JP2018035409A - High-strength hot-forged non-heat-treated steel component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength hot-forged non-heat-treated steel component.SOLUTION: A high-strength hot-forged non-heat-treated steel component contains, as its chemical components, in unit mass%, C: 0.50-0.65%, Si: 0.60-1.20%, Mn: 0.60-1.00%, P: 0.040-0.060%, S: 0.060-0.100%, Cr: 0.05-0.20%, V: 0.25-0.40%, and N: 0.0020-0.0080%, and further contains one or more selected from the group consisting of Sb: 0.0001-0.0050%, Sn: 0.0001-0.0050%, Te: 0.0001-0.0050% and Pb: 0.0001-0.0050%, with the balance being Fe and inevitable impurities, wherein the steel structure is ferrite-perlite, and the ferrite structure of them has an area ratio of 20 area% or more.SELECTED DRAWING: Figure 1

Description

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

  For automobile engine parts and undercarriage parts, hot forging is performed, followed by heat treatment such as quenching and tempering (hereinafter, heat treated parts are referred to as tempered parts), or without applying heat treatment. (Hereinafter, parts that are not heat-treated are referred to as non-heat treated parts), and the mechanical properties necessary for the applied parts are ensured. Recently, from the viewpoint of economic efficiency in the manufacturing process, many parts manufactured by omitting tempering, that is, non-tempered parts are widely used.

  Examples of parts for automobile engines include a connecting rod (hereinafter referred to as a connecting rod). This component is a component that transmits power when the reciprocating motion of the piston is converted into the rotational motion by the crankshaft in the engine. The connecting rod clamps an eccentric portion called a pin portion of the crankshaft sandwiched between the cap portion and the 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 order to make the fastening of the cap part and the rod part efficient, in recent years, a fracture separation type connecting rod has been widely used.

  The fracture separation type connecting rod is a steel material formed into a shape in which the cap part and the rod part are integrated by hot forging etc., and then a notch is made in the part corresponding to the boundary between the cap part and the rod part, A method of breaking and separating is adopted. In this construction method, the fracture surfaces separated by breakage at the mating surfaces of the cap portion and the rod portion are fitted to each other, so that machining of the mating surfaces is unnecessary and processing for alignment can be omitted as necessary. . As a result, the machining process for the parts can be greatly reduced, and the economic efficiency during the production of the parts can be greatly improved. In the fracture separation type connecting rod manufactured by such a construction method, the fracture form of the fracture surface is brittle, the deformation near the fracture surface due to fracture separation is small, and the amount of chipping due to fracture separation is small, that is, It is required that the fracture separation is good.

  As a steel material to be used for the fracture separation type connecting rod, DIN standard C70S6 is widely used in Europe and the United States. This is a high carbon non-tempered steel containing 0.7% by mass of C, and its metal structure is a pearlite structure with low ductility and toughness in order to suppress dimensional change during fracture separation. C70S6 is excellent in fracture separability due to its small plastic deformation in the vicinity of the fractured surface at the time of fracture, while it has a coarser structure than the ferrite-pearlite structure of medium carbon non-tempered steel that is the current steel for connecting rods. The yield ratio (= yield strength / tensile strength) is low, and there is a problem that it cannot be applied to a high-strength connecting rod that requires high buckling strength.

  In order to increase the yield ratio of steel, it is necessary to reduce the carbon content and increase the ferrite fraction. However, increasing the ferrite fraction improves the ductility of the steel material, increases the amount of plastic deformation during fracture separation, increases the shape deformation of the connecting rod sliding portion fastened to the pin portion of the crankshaft, and connects the connecting rod. There arises a problem in the performance of the parts such as the roundness of the part is lowered.

  Several non-tempered steels have been proposed as steel materials suitable for high strength fracture separation type connecting rods. For example, Patent Document 1 and Patent Document 2 describe a technique for improving fracture separability by adding a large amount of an embrittlement element such as Si or P to a steel material and reducing the ductility and toughness of the material itself. ing. Patent Document 3 and Patent Document 4 describe a technique for improving the fracture separability of a steel material by reducing precipitation ductility and toughness of ferrite using precipitation strengthening by second phase particles. Furthermore, Patent Documents 5 to 7 describe techniques for improving the fracture separability of steel materials by controlling the form of Mn sulfide.

  On the other hand, in recent years, with the increase in engine output due to the widespread use of high-power diesel engines or turbo engines, there is an increasing need for higher strength connecting rods. As one of the 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 precipitation of steel by fine VC has been used. Among the elements that generate alloy carbides, V has a large amount of solid solution in the steel material by heating before hot forging (around 1250 ° C.), and a large amount of precipitation strengthening can be obtained. However, there is a limit to the amount of V dissolved in steel materials, and it is difficult to further increase the strength by precipitation strengthening of VC.

  As described above, the present situation is that no steel that can meet the recent demands for increasing the strength of connecting rods and that can produce a fractured separating type connecting rod having excellent strength has not been obtained.

Japanese Patent No. 3637375 Japanese Patent No. 3756307 Japanese Patent No. 3355132 Japanese Patent No. 3988661 Japanese Patent No. 4314851 Japanese Patent No. 3671688 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-tempered 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-tempered steel part having excellent strength, excellent yield strength and yield ratio (in the following description, “high-strength hot We have eagerly studied how to realize “forged non-tempered steel parts” as simply “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 steel having a ferrite and pearlite structure, the yield ratio decreases. This is because the increase in yield strength is small relative to the increase in tensile strength associated with an increase in C content. This is because steel with a ferrite-pearlite structure mainly composed of ferrite and having a relatively low C content exhibits a yield point phenomenon (discontinuous yield) at the time of tensile fracture, whereas it has a pearlite-based ferrite-pearlite structure. This is because a steel having a high target C content has a continuous yield with a smooth transition from elastic deformation to plastic deformation at the time of tensile fracture.

  (B) The inventors have repeatedly studied a means for suppressing the decrease in the ferrite content while increasing the C content. As a result, it has been 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 area% or more, a yield point phenomenon occurs even at a high carbon composition, and a high yield ratio can be obtained.

  (C) The inventors of the present invention repeatedly studied another means for finely dispersing Mn sulfide while maintaining the break separation property by setting the contents of Mn and S within a predetermined range. As a result, it has been found that it is effective to contain one or more selected from the group consisting of trace amounts 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 and pearlite structure, the inventors have improved the yield ratio by combining precipitation strengthening with VC. We found that the strength was improved.

  Based on the knowledge of (a) to (d) as described above, Mn sulfide is contained by containing one or more selected from the group consisting of trace amounts of Sb, Sn, Te and Pb in the steel. By finely dispersing the steel, the yield ratio of the steel is improved if the area ratio of the ferrite structure in the ferrite / pearlite structure is 20 area% or more while having a high carbon composition with a C content of 0.50% or more. It has been found that the yield strength can be improved, and the present invention has been made.

The gist of the invention is as follows.
(1) The high strength hot forged non-heat treated steel part according to one aspect of the present invention has a chemical composition of unit mass%, 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 ~ 0.0050% and Pb: contain 0.001% to 0.0050% or more selected from the group consisting of 0.0001% and 0.0050%, the balance is Fe and inevitable impurities, the steel structure is ferrite pearlite Yes, of which the area ratio of the ferrite structure is 20% by area or more
(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 do.
(3) In the present invention, the chemical component is further 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%. It can contain seeds or two or more.

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

It is a disassembled perspective view which shows the fracture | rupture isolation | separation type | mold connecting rod which is an example of the high intensity | 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 separation type 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 separation type connecting rod 1 of this example is composed of a semicircular arc-shaped upper half halved body 2 and a semicircular arc-shaped lower half halved body 3 that are divided vertically. ing.
Screw holes 5 having thread grooves for fixing to the lower half half 3 are formed on both end sides of the semicircular arc portion 2A of the upper half half 2. Insertion holes 6 for fixing to the upper half halves 2 are formed at both ends of the semicircular arc portion 3A of the lower half halves 3 respectively.

  The semicircular arc portion 2A of the upper half half 2 and the semicircular arc portion 3A of the lower half half 3 are aligned in an annular shape, and the coupling bolts 7 are inserted into the insertion holes 6 and the screw holes 5 on both ends. The annular big end portion 8 is configured by screwing. An annular small end portion 9 is formed on the upper end side of the rod portion 2 </ b> B of the upper side half body 2.

  1 is incorporated in an internal combustion engine in order to convert the reciprocating motion of a piston of an internal combustion engine such as an automobile engine into a rotational 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 separation type connecting rod 1 of the present embodiment is formed from a high-strength hot-forged non-heat treated steel having the components, structure, and Mn sulfide dispersed state described below, and a semicircular arc portion of the upper half halved body 2 2A and the semicircular arc portion 3A of the lower half half 3 are formed by brittle fracture of a portion that was originally one annular component. As an example of the manufacturing method of the fracture separation type connecting rod 1, a notch is provided in a part of a hot forged product, and the fracture is separated brittlely starting from the notch. And a butting surface 3 a of the semicircular arc portion 3 </ b> A of the lower half half 3. Since these abutting surfaces 2a and 3a are originally formed by breaking and separating one member, they can be abutted with good alignment accuracy.

  The break-separated connecting rod 1 having this structure eliminates the need for new processing of the abutting surface and positioning pins, and greatly simplifies the manufacturing process.

Hereinafter, the non-tempered steel for high-strength hot forging constituting the fracture separation type connecting rod 1 will be described.
For example, the fracture separation type connecting rod 1 has a chemical component of unit mass%, 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: 0.00. 0020-0.0080%, Sb: 0.0001-0.0050%, Sn: 0.0001-0.0050%, Te: 0.0001-0.0050% and Pb: 0.0001- It contains one or more selected from the group consisting of 0.0050%, the balance consists of Fe and inevitable impurities, the steel structure is ferrite pearlite, of which the area ratio of the ferrite structure is 20 area% It consists of the above steel. A steel with this composition is hot forged, air-cooled, and made into non-tempered steel, thereby obtaining a high-strength hot-forged non-tempered steel part with the above-mentioned target composition.

<Steel component>
First, the reasons for limiting the component composition of the high-strength hot-forged non-tempered steel part according to this embodiment will be described.

(C: 0.50 to 0.65%)
C has the effect of ensuring the tensile strength of the high-strength hot-forged non-tempered steel part. In order 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 ferrite content contained in the steel metal structure (steel structure) is usually less than 20 area%, and the yield ratio of the steel is lowered. However, since the steel according to the present embodiment contains Mn, S, Sb, Sn, Te and Pb within a predetermined range as will be described later, Mn sulfide is finely dispersed. The ferrite content can be 20% by area or more while maintaining 50% or more. A preferable 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 leads to a decrease in the ratio of yield strength to yield strength (yield ratio). Therefore, the upper limit of the C content is set to 0.65%. The upper limit with preferable C content is 0.63%, 0.60%, or 0.59%.

  In the present embodiment, regarding the element content, when a range is described as 0.50 to 0.65%, unless otherwise specified, the range includes upper and lower limit numerical values. 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 lowers the ductility and toughness of steel. The decrease in ductility and toughness of steel reduces the amount of plastic deformation near the fracture surface at the time of fracture and improves fracture separability. In order 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, so the upper limit of Si content is 1.20%. A preferable lower limit of the Si content is 0.70%, 0.80%, or 0.85%. The upper limit with preferable Si content is 1.00%, 0.95%, or 0.90%.

(Mn: 0.60 to 1.00%)
Mn strengthens ferrite by solid solution strengthening, and lowers the ductility and toughness of steel. The decrease in the ductility and toughness of the steel reduces the amount of plastic deformation near the fracture surface at the time of fracture, and improves the fracture separability. Mn combines with S to form Mn sulfide. This Mn sulfide becomes the nucleus of ferrite transformation in the cooling process after forming a part by hot forging, and has the effect of increasing the amount of ferrite. On the other hand, when Mn is contained excessively, the ferrite becomes too hard and the frequency of occurrence of chipping at the time of fracture increases. In view of these, the range of Mn content is 0.60 to 1.00%. A preferable lower limit of the Mn content is 0.70%, 0.80%, or 0.85%. The upper limit with preferable 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 the steel and improving the fracture separability. However, P causes the above-described effects and at the same time, the effect of causing embrittlement of the grain boundaries and facilitating chipping of the fracture surface. Considering the above, the range of the P content is 0.040 to 0.060%. The minimum with preferable P content is 0.042%, 0.045%, or 0.048%. A preferable upper limit of the P content is 0.055%, 0.053%, or 0.050%.

(S: 0.060-0.100%)
S combines with Mn to form Mn sulfide. This Mn sulfide becomes the nucleus of ferrite transformation in the cooling process after forming a part by hot forging, and has the 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, when S is contained excessively, the amount of plastic deformation in the vicinity of the fracture surface at the time of fracture division may increase, and the fracture separability may decrease. In addition to this, when S is excessively contained, chipping of the fracture surface may be promoted. From the above, the range of S content is set to 0.060 to 0.100%. A preferable lower limit of the S content is 0.070%, 0.072%, or 0.075%. The upper limit with preferable S content is 0.095%, 0.090%, or 0.085%.

  Fine Mn sulfide becomes the nucleus of ferrite transformation in the cooling process after forming a part by hot forging. Therefore, precipitation of fine Mn sulfide has the effect of contributing to ferrite structure formation. The total amount of Mn sulfide precipitated in the steel does not vary greatly in the composition range of the steel of this embodiment, but the situation in which fine Mn sulfide is precipitated is extended by processing by forging and refined. This 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 major axis of Mn sulfide by the length of the minor axis of Mn sulfide. The average aspect ratio of the Mn sulfide is an average value of the aspect ratio of the Mn sulfide measured by observing the structure from the vertical direction of the parallel section in the longitudinal direction.
In this sense, the average aspect ratio of the fine Mn sulfide is desirably in the range of about 1.1 to 1.4.

(Cr: 0.05-0.20%)
Cr, like Mn, strengthens ferrite by solid solution strengthening and decreases ductility and toughness. The decrease 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 separability. However, when Cr is contained excessively, the lamellar spacing of pearlite is reduced, and the ductility and toughness of pearlite are increased. For this reason, when Cr is excessively contained, the amount of plastic deformation near the fracture surface at the time of fracture increases, and the fracture separability decreases. Further, when Cr is excessively contained, a bainite structure is easily formed, and a decrease in yield strength due to a decrease in yield ratio and a significant decrease in fracture separation are observed. Therefore, the range of Cr content is made 0.05 to 0.20%. In view of the above effects, the preferable upper limit of the Cr content is 0.17%, 0.15%, or 0.13%. Moreover, the minimum with preferable 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, strengthens ferrite, and lowers the ductility and toughness of steel. The decrease 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 separability of hot forged parts. V has the effect of increasing the yield ratio of hot forged parts by precipitation strengthening of carbides or carbonitrides. In order 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, since the effect is saturated even if V is contained excessively, the upper limit of V content is 0.40%. The upper limit of the V content is preferably 0.35%, 0.33%, or 0.31%.

(N: 0.0020 to 0.0080%)
N promotes ferrite transformation by mainly forming V nitride or V carbonitride during cooling after hot forging and acting as a transformation nucleus of ferrite. Thereby, N has the effect of suppressing the formation of a bainite structure that significantly impairs the fracture separability of the hot forged parts. In order to obtain this effect, the lower limit of the N content is set to 0.0020%. If N is contained excessively, the hot ductility is lowered and cracking or flaws are likely to occur during hot working, so 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 value of the N content may be 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)
In addition to the above components, the high-strength hot-forged non-heat treated steel part according to the present embodiment is one or two or more selected from the group consisting of Sb, Sn, Te, and Pb. It is characterized by containing in a range of ˜0.0050%. In these elements, Mn sulfide is finely dispersed as the solidification structure of steel is refined. In order to obtain the effect of finely dispersing Mn sulfide, the content of these elements needs to be 0.0001% or more. However, if these elements are contained excessively, these elements precipitate on the Mn sulfide and lose the effect as the core of 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%.

(Ti: 0.10% or less and Nb: one or two selected from the group consisting of 0.05% or less)
Ti and Nb mainly form carbides or carbonitrides during cooling after hot forging, strengthen the ferrite by precipitation strengthening, and lower the ductility and toughness of the 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, in order to obtain the above-described effect, the lower limit of the Ti content may be 0.05%, and the lower limit of the Nb content may be 0.01%. However, if these elements are contained excessively, the effect is saturated, so the upper limit of Ti content is 0.10% and the upper limit of Nb content is 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 and serve as crystallization nuclei for Mn sulfide, which has the effect of uniformly and finely dispersing Mn sulfide. Therefore, the lower limit values of Ca, Zr and Mg may be 0.001%. On the other hand, if the content of any element exceeds 0.005%, hot workability deteriorates and hot rolling becomes difficult. From these things, the upper limit of each content of Ca, Zr and Mg is made 0.005%.

  The balance of the chemical components of the high-strength hot-forged non-tempered steel part according to this embodiment includes iron (Fe) and unavoidable impurities. Inevitable impurities are components mixed in by various factors of raw materials such as ore or scrap, or manufacturing process when industrially manufacturing steel, and high strength hot forging according to this embodiment. It means that it is allowed as long as it does not adversely affect non-tempered steel parts.

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

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

  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 selected from one or two selected from the group consisting of Sb, Sn, Te and Pb. It is enhanced by using Mn sulfide finely dispersed by more than seeds. The present inventors have found that when the area ratio of the ferrite structure is 20 area% or more with respect to the entire steel, a yield point phenomenon occurs even at a high carbon composition, and a high yield ratio can be obtained. Therefore, the lower limit of the area ratio of the ferrite structure with respect to the entire steel is set to 20 area%. The lower limit of the area ratio of the ferrite structure relative to the entire steel may be 22 area%, 23 area%, or 25 area%.

In addition, it is preferable that the remainder of the steel structure of the present embodiment is a pearlite structure except for the ferrite structure, and other structures such as a bainite structure are not generated.
However, if it is within the range of less than 2 area%, inclusion of a structure other than ferrite and pearlite such as bainite and martensite is allowed. Further, since it is preferable that the amount of the ferrite structure is large, the upper limit value of the area ratio of the ferrite structure with respect to the entire steel is not particularly limited, but the range of chemical components of the high-strength hot forged non-heat treated steel part according to this embodiment In general, the upper limit of the area ratio of the ferrite structure is generally about 50 area%. The upper limit of the area ratio of the ferrite structure relative to the entire steel may be 35 area%, 30 area%, or 28 area%.

  With the fracture separation type connecting rod 1 as a high-strength hot-forged non-heat treated steel part according to this embodiment, new processing of the butt surface and positioning pins are unnecessary, and the manufacturing process can be greatly simplified.

The method for producing a high-strength hot-forged non-tempered steel part according to the present embodiment is obtained by casting, hot-rolling steel having chemical components of the above-described high-strength hot-forged non-tempered steel part according to the present embodiment. And a hot forging step.
Casting conditions are not particularly limited, and may be normal 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 such that the cooling rate is changed during cooling after forging, that is, when cooling by air or by blast cooling by a blast cooling device. It changes with. 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, the cooling rate needs to be 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 the cooling means after forging.

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

  The use of the high-strength hot-forged non-tempered steel part according to the present embodiment is not particularly limited. However, when applied to a machine part that is used by breaking and dividing, for example, a break-separated connecting rod, a particularly advantageous effect is achieved. .

  The invention is described in detail below by means 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 molten steel having the composition shown in Table 1 and Table 2 below was manufactured by continuous casting, and, as necessary, a rolling raw material of 162 mm square was obtained through a soaking diffusion process and a block rolling process.
Next, it was made a steel bar shape with a diameter of 45 mm by hot rolling. The underlined portion in Table 2 indicates an example outside the scope of the present invention.

  Next, in order to examine the structure and mechanical properties, a test piece corresponding to a forged connecting rod was produced by hot forging. Specifically, a steel bar having a diameter of 45 mm is heated to 1150 to 1280 ° C., and then forged perpendicularly to the length direction of the steel bar to a thickness of 20 mm, by air cooling by standing 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 created. A JIS No. 4 tensile test piece was processed from the forged material after cooling.

  The tensile test was carried out at a normal temperature of 20 mm / min in accordance with JIS Z 2241. Those whose yield strength did not reach 900 MPa were judged to be inferior in strength.

  A 10 mm square sample was cut out from the same site as the tensile test piece, and the form of Mn sulfide and the ferrite / pearlite structure in the steel were observed from the vertical direction of the longitudinal direction.

  In order to measure the aspect ratio of Mn sulfide in steel, after polishing on a mirror surface, 10 micrographs of the structure of 1000 times are taken with an optical microscope, and the average aspect ratio of Mn sulfide is a small general-purpose image analyzer ( Luzex (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 size of Mn sulfide is smaller when the average aspect ratio of Mn sulfide is closer to 1. The number density is large. That is, the size and dispersion state of Mn sulfide were determined by comparing the average aspect ratio values of Mn sulfide. In addition, in order to measure the area ratio of the ferrite structure in the ferrite / pearlite structure, corrosion was performed with a nital etchant, and five 200-fold structure photographs were taken with an optical microscope. The area ratio was determined by a small general-purpose image analyzer (Luzex: registered trademark, manufactured by Nireco Corporation). The metal structures of the examples shown in Tables 1 and 2 were substantially composed of ferrite and pearlite.

In Table 1, steel no. Examples of the present invention of A to AK are high strength hot forged non-heat treated steel parts each having a yield strength of 900 MPa or more within the specified range of steel chemical components.
On the other hand, in Table 2, the 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.
Since the comparative example AM has a low C content, the required yield strength cannot be obtained.
Since 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, the area ratio of the ferrite structure in the ferrite-pearlite structure is less than 20 area%. It is. For this reason, the yield ratio is low, and the required yield strength cannot be obtained.
Since Comparative Example AR has a high Cr content and Comparative Example BH has a low N content, a bainite structure is generated. For this reason, 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 finely dispersing Mn sulfide by Sb, Sn, Te and Pb, and the area ratio of the ferrite structure in the ferrite / pearlite structure is 20 Less than area%. For this reason, the yield ratio is low, and the required yield strength cannot be obtained.
In Comparative Examples AU to BG, the content of any of Sb, Sn, Te, and Pb is large. On the contrary, the effect of finely dispersing Mn sulfide by Sb, Sn, Te, and Pb is reduced, and the ferrite structure in the ferrite / pearlite structure The area ratio is less than 20 area%. For this reason, the yield ratio is low, and the required yield strength cannot be obtained.

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

Claims (3)

Chemical component is unit mass%,
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: 0.05-0.20%,
V: 0.25 to 0.40%,
N: 0.0020 to 0.0080% is contained, 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%
1 type or 2 types or more selected from the group consisting of: The balance consists of Fe and inevitable impurities, the steel structure is ferrite pearlite, and the area ratio of the ferrite structure is 20 area% or more Features high strength hot forged non-tempered steel parts. Furthermore, the chemical component is unit mass%,
The high-strength hot-forged non-tempered 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. Furthermore, the chemical component is unit mass%,
Ca: 0.005% or less,
The non-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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