, or 0.050%.

(S: 0.060 to 0.100%)
S combines with Mn to form Mn sulfide. The Mn sulfide serves as a nucleus of ferrite transformation in a cooling process after component forming by hot forging, and has an effect of increasing the amount of ferrite. In order to obtain the effect, the lower limit of the S content needs to be 0.060%. On the other hand, if S is excessively contained, the amount of plastic deformation in the vicinity of the fractured surface at the time of fracture splitting increases, and the case where fracture separability may decrease may occur. In addition, when S is excessively contained, the fracture surface may be chipped. From the above, the range of the S content is set to 0.060 to 0.100%. A preferred lower limit of the S content is 0.070%, 0.072%, or 0.075%. A preferred upper limit for the S content is 0.095%, 0.090%, or 0.085%.

  In addition, since fine Mn sulfide becomes a core of ferrite transformation in a cooling process after component forming by hot forging, precipitation of fine Mn sulfide has an effect of contributing to formation of a ferrite structure. The total amount of Mn sulfides precipitated in the steel does not fluctuate greatly in the composition range of the steel of the present embodiment, but the situation in which fine Mn sulfides are precipitated has been extended by forging to be refined. It can be expressed by the small average aspect ratio of Mn sulfide.

The aspect ratio of Mn sulfide is a value obtained by dividing the length of the long axis of Mn sulfide by the length of the short axis of Mn sulfide. The average aspect ratio of Mn sulfide is an average value of the aspect ratio of Mn sulfide measured by observing a structure in a longitudinal parallel section from a vertical direction.
In this sense, the average aspect ratio of the fine Mn sulfide is preferably in the range of about 1.1 to 1.4.

(Cr: 0.05-0.20%)
Cr strengthens ferrite by solid solution strengthening like Mn, and reduces ductility and toughness. The reduction in ductility and toughness reduces the amount of plastic deformation in the vicinity of the fracture surface at the time of fracture, and improves fracture separation. However, when Cr is contained excessively, the lamella spacing of pearlite becomes small, and the ductility and toughness of pearlite are rather increased. Therefore, when Cr is contained excessively, the amount of plastic deformation near the fracture surface at the time of fracture increases, and fracture separability decreases. Furthermore, when Cr is excessively contained, a bainite structure is easily generated, and a decrease in yield strength due to a decrease in yield ratio and a remarkable decrease in break separation properties are observed. Therefore, the range of the Cr content is set to 0.05 to 0.20%. In view of the above effects, a preferable upper limit of the Cr content is 0.17%, 0.15%, or 0.13%. Further, a preferable lower limit of the Cr content is 0.07%, 0.08%, or 0.10%.

(V: 0.25 to 0.40%)
V mainly forms carbides or carbonitrides during cooling after hot forging to strengthen ferrite and reduce ductility and toughness of steel. The reduction in ductility and toughness reduces the amount of plastic deformation in the vicinity of the fracture surface at the time of fracture, and improves the fracture separation of the hot forged part. V has the effect of increasing the yield ratio of hot forged parts by strengthening the precipitation of carbides or carbonitrides. To obtain these effects, the lower limit of the V content needs to be 0.25%. The lower limit of the V content is preferably 0.27% or 0.30%. On the other hand, the effect is saturated even if V is excessively contained, so the upper limit of the V content is 0.40%. The upper limit of the V content is preferably 0.35%, 0.33%, or 0.31%.

(N: 0.0020-0.0080%)
N promotes ferrite transformation by mainly forming V nitride or V carbonitride upon cooling after hot forging and acting as a transformation nucleus of ferrite. Accordingly, N has an effect of suppressing the formation of a bainite structure that significantly impairs the fracture separability of the hot forged part. To obtain this effect, the lower limit of the N content is set to 0.0020%. If N is excessively contained, hot ductility is reduced and cracks or flaws are likely to occur during hot working. Therefore, the upper limit of the N content is set to 0.0080%. The lower limit of the N content may be 0.0040%, 0.0042%, or 0.0045%. The upper limit of the N content may be set to 0.0075%, 0.0070%, or 0.0060%.

(Sb: 0.0001 to 0.0050%, Sn: 0.0001 to 0.0050%, Te: 0.0001 to 0.0050%, and Pb: 0.0001 to 0.0050%. One or more)
The high-strength hot-forged non-heat-treated steel part according to the present embodiment includes, in addition to the above components, one or more selected from the group consisting of Sb, Sn, Te, and Pb, each of which is 0.0001. It is characterized in that it is contained within the range of -0.0050%. With these elements, Mn sulfide is finely dispersed with the refinement of the solidification structure of steel. In order to obtain an effect of finely dispersing Mn sulfide, the content of these elements needs to be 0.0001% or more. However, if these elements are excessively contained, these elements precipitate on Mn sulfide and lose their effect as nuclei for ferrite transformation, so the upper limit of the content of these elements is made 0.0050%. In view of the above effects, the upper limit of the total content of Sb, Sn, Te, and Pb is preferably 0.0050%. The upper limit of the total content of Sb, Sn, Te and Pb is more preferably 0.0030%.

(One or two selected from the group consisting of Ti: 0.10% or less and Nb: 0.05% or less)
Ti and Nb mainly form carbides or carbonitrides during cooling after hot forging, strengthen ferrite by precipitation strengthening, and lower the ductility and toughness of steel. The reduction in ductility and toughness has the effect of reducing the amount of plastic deformation in the vicinity of the fracture surface at the time of fracture and improving fracture separation. Therefore, the lower limit of the Ti content may be set to 0.05% and the lower limit of the Nb content may be set to 0.01% in order to obtain the above-described effects. However, if these elements are excessively contained, the effect is saturated. Therefore, the upper limit of the Ti content is set to 0.10%, and the upper limit of the Nb content is set to 0.05%.

(One or more selected from the group consisting of Ca: 0.005% or less, Zr: 0.005% or less, and Mg: 0.005% or less)
Ca, Zr, and Mg all form oxides, become crystallization nuclei of Mn sulfide, and have the effect of uniformly and finely dispersing Mn sulfide. Therefore, the lower limit of each of Ca, Zr and Mg may be set to 0.001%. On the other hand, if the content of any of the elements exceeds 0.005%, hot workability deteriorates and hot rolling becomes difficult. For these reasons, the upper limits of the contents of Ca, Zr and Mg are set to 0.005%.

  The balance of the chemical components of the high-strength hot-forged non-heat treated steel part according to the present embodiment contains iron (Fe) and unavoidable impurities. The unavoidable impurities are components that are mixed due to raw materials such as ores or scraps or various factors in the manufacturing process when steel is industrially manufactured, and are high-strength hot forging according to the present embodiment. Means acceptable within a range that does not adversely affect non-heat treated steel parts.

  Next, the reasons for limiting the above-described organization will be described.

<Steel structure is ferrite / pearlite, of which the area ratio of the ferrite structure is 20 area% or more>
In a steel having a normal ferrite-pearlite structure, the yield ratio decreases as the C content increases. This is because the increase in the yield strength as the C content increases is smaller than the increase in the tensile strength as the C content increases. This is because the yield point phenomenon (discontinuous yielding) occurs in a ferrite-pearlite structure mainly composed of ferrite having a small C content, whereas in a ferrite-pearlite structure mainly composed of pearlite having a large C content, the yield is caused by elastic deformation. This is because the transition to plastic deformation results in a smooth continuous yield.

  On the other hand, in the high-strength hot-forged non-heat treated steel part according to the present embodiment, the area ratio of the ferrite structure in the ferrite-pearlite structure is one or two selected from the group consisting of Sb, Sn, Te, and Pb. Increased by using Mn sulfide finely dispersed by the seeds or more. The present inventors have found that by setting the area ratio of the ferrite structure to 20% by area or more with respect to the entire steel, a yield point phenomenon occurs even with a high carbon composition, and a high yield ratio can be obtained. Therefore, the lower limit of the area ratio of the ferrite structure to the entire steel is set to 20 area%. The lower limit of the area ratio of the ferrite structure to the entire steel may be 22 area%, 23 area%, or 25 area%.

It is preferable that the rest of the structure of the steel of this embodiment except for the ferrite structure is substantially a pearlite structure, and other structures such as a bainite structure are not generated.
However, as long as it is within the range of less than 2% by area, the inclusion of structures other than ferrite such as bainite and martensite and pearlite is allowable. Since the amount of the ferrite structure is preferably larger, the upper limit of the area ratio of the ferrite structure to the entire steel is not particularly limited, but the range of the chemical composition of the high-strength hot-forged non-heat-treated steel part according to the present embodiment. Within the range, the upper limit of the area ratio of the ferrite structure is usually about 50 area%. The upper limit of the area ratio of the ferrite structure to the entire steel may be 35 area%, 30 area%, or 28 area%.

  In the case of the break-separable connecting rod 1 as a high-strength hot-forged non-heat-treated steel part according to the present embodiment, new processing of the abutting surfaces and positioning pins are not required, and the manufacturing process can be greatly simplified.

The method for producing a high-strength hot-forged non-heat-treated steel part according to the present embodiment includes casting and hot-rolling steel having the chemical composition of the high-strength hot-forged non-heat-treated steel part according to the present embodiment. And hot forging.
The casting conditions are not particularly limited, and may be ordinary conditions.
The hot rolling conditions are not particularly limited, and may be normal conditions.

  In hot forging, it is necessary to reduce the cooling rate after forging. The area ratio of the ferrite structure of the high-strength hot forged non-heat treated steel part according to the present embodiment is changed at the time of cooling after forging, that is, at the time of air cooling by standing cooling or at the time of blast cooling by a blast cooling device. To change. For example, by increasing the cooling rate to a range of 3.5 ° C./second or more, the area ratio of the ferrite structure becomes less than 20 area%. Therefore, at the time of cooling after forging, it is necessary to set the cooling rate to less than 3.5 ° C./second, and it is preferable that the cooling means after forging is naturally cooled. It is not preferable to use so-called forced cooling such as water cooling and blast cooling as a cooling means after forging.

  According to the present embodiment, it is possible to provide a high-strength hot-forged non-heat-treated steel part such as a fracture-separating high-strength connecting rod having excellent strength, excellent yield strength, and yield ratio. The high-strength hot-forged non-heat-treated steel part according to the present embodiment has excellent break separation properties.

  The application of the high-strength hot-forged non-heat-treated steel part according to the present embodiment is not particularly limited. However, when it is applied to a mechanical part used by breaking and splitting, for example, a break-separating connecting rod, a particularly suitable effect is exhibited. .

  The present invention will be described in detail below by way of examples. These examples are for explaining the technical significance and effects of the present invention, and do not limit the scope of the present invention.

Converter steel smelting steels having the compositions shown in Tables 1 and 2 below were produced by continuous casting, and if necessary, were subjected to a soaking process and a slab rolling process to obtain a 162 mm square rolled material.
Next, a steel bar shape having a diameter of 45 mm was formed by hot rolling. The underlined portions in Table 2 indicate examples outside the scope of the present invention.

  Next, in order to examine the structure and mechanical properties, a test piece equivalent to a forged connecting rod was prepared by hot forging. Specifically, after heating a steel bar having a diameter of 45 mm to 1150 to 1280 ° C., it is forged perpendicularly to the longitudinal direction of the steel bar to a thickness of 20 mm, and is air-cooled by standing cooling, or by blast cooling by a blast cooling device. Cooled to room temperature. By changing the cooling rate, the area ratio of the ferrite structure in the ferrite / pearlite structure was determined. From the cooled forged material, a JIS No. 4 tensile test piece was processed.

  The tensile test was performed at room temperature at a speed of 20 mm / min in accordance with JIS Z 2241. If the yield strength did not reach 900 MPa, the strength was judged to be inferior.

  A 10 mm square sample was cut out from the same site as the tensile test piece, and the morphology of Mn sulfide and the ferrite-pearlite structure in the steel were observed from a direction perpendicular to the longitudinal direction.

  In order to measure the aspect ratio of Mn sulfide in steel, after polishing to a mirror surface, 10 microstructure photographs were taken with an optical microscope at 1000 times magnification, and the average aspect ratio of Mn sulfide was measured using a small general-purpose image analyzer ( Luzex (Luzex): a registered trademark, manufactured by Nireco Corporation. Since the minor axis length of Mn sulfide is determined by the forging ratio, and the amount of Mn sulfide is determined by the amount of S, the smaller the average aspect ratio of Mn sulfide is, the smaller the size of Mn sulfide is. The number density is large. That is, the size and the dispersion state of the Mn sulfide were determined by comparing the values of the average aspect ratio of the Mn sulfide. Further, in order to measure the area ratio of the ferrite structure in the ferrite-pearlite structure, corrosion was performed with a nital corrosive solution, and five 200-fold structure photographs were taken with an optical microscope. The area ratio was determined using a small general-purpose image analyzer (Luzex: registered trademark, manufactured by Nireco Corporation). The metal structure of each example shown in Tables 1 and 2 was substantially composed of ferrite and pearlite.

In Table 1, steel No. The present invention examples A to AK are all high-strength hot-forged non-heat-treated steel parts having a yield strength of 900 MPa or more, all within the specified range of steel chemical components.
In contrast, in Table 2, Comparative Example AL has a low yield ratio and a low yield strength because the area ratio of the ferrite structure in the ferrite-pearlite structure is 20 area% or less.
In Comparative Example AM, the required yield strength was not obtained because the C content was low.
Comparative Example AN has a high C content, Comparative Example AO has a low Mn content, and Comparative Example AQ has a low S content, so that the area ratio of the ferrite structure in the ferrite / pearlite structure is less than 20 area%. It is. Therefore, the yield ratio is low, and the required yield strength cannot be obtained.
Comparative Example AR has a large Cr content, and Comparative Example BH has a small N content, so that a bainite structure is generated. Therefore, the yield ratio is low, and the required yield strength cannot be obtained.
Since Comparative Example AT does not contain Sb, Sn, Te and Pb, there is no effect of fine dispersion of Mn sulfide by Sb, Sn, Te and Pb, and the area ratio of the ferrite structure in the ferrite / pearlite structure is 20%. Area%. Therefore, the yield ratio is low, and the required yield strength cannot be obtained.
Comparative Examples AU to BG have a large content of any one of Sb, Sn, Te, and Pb, and on the contrary, the effect of dispersing Mn sulfide finely dispersed by Sb, Sn, Te, and Pb is reduced, and the ferrite structure in the ferrite-pearlite structure is reduced. Is less than 20 area%. Therefore, the yield ratio is low, and the required yield strength cannot be obtained.

DESCRIPTION OF SYMBOLS 1 ... Break-separation type connecting rod (high-strength hot-forged non-heat-treated steel part), 2 ... Upper half body, 2A ... Semi-arc part, 2a ... Butt surface, 3 ... Lower half body, 3A: semi-circular portion, 3a: butting surface, 5: screw hole, 6: insertion hole, 7: coupling bolt, 8: big end portion, 9 ... Small end part.

Claims (3)

The chemical component is in unit mass%,
C: 0.50 to 0.65%,
Si: 0.60 to 1.20%,
Mn: 0.60-1.00%,
P: 0.040 to 0.060%,
S: 0.060 to 0.100%,
Cr: 0.05 to 0.20%,
V: 0.25 to 0.40%,
N: 0.0020 to 0.0080%, Sb: 0.0001 to 0.0050%,
Sn: 0.0001 to 0.0050%,
Te: 0.0001-0.0050% and Pb: 0.0001-0.0050%
Containing one or more selected from the group consisting of: Fe and unavoidable impurities, and the steel structure is ferrite / pearlite, of which the area ratio of the ferrite structure is 20% by area or more. High strength hot forged non-heat treated steel parts characterized by: Further, the chemical component is expressed in unit mass%,
The high-strength hot-forged non-heat treated steel according to claim 1, comprising one or two selected from the group consisting of Ti: 0.10% or less and Nb: 0.05% or less. parts. Further, the chemical component is expressed in unit mass%,
Ca: 0.005% or less,
The high-strength hot forging according to claim 1 or 2, comprising one or more selected from the group consisting of Zr: 0.005% or less and Mg: 0.005% or less. Tempered steel parts.

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