WO2017110883A1

WO2017110883A1 - Steel

Info

Publication number: WO2017110883A1
Application number: PCT/JP2016/088123
Authority: WO
Inventors: 真也寺本; 聡志賀
Original assignee: 新日鐵住金株式会社
Priority date: 2015-12-25
Filing date: 2016-12-21
Publication date: 2017-06-29
Also published as: CN108474068A; JP6620822B2; KR20180087371A; KR102101233B1; EP3395975A4; EP3395975A1; US20180371593A1; JPWO2017110883A1

Abstract

A steel according to an aspect of the present invention contains, in mass%, 0.10-0.25% of C, 0.60-1.20% of Si, 0.60-1.00% of Mn, 0.040-0.060% of P, 0.060-0.100% of S, 0.05-0.20% of Cr, 0.0001-0.0050% of Bi, 0.0020-0.0150% of N, 0-0.010% of V, 0-0.0050% of Al, 0-0.020% of Ti, 0-0.005% of Ca, 0-0.005% of Zr, and 0-0.005% of Mg, with the remainder comprising Fe and impurities.

Description

steel

The present invention relates to steel, and more particularly, to a non-heat treated steel for hot forging excellent in fracture separation.
This application claims priority based on Japanese Patent Application No. 2015-253563 filed in Japan on December 25, 2015, the contents of which are incorporated herein by reference.

Car engine parts and undercarriage parts are formed by hot forging, 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 that are not tempered, that is, non-tempered parts are widely used.

Examples of automotive engine parts include connecting rods (hereinafter referred to as connecting rods). 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 increase the efficiency of fastening between the cap part and the rod part, a break-separated connecting rod has been often used in recent years.

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 this part 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. The fracture separation type connecting rod manufactured by such a method has a brittle fracture surface, a small deformation near the fracture surface due to fracture separation, and a small amount of chipping during fracture separation. That is, it is required that the break separation property is good.

As a steel material to be used for the fracture separation type connecting rod, the 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 having 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 increases the ductility of the steel material, increases the amount of plastic deformation at the time of fracture separation, increases the shape deformation of the connecting rod sliding part fastened to the pin part of the crankshaft, and roundness This causes a problem in the performance of the parts such as a decrease in the number of parts.

Several non-tempered steels have been proposed as steel materials suitable for high-strength fracture-separated connecting rods. For example, Patent Document 1 and Patent Document 2 include a technique for improving the fracture separability of a steel material by adding a large amount of an embrittlement element such as Si or P to the steel material and reducing the ductility and toughness of the steel material itself. Are listed. 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 by utilizing precipitation strengthening of second phase particles. Further, Patent Documents 5 to 7 describe techniques for improving the fracture separability of steel materials by controlling the form of Mn sulfide. Patent Document 8 describes a technique for improving the fracture separation property of steel by cooling the steel to −60 ° C. or lower and then performing fracture separation.
However, the situation is that none of the techniques is sufficiently satisfactory with respect to break separation.

In the techniques described in Patent Document 1, Patent Document 2, and Patent Document 6, it is required to contain a large amount of C in order to increase the strength of the steel. When steel having such characteristics is subjected to fracture separation, the amount of chipping at the fracture surface increases and the fracture separation property is insufficient. However, Patent Document 1, Patent Document 2, and Patent Document 6 do not discuss any means for suppressing the amount of chipping.

In the technique of Patent Document 3, it is required to limit the Mn content to less than a predetermined value in order to reduce the ductility of steel. However, Mn is an element that is effective for forming irregularities on the fracture surface caused by fracture separation and enhancing the fitting property of the fracture surface. When the steel described in Patent Document 3 is subjected to fracture separation, a sufficient size and number of irregularities are not formed on the fracture surface, so that fracture separation is insufficient. However, in Patent Document 3, no consideration is given to the fitting property of the fracture surface.

In the techniques of Patent Document 4, Patent Document 5 and Patent Document 7, it is required to contain V and / or Ti in order to embrittle ferrite in steel and improve fracture separation. However, the present inventors have found that when V or Ti is added to steel to such an extent that ferrite is embrittled, segregation of these elements occurs and chipping occurs in a region where the concentration of V or Ti is high. When the steels described in Patent Document 4, Patent Document 5 and Patent Document 7 are subjected to fracture separation, since the amount of chipping cannot be suppressed, the fracture separability is insufficient. However, Patent Document 4, Patent Document 5 and Patent Document 7 do not discuss any segregation of ferrite embrittlement elements such as V and Ti.

In the technique described in Patent Document 8, in order to enhance the mechanical properties of steel, it is required to set an index Ceq indicating the post-forging hardness of steel to a predetermined value or more. Steel having such characteristics has a large amount of chipping at the time of break separation, and the break separation property is impaired. Breaking separation at a low temperature of −60 ° C. or less, which is proposed in Patent Document 8, lowers the economic efficiency at the time of manufacturing parts.

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 Japanese Unexamined Patent Publication No. 2004-183094

As described above, the fracture separability is evaluated by, for example, the deformation amount at the fracture surface, the brittle fracture surface ratio at the fracture surface, the size and number of irregularities at the fracture surface, and the amount of chipping at the fracture surface. Suppression of the deformation amount and improvement of the brittle fracture surface ratio are achieved by lowering the toughness of the steel. For example, in a steel having a low Charpy impact value, which is an index of toughness, it is normal that suppression of deformation and improvement of the brittle fracture surface ratio are achieved. According to the prior art, by adding V and Ti or the like to steel to cause precipitation strengthening in ferrite, the toughness of the steel is reduced, and the suppression of deformation and the improvement of the brittle fracture surface ratio have been achieved. . However, these elements, particularly V, are easily segregated. When the amount of these embrittlement elements required to improve the fracture separation is added to the steel, excessive embrittlement occurs in the segregation part of these elements (the part where the concentration of these elements is higher than the surrounding area). Chipping occurs. Thereby, the amount of chipping at the time of break separation increases, and break separation properties are impaired. Therefore, it is necessary to ensure break separation without using an element that increases the amount of chipping such as V.

Also, a high yield ratio is required for steel used as a material for machine parts such as high-strength connecting rods that require high buckling strength.

In view of the above circumstances, an object of the present invention is to provide a non-heat treated steel for hot forging excellent in fracture separability and yield ratio. Specifically, an object is to provide a steel that can achieve both a reduction in toughness and a reduction in the amount of chipping, and also has an excellent yield ratio.

In order to solve the above-mentioned problems, the present inventor has earnestly studied a method for realizing a non-heat treated steel for hot forging excellent in fracture separability, and as a result, obtained the following knowledge (a) and (b). It was.

(A) It has been found that the toughness is remarkably lowered by containing a small amount of Bi in the steel. This is because Bi dissolved in steel significantly embrittles ferrite. Due to this effect, a steel material having a low carbon composition that is inferior in break separation can be used as a non-heat treated steel for break separation.

(B) It was found that the toughness can be reduced by containing a small amount of Bi in the steel without containing V which is easily segregated. This is because the effect of embrittlement of ferrite is significantly larger in the case of solid solution Bi than the precipitation strengthening by VC.

Based on the knowledge of (a) and (b) as described above, the steel material having a low carbon composition also has sufficient fracture separability by containing a trace amount Bi without containing a conventionally known ferrite embrittlement element such as V. It has been found that it can be improved, and the present invention has been made.

The gist of the invention is as follows.
(1) The steel according to one embodiment of the present invention is unit mass%, C: 0.10 to 0.25%, 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%, Bi: 0.0001 to 0.0050%, N: 0.0020 ~ 0.0150%, V: 0 ~ 0.010%, Al: 0 ~ 0.0050%, Ti: 0 ~ 0.020%, Ca: 0 ~ 0.0050%, Zr: 0 ~ 0.0050% And Mg: 0 to 0.0050%, with the balance being Fe and impurities.
(2) The steel described in (1) is unit mass%, Ca: 0.0005 to 0.0050%, Zr: 0.0005 to 0.0050%, and Mg: 0.0005 to 0.0050. % May be contained.
(3) The steel described in (1) or (2) above may contain N: 0.0020 to 0.0090% in unit mass%.
(4) The steel according to any one of (1) to (3) may contain Al: 0 to 0.0008% in unit mass%.
(5) The steel according to any one of the above (1) to (4) may contain V: 0 to 0.004% in unit mass%.

According to the present invention, it is possible to provide a non-tempered steel for hot forging that achieves both a reduction in toughness and a reduction in the amount of chipping, and is excellent in fracture separation and yield ratio.

It is a disassembled perspective view which shows the connecting rod which is an example of the use of the steel which concerns on 1 aspect of this invention.

<Steel component>
First, the reasons for limiting the component composition of steel according to this embodiment will be described. Hereinafter, the unit “%” of the alloying element content of steel means “mass%” unless otherwise specified.

C: 0.10 to 0.25%
C has an effect of securing the tensile strength of steel. In order to obtain the required strength, the lower limit of the C content needs to be 0.10%. On the other hand, when C is contained excessively, the frequency of occurrence of chipping of the fracture surface increases, so the upper limit of the C content is 0.25%. The lower limit of the C content may be 0.12%, 0.15%, or 0.19%. The upper limit of the C content may be 0.23%, 0.22%, or 0.21%.

Si: 0.60 to 1.20%
Since Si strengthens ferrite by solid solution strengthening and decreases the ductility and toughness of the steel, it improves the fracture separability of the steel. 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 contained excessively, the frequency of occurrence of chipping of the fracture surface increases, so the upper limit of the Si content is 1.20%. The lower limit value of the Si content may be 0.70%, 0.75%, or 0.80%. The upper limit of the Si content may be 1.00%, 0.90%, or 0.85%.

Mn: 0.60 to 1.00%
Since Mn strengthens ferrite by solid solution strengthening and lowers the ductility and toughness of the steel, it improves the fracture separability of the steel. Mn combines with S to form Mn sulfide. When cracking and separating steel parts made of steel of this embodiment, cracks propagate along the Mn sulfide elongated in the rolling direction, so that the Mn sulfide has a larger unevenness on the fracture surface and fits the fracture surface. This has the effect of preventing displacement. On the other hand, when Mn is excessively contained, the frequency of occurrence of chipping of the fracture surface increases because the ferrite becomes too hard. In view of these, the range of Mn content is 0.60 to 1.00%. The lower limit value of the Mn content may be 0.70%, 0.80%, or 0.82%. The upper limit value of the Mn content may be 0.90%, 0.87%, or 0.85%.

P: 0.040 to 0.060%
P decreases the ductility and toughness of ferrite and pearlite, and embrittles the steel. Usually, P is regarded as an impurity element which is not preferably contained. In steel that is a material of a part manufactured by a manufacturing method that does not include fracture separation, the P content is usually about 0.020% or less in order to prevent embrittlement of the part. However, in the steel according to the present embodiment for the purpose of improving the break separation property, P is beneficial because it has the effect of improving the break separation property. Therefore, in the steel according to the present embodiment, it is necessary that the P content be 0.040% or more, which greatly exceeds the range included in ordinary steel as impurities. However, an excessive amount of P causes embrittlement of the crystal grain boundary and easily causes chipping of the fracture surface. Considering the above, the range of P content is 0.040 to 0.060%. The lower limit value of the P content may be 0.042%, 0.045%, or 0.048%. The upper limit of the P content may be 0.058%, 0.055%, or 0.050%.

S: 0.060 to 0.100%
S combines with Mn to form Mn sulfide. When cracking and separating a steel part made of steel according to this embodiment, cracks propagate along the Mn sulfide elongated in the rolling direction, so S increases the irregularities of the fracture surface and fits the fracture surface. This has the effect of preventing displacement. 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 separation increases, and the fracture separability may decrease. In addition to this, when S is excessively contained, chipping of the fracture surface may be promoted. For these reasons, the S content range is 0.060 to 0.100%. The lower limit value of the S content may be 0.070%, 0.075%, or 0.080%. The upper limit value of the S content may be 0.090%, 0.088%, or 0.085%.

Cr: 0.05-0.20%
Cr, like Mn, strengthens ferrite by solid solution strengthening and reduces the ductility and toughness of the steel, and thus improves the fracture separability of the steel. However, when Cr is excessively contained, the lamellar spacing of pearlite is reduced, and on the contrary, the ductility and toughness of pearlite are increased, so that the fracture separation property of steel is lowered. Further, when Cr is excessively contained, a bainite structure is likely to be generated, resulting in a decrease in yield strength due to a decrease in yield ratio and a significant decrease in fracture separability. Therefore, the Cr content range is 0.05 to 0.20%. The lower limit of the Cr content may be 0.07%, 0.09%, or 0.10%. The upper limit of the Cr content may be 0.17%, 0.16%, or 0.15%.

Bi: 0.0001 to 0.0050%
Bi is an important element in the steel according to the present embodiment. When the steel contains a small amount of Bi, the solid solution Bi embrittles the ferrite and lowers the ductility and toughness of the steel, so that the fracture separability of the steel is improved. What should be noted here is that the ferrite embrittlement effect of Bi is expressed in a very small amount. As the inventors have found out, in order to obtain the above-described effect, the Bi content should be 0.0001% or more. It has not been reported so far that such a small amount of Bi improves the break-separability of steel. In addition, when Bi was used to embrittle ferrite, an increase in the amount of chipping was not confirmed. The cause of this is unknown, but it was estimated that the amount of Bi was so fine that the effect of Bi segregation was so small that it could be ignored.

However, if the Bi content exceeds 0.0050%, the embrittlement effect of ferrite by Bi is saturated and the yield strength is reduced. For these reasons, the Bi content is 0.0001% to 0.0050% in the steel according to the present embodiment. The lower limit value of Bi content may be 0.0025%, 0.0028%, or 0.0030%. The upper limit of Bi content may be 0.0045%, 0.0042%, or 0.0040%.

N: 0.0020 to 0.0150%
If N contains V or Ti in the steel, these nitrides or carbonitrides are formed, but other Ns exist in a solid solution state in the steel. Solid solution N (that is, N in a state of solid solution in steel) embrittles ferrite and lowers the ductility and toughness of the steel, so that the fracture separation of the steel is improved. 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 N content is 0.0150%. The lower limit value of the N content may be 0.0050%, 0.0070%, or 0.0080%. The upper limit of the N content may be 0.0100%, 0.0095%, or 0.0090%.

V: 0 to 0.010%
V has the effect of forming carbides or carbonitrides to cause precipitation strengthening in the ferrite, reducing the ductility and toughness of the ferrite, and thereby reducing the amount of deformation during fracture separation. Therefore, according to the prior art, V may be contained in steel that requires high fracture separation. However, in order to sufficiently obtain the above effect using V, the V content needs to be about 0.10% or more. When about 0.10% or more of V is contained in the steel, segregation of V occurs, and the ductility and toughness of the ferrite are excessively reduced in a region where the V concentration is high, so that chipping is likely to occur at the time of fracture separation of the steel. That is, V can reduce the amount of deformation during break separation, but increases the amount of chipping during break separation.

Since the steel according to the present embodiment contains the above-described trace amount of Bi, V is not required for improving the break separation property. Therefore, the lower limit of the V content is 0%. In order to reduce the amount of chipping, it is preferable not to contain V. However, when steel according to the present embodiment is manufactured using scrap as a material, V may be mixed. In this case, V of 0.010% or less is allowed because it does not increase the amount of chipping. The upper limit value of the V content may be 0.007%, 0.005%, 0.004%, or 0.002%. If scrap is not used as a steel material, the content of V mixed in the steel as impurities is usually 0.010% or less. Moreover, in the technical field to which the steel according to the present embodiment belongs, V of 0.010% or less is usually regarded as an impurity that does not substantially affect the properties of the steel. In a mill sheet or the like, 0.010% or less of V is considered to have a content of 0%, and the disclosure thereof may be omitted.

Al: 0 to 0.0050%
Since the steel according to the present embodiment can exhibit its effect without containing Al, the lower limit value of the Al content is 0%. Further, 0.0050% or more of Al forms Al oxide in the steel, and this Al oxide may impair the machinability of the steel. For the above reasons, the upper limit of the Al content of the steel according to this embodiment is set to 0.0050%. The upper limit value of the Al content may be 0.0040%, 0.0010%, or 0.0008%. In the technical field to which the steel according to this embodiment belongs, 0.0050% or less of Al is usually regarded as an impurity that does not substantially affect the properties of the steel. Accordingly, in a mill sheet or the like, 0.0050% or less of Al is considered to have a content of 0%, and the disclosure thereof may be omitted.

Ti: 0 to 0.020%
Ti, like V described above, has the effect of forming nitrides and causing precipitation strengthening in ferrite, reducing the ductility and toughness of ferrite, and thereby reducing the amount of deformation during fracture separation. However, like V described above, Ti may increase the amount of chipping during fracture separation.
Since the steel according to the present embodiment contains the above-described trace amount of Bi, Ti is not required for improving the break separation property. Therefore, the lower limit of the Ti content is 0%. In order to reduce the amount of chipping, it is preferable not to contain Ti. However, when the steel according to this embodiment is manufactured using scrap as a material, Ti may be mixed. In this case, 0.020% or less of Ti is allowed because it does not increase the amount of chipping. The upper limit value of the Ti content may be 0.010%, 0.005%, or 0.002%. If scrap is not used as a steel material, the content of Ti mixed into the steel as impurities is usually 0.020% or less. Further, in the technical field to which the steel according to the present embodiment belongs, 0.020% or less of Ti is usually regarded as an impurity that does not substantially affect the properties of the steel. In a mill sheet or the like, 0.020% or less of Ti is considered to have a content of 0%, and the disclosure thereof may be omitted.

Ca: 0 to 0.0050%, Zr: 0 to 0.0050%, Mg: 0 to 0.0050%
Since the steel according to the present embodiment can exhibit its effect without containing Ca, Zr, and Mg, the lower limit of the content of Ca, Zr, and Mg is 0%. However, Ca, Zr, and Mg all form oxides and become the crystallization nuclei of MnS and have the effect of uniformly and finely dispersing MnS. When cracking and separating a steel part made of steel according to this embodiment, cracks propagate along MnS elongated in the rolling direction, so that the larger the Mn sulfide, the larger the irregularities of the fracture surface, while the ductility and High toughness and low break separation. By finely dispersing MnS, it is easy to propagate in the crack propagation direction, and the effect of improving break separation is obtained. In order to obtain this effect, the steel according to the present embodiment contains one or more selected from the group consisting of 0.0005% or more of Ca, 0.0005% or more of Zr, and 0.0005% or more of Mg. You may do it. On the other hand, when the content of Ca, Zr, or Mg exceeds 0.0050%, the hot workability of the steel deteriorates, and the hot rolling of the steel becomes difficult. For these reasons, the upper limit of each of Ca, Zr, and Mg contents is set to 0.0050%.

The balance of the chemical components of the steel according to this embodiment includes iron and impurities. Impurities are components that are mixed due to various factors of raw materials such as ore or scrap, or manufacturing process when industrially manufacturing steel materials, and a range that does not adversely affect the steel according to the present embodiment. Means what is allowed.

The structure of the steel according to the present embodiment is a so-called ferrite pearlite structure that is substantially composed of ferrite and pearlite and may contain slight inclusions. This structure is obtained by controlling the chemical composition of steel within the above-described range. Therefore, although it is not necessary to explicitly limit the structure of the steel according to the present embodiment, for example, the structure of the steel according to the present embodiment may be defined as a structure including a total of 99 area% or more of ferrite and pearlite. .

The non-tempered steel described above has improved the fracture separability by adding V to the conventional material to improve the brittleness of the ferrite. On the other hand, the ferritic brittleness due to the effect of adding a small amount of Bi without adding V. The break separation property is improved.

The application of the steel according to the present embodiment described above is not particularly limited. However, since the steel according to the present embodiment has a good breaking separation property, it is suitable to be used as a material for machine parts (breaking separation type parts) obtained by a manufacturing method including a breaking separation process. It is particularly suitable for use as a material for the connecting rod. If it is the fracture separation type connecting rod 1 as a steel part which consists of non-heat treated steel of this embodiment, the new process of a butt surface and a positioning pin will become unnecessary, and the simplification of a manufacturing process can be achieved significantly.

FIG. 1 is an exploded perspective view showing an example of a fracture separation type connecting rod formed of steel according to the present invention.

1 is composed of a semicircular arc-shaped upper-side half-divided body 2 and a semicircular-arc-shaped lower-side half-divided body 3 that are separated vertically. Screw holes 5 having screw grooves for fixing to the lower half half 3 are respectively formed at both ends of the semicircular arc 2A of the upper side half 2. The semicircular arc 3A of the lower side half 3 is formed. Insertion holes 6 for fixing to the upper-side halves 2 are formed on both ends of each.

The semicircular arc portion 2A of the upper half halves 2 and the semicircular arc portion 3A of the lower halves 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. 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 2 </ b> B of the upper half 2.

1 is incorporated in an internal combustion engine to convert a reciprocating motion of a piston of an internal combustion engine such as an automobile engine into a rotational motion, and a small end portion 9 is connected to a piston (not shown). The big end portion 8 is connected to a connecting rod journal (not shown) of the internal combustion engine.

The semicircular arc part 2A of the upper half half body 2 of the fracture separation type connecting rod 1 and the semicircular arc part 3A of the lower side half half body 3 are formed by brittle fracture of a part that was originally one annular part. As an example, a notch is formed in a part of a hot forged product, and the notch is used as a starting point for breaking and separating in a brittle manner, so that the butted surface 2a of the semicircular arc portion 2A of the upper side half 2 and the lower side half The abutting surface 3a of the three semicircular arc portions 3A is formed. Since these abutting

surfaces

2a and 3a are formed by breaking and separating the steel according to the present embodiment having a good breaking separation property, the butting can be performed with a 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 present invention will be described in detail below by 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.

The converter molten steel having the composition shown in Table 1 below was manufactured by continuous casting, and if necessary, a rolling raw material of 162 mm square was obtained through a soaking diffusion treatment and a block rolling process. Next, the rolled material was hot-rolled to obtain a steel bar shape having a diameter of 45 mm. Values underlined in Table 1 are values outside the scope of the present invention. In addition, the symbol “-” in Table 1 indicates that the element related to the symbol was not added in the manufacturing stage and contained only an amount below a level normally regarded as an impurity. Note that the contents of V, Al, and Ti in Examples 1 to 23 and Comparative Examples a to h were very small to the extent that they were regarded as impurities according to the technical common sense in the technical field to which the present invention belongs. In order to confirm the action and effect of the invention, measurements were carried out in particular detail, and the values are shown in Table 1.

Next, in order to examine the fracture separability, mechanical properties, and structure, a test piece corresponding to a forged connecting rod was created by hot forging the above bar steel. Specifically, a steel bar having a diameter of 45 mm was heated to 1150 to 1280 ° C., then forged perpendicularly to the length direction of the steel bar to a thickness of 20 mm, and cooled to room temperature by blast cooling with a blast cooling device. A JIS No. 4 tensile test piece and a Charpy impact test piece were processed from the forged material after cooling. The Charpy impact test piece was subjected to V-notch processing of 45 degrees with a depth of 2 mm and a tip curvature of 0.25 mm.

The fracture separation property is said to be good when the fracture form of the fracture surface is brittle, the amount of deformation near the fracture surface due to fracture separation is small, and the amount of chipping during fracture separation is small. In steels with low Charpy impact values, it is normal that suppression of deformation and improvement of brittle fracture surface ratio are achieved. Therefore, the present inventors adopted the Charpy impact value as an index for evaluating the fracture form of the fracture surface and the deformation near the fracture surface. The Charpy impact test was repeated 5 times at room temperature based on JIS Z 2242 on the above Charpy impact test piece, and the average value of the five values obtained was taken as the Charpy impact value of the test piece. The steel having a Charpy impact value of 9 J / cm ² or less was judged to have achieved the suppression of deformation and the improvement of the brittle fracture surface ratio.
The method for measuring the amount of chipping was as follows. It is a plate of 80 mm x 80 mm and thickness 18 mm, and has a hole with a diameter of 50 mm at the center, and on the inner surface of this hole, a position of ± 90 degrees with respect to the length direction of the steel bar that is the material before forging Test pieces for fracture separation evaluation having a V notch of 45 degrees with a depth of 1 mm and a tip curvature of 0.5 mm were prepared at two locations. Furthermore, a through-hole having a diameter of 8 mm was formed as a bolt hole in the test piece for evaluation of fracture separability so that the center line is located at a location 8 mm from the side surface on the notch processing side. The test specimen for evaluation of break separation was broken using a break separation evaluation test apparatus. The test apparatus for evaluation of breaking separation is composed of a split mold and a falling weight tester. The split mold is a shape in which a 46.5 mm diameter cylinder formed on a rectangular steel material is divided into two along the center line. One side is fixed and one side moves on the rail. Wedge holes are machined on the mating surfaces of the two semi-cylinders. At the time of the break test, a hole with a diameter of 50 mm of the test piece is fitted into this split mold with a diameter of 46.5 mm, and a wedge is placed on the falling weight. The falling weight has a mass of 200 kg and is a mechanism that falls along the guide. When the falling weight is dropped, a wedge is driven and the test piece is pulled and broken in two. Note that the periphery of the test piece is fixed so as to be pressed against the split mold so that the test piece is not released from the split mold at the time of breaking. In this test, fracture was performed at a falling weight height of 100 mm. The fractured surfaces thus obtained were brought together, and the fractured steel was bolted and assembled with a torque of 20 N · m, and then the work of loosening the bolts and separating the fractured surfaces was repeated 10 times. The total weight of the pieces dropped off by this operation was defined as the amount of chipping of the steel. A steel having a chip generation amount of less than 1.00 mg was judged to have suppressed the chip generation amount.

The tensile test was performed on the above-mentioned JIS No. 4 tensile test piece at a speed of 20 mm / min at room temperature in accordance with JIS Z 2241. A sample having a yield ratio of 0.75 or more was judged as a sample having a good yield ratio.

Further, a 10 mm square sample was cut out from the same site as the Charpy impact test piece and the tensile test piece, subjected to nital corrosion, and the structure was observed.

Table 2 shows the test results. Steel No. In all of the inventive examples 1 to 23, the steel chemical composition was within the specified range of the present invention, so the Charpy impact value could be 9 J / cm ² or less, and the amount of chipping was further suppressed. That is, Steel No. 1 to 23 had good breaking separation. Furthermore, steel no. Since 1 to 23 had a high yield ratio, they could be used as materials for machine parts that required high buckling strength.

On the other hand, Comparative Example a had a low C content and therefore had a low tensile strength and a high Charpy impact value.
In Comparative Examples b to d, since the content of Si, Mn or P was small, the brittle effect of ferrite was small and the Charpy impact value was high.
In Comparative Example e, since the Cr content was large, a bainite structure was partially formed in addition to the ferrite / pearlite structure, and thus the Charpy impact value was high and the yield ratio was further impaired.
Since Comparative Example f did not contain Bi, there was no brittle effect of ferrite, and the Charpy impact value was high.
Since Comparative Example g contains Bi, the effect of embrittlement of ferrite was obtained and the Charpy impact value was low, but since the Bi content was large, the yield strength and yield ratio were low.
In Comparative Example h, since the V content was large, segregation of V occurred, the toughness of ferrite decreased excessively in the region where the V concentration was high, and the amount of chipping at the time of fracture separation of the steel increased.

The steel according to the present invention can achieve both a reduction in toughness and a reduced amount of chipping, and is also excellent in yield ratio. Therefore, when the steel according to the present invention is used as a non-heat treated steel for hot forging, which is a material of a machine part obtained by a production method including a fracture separation process, a machine part having a high buckling strength can be produced. In addition, the economic efficiency at the time of manufacturing parts can be greatly improved.

1 ... Breaking type connecting rod (steel part)
2 ... Upper side half 2A ... Semi arc part 2a ... Abutting surface 2B ... Rod part 3 ... Lower side half part 3A ... Semi arc part 3a ... Abutting surface 5 ... Screw hole 6 ... Insertion hole 7 ... Coupling bolt 8 ... Big end part 9 ... Small end part

Claims

In unit mass%
C: 0.10 to 0.25%,
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%,
Bi: 0.0001 to 0.0050%,
N: 0.0020 to 0.0150%,
V: 0 to 0.010%,
Al: 0 to 0.0050%,
Ti: 0 to 0.020%,
Ca: 0 to 0.0050%,
Zr: 0 to 0.0050% and Mg: 0 to 0.0050%
A steel characterized in that the balance is made of Fe and impurities.
In unit mass%
Ca: 0.0005 to 0.0050%,
Zr: 0.0005 to 0.0050%, and Mg: 0.0005 to 0.0050%
The steel according to claim 1, comprising at least one of the above.
In unit mass%
N: 0.0020 to 0.0090%
The steel according to claim 1, comprising:
In unit mass%
Al: 0 to 0.0008%
The steel according to any one of claims 1 to 3, characterized by comprising:
In unit mass%
V: 0 to 0.004%
The steel according to any one of claims 1 to 4, characterized by comprising: