EP3395975A1

EP3395975A1 - Steel

Info

Publication number: EP3395975A1
Application number: EP16878770.3A
Authority: EP
Inventors: Shinya Teramoto; Akira Shiga
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2015-12-25
Filing date: 2016-12-21
Publication date: 2018-10-31
Also published as: EP3395975A4; KR20180087371A; WO2017110883A1; US20180371593A1; KR102101233B1; CN108474068A; JP6620822B2; JPWO2017110883A1

Abstract

A steel according to one aspect of the present invention contains, 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% to 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 0.005%, Zr: 0% to 0.005%, Mg: 0% to 0.005%, and a remainder including Fe and impurities.

Description

[Technical Field of the Invention]

The present invention relates to steel and particularly to a forging steel free of quenching and tempering for hot forging having excellent fracture splitability.
Priority is claimed on Japanese Patent Application No. 2015-253563, filed December 25, 2015 , the content of which is incorporated herein by reference.

[Related Art]

Automotive engine parts and suspension parts are formed by hot forging and then subjected to a heat treatment such as quenching or tempering (hereinafter, parts on which a heat treatment is carried out will be referred to as quenching and tempering component) or skip a heat treatment (hereinafter, parts on which a heat treatment is not carried out will be referred to as non quenching and tempering component), thereby ensuring mechanical properties necessary for parts on which a heat treatment is carried out. Recently, from the viewpoint of the economic efficiency in manufacturing steps, parts that skip thermal refining, that is, non quenching and tempering component have been widely distributed.
An example of an automotive engine parts is a connecting rod (hereinafter, referred to as a connecting rod). These parts are parts that transmit power when the reciprocal motion of a piston in an engine is converted to a rotary motion by a crankshaft. The connecting rod fastens an eccentric portion that is referred to as a pin portion of the crankshaft by pinching the eccentric portion with the cap portion and the rod portion of the connecting rod and transmits power through a mechanism of the rotary sliding of the pin portion and the fastening portion of the connecting rod. In recent years, in order to efficiently fasten the cap portion and the rod portion, fracture split connecting rods have been widely used.
A fracture split connecting rod is a connecting rod for which a method in which steel is formed to a shape in which the cap portion and the rod portion are integrated together by means of hot forging or the like, then, a notch is made in a portion corresponding to the boundary between the cap portion and the rod portion, and this portion is fractured and splitted is used. In this method, since fractured and splitted fracture surfaces are fitted to each other in the mating surfaces of the cap portion and the rod portion, machining of the mating surfaces is not required, and thus it is possible to skip processing for positioning as necessary. Therefore, it is possible to significantly reduce processing steps of components, and the economic efficiency at the time of manufacturing components significantly improves. For fracture split connecting rods manufactured using the above method, it is necessary that the fracture form of fractured surfaces is brittle, the deformation amount in the vicinity of fracture surfaces caused by fracturing and splitting is small, and the chipping generation amount during fracturing and splitting is small, that is, the fracture splitability is favorable.
In Europe and the United States of America, as steel for fracture split connecting rods, C70S6 according to DIN standards is distributed. This is forging high carbon steel free of quenching and tempering including 0.7 mass% of C and is provided with a metallographic structure made of a pearlite structure having low ductility and low toughness in order to suppress dimensional changes during fracturing and splitting. C70S6 allows only a small plastic deformation amount in the vicinity of fractured surfaces during fracturing and thus has excellent fracture splitability. On the other hand, C70S6 has a structure that is coarser than the ferrite/pearlite structure of forging medium carbon steel free of quenching and tempering which is steel for the current connecting rods and thus has a low yield ratio (= yield strength/tensile strength) and has a problem of being inapplicable to high-strength connecting rods requiring 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 fraction of ferrite. However, when the fraction of ferrite is increased, the ductility of steel is improved, the plastic deformation amount during fracturing and splitting is increased, the shape deformation of a connecting rod sliding portion that is fastened to the pin portion of the crankshaft is increased, and a problem with parts properties such as a decrease in circularity is caused.
Several kinds of forging steels free of quenching and tempering have been proposed as preferred steel for high-strength fracture split connecting rods. For example, Patent Document 1 and Patent Document 2 disclose a technique for improving the fracture splitability of the steel by adding a large amount of an embrittlement element such as Si or P to steel and degrading the ductility and toughness of the steel. Patent Document 3 and Patent Document 4 disclose a technique for improving the fracture splitability of steel by degrading the ductility and toughness of ferrite using the precipitation strengthening of second phase particles. Furthermore, Patent Documents 5 to 7 disclose a technique for improving the fracture splitability of steel by controlling the form of a Mn sulfide. Patent Document 8 discloses a technique for improving the fracture splitability of steel by cooling the steel to -60°C or lower and fracturing and splitting the steel.
However, currently, it cannot be said that any of these techniques are sufficiently satisfactory in terms of the fracture splitability.
In the techniques disclosed in Patent Document 1, Patent Document 2, and Patent Document 6, in order to increase the strength of steel, the addition of a large amount of C is required. In a case where steel having the above properties is fractured and splitted, the chipping generation amount on the fractured surfaces increases, and thus the fracture splitability is unsatisfactory. However, in Patent Document 1, Patent Document 2, and Patent Document 6, there are no studies regarding means for suppressing the chipping generation amount.
In the technique of Patent Document 3, in order to degrade the ductility of steel, it is necessary to limit the Mn content to less than a predetermined value. However, Mn is an effective element to form unevenness on fractured surfaces generated by fracturing and splitting and enhance the fittability of the fractured surfaces. In a case where the steel disclosed in Patent Document 3 is fractured and splitted, a sufficient number of sufficiently large unevenness are not formed on the fractured surfaces, and thus the fracture splitability is unsatisfactory. However, in Patent Document 3, there is no study regarding the fittability of fractured surfaces.
In the techniques of Patent Document 4, Patent Document 5, and Patent Document 7, the containing V and/or Ti is required in order to enhance the fracture splitability by embrittling ferrite in steel. However, the present inventors found that, in a case where V or Ti is added to steel to an extent that ferrite is embrittled, segregation of these elements is occurred, and chipping is generated in regions having a high concentration of V or Ti. In a case where the steel disclosed in Patent Document 4, Patent Document 5, and Patent Document 7 is fractured and splitted, it is not possible to suppress the chipping generation amount, and thus the fracture splitability is unsatisfactory. However, in Patent Document 4, Patent Document 5, and Patent Document 7, there are no studies regarding the segregation of ferrite embrittlement elements such as V and Ti.
In the technique of Patent Document 8, in order to enhance the mechanical properties of steel, it is necessary to set an index Ceq, which indicates hardness after the forging of steel, to a predetermined value or more. In steel having the above properties, the chipping generation amount during fracturing and splitting is large, and the fracture splitability is impaired. Fracturing and splitting at a low temperature of - 60°C or lower which is proposed in Patent Document 8 decrease the economic efficiency at the time of manufacturing products.

[Prior Art Document]

[Patent Document]

[Patent Document 1] Japanese Patent No. 3637375
[Patent Document 2] Japanese Patent No. 3756307
[Patent Document 3] Japanese Patent No. 3355132
[Patent Document 4] Japanese Patent No. 3988661
[Patent Document 5] Japanese Patent No. 4314851
[Patent Document 6] Japanese Patent No. 3671688
[Patent Document 7] Japanese Patent No. 4268194
[Patent Document 8] Japanese Unexamined Patent Application, First Publication No. 2004-183094

[Disclosure of the Invention]

[Problems to be Solved by the Invention]

As described above, the fracture splitability is evaluated by, for example, the deformation amount on fractured surfaces, the ratio of brittle fracture surfaces to fractured surfaces, the sizes and number of unevenness on fractured surfaces, the chipping generation amount on fractured surfaces, and the like. The suppression of the deformation amount and the improvement in the ratio of brittle fracture surfaces are achieved by degrading the toughness of steel. For example, in steel having a low Charpy impact value which is an index of toughness, it is common that the suppression of the deformation amount and the improvement in the ratio of brittle fracture surfaces are achieved. According to the related art, the suppression of the deformation amount and the improvement in the ratio of brittle fracture surfaces have been achieved by adding V, Ti, and the like to steel and causing precipitation strengthening in ferrite, thereby degrading the toughness of steel. However, these elements, particularly, V is an element that is easily segregated. In a case where an amount of these embrittlement elements necessary to improve the fracture splitability is added to steel, embrittlement excessively occurs in the segregation portions of these elements (portions in which the concentration of these elements is higher than those of peripheral portions), and chipping is generated during fracturing and splitting. Therefore, the chipping generation amount during fracturing and splitting is increased, and the fracture splitability is impaired. Therefore, it is necessary to ensure the fracture splitability without using elements that increase the chipping generation amount such as V.
In addition, for steel used as a material for mechanical parts such as high-strength connecting rods requiring a high buckling strength, a high yield ratio is also required.
The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide forging steel free of quenching and tempering for hot forging being excellent in terms of the fracture splitability and the yield ratio. Specifically, the object is to provide steel which is capable of achieving both the degradation in the toughness and the suppression of the chipping generation amount, furthermore, excellent in terms of the yield ratio.

[Means for Solving the Problem]

In order to achieve the above object, the present inventors carried out intensive studies regarding a method for realizing forging steel free of quenching and tempering for hot forging having excellent fracture splitability and consequently obtained the following knowledge (a) and (b).

(a) It was found that the toughness is significantly degraded by containing a small amount of Bi in steel. This is because a solid solution of Bi in the steel significantly embrittles ferrite. Due to this effect, it becomes possible to use steel having a low carbon composition which has poor fracture splitability as forging steels free of quenching and tempering for fracturing and splitting.
(b) It was found that the toughness is degraded by containing a small amount of Bi in steel, even if V which is easily segregated is not contained in the steel. This is because the solid solution of Bi has a significantly greater effect of embrittling ferrite than the precipitation strengthening by VC.

On the basis of the above knowledge (a) and (b), the present inventors found that, even for steel having a low carbon composition, it is possible to significantly improve the fracture splitability by containing a small amount of Bi without containing any ferrite embrittlement elements known in the related art such as V and completed the present invention.
The gist of the invention is as described below.

(1) A steel according to one aspect of the present invention contains, 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% to 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%, Mg: 0% to 0.0050%; and a remainder including Fe and impurities.
(2) The steel according to (1) may further contain, in unit mass%, one or more of Ca: 0.0005% to 0.0050%, Zr: 0.0005% to 0.0050%, and Mg: 0.0005% to 0.0050%.
(3) The steel according to (1) or (2) may further contain, in unit mass%, N: 0.0020% to 0.0090%.
(4) The steel according to any one of (1) to (3) may further contain, in unit mass%, Al: 0% to 0.0008%.
(5) The steel according to any one of (1) to (4) may further contain, in unit mass%, V: 0% to 0.004%.

[Effects of the Invention]

According to the present invention, it is possible to provide a forging steel free of quenching and tempering for hot forging in which both the degradation in the toughness and the suppression of the chipping generation amount are achieved and the fracture splitability and the yield ratio are excellent.

[Brief Description of the Drawings]

FIG. 1 is an exploded perspective view showing a connecting rod which is an application example of steel according to one aspect of the present invention.

[Embodiments of the Invention]

First, the reasons for limiting the chemical composition of steel according to the present embodiment will be described. Hereinafter, unless particularly otherwise described, the unit "%" for the amounts of alloying elements in the steel means "mass%".

C: 0.10% to 0.25%

C has an effect for ensuring the tensile strength of steel. In order to obtain a required strength, it is necessary that the lower limit of the C content is set to 0.10%. On the other hand, when C is contained in the steel excessively, the frequency of chipping generation on fractured surfaces is increased, and thus the upper limit of the C content is set to 0.25%. The lower limit of the C content may be set to 0.12%, 0.15%, or 0.19%. The upper limit of the C content may be set to 0.23%, 0.22%, or 0.21%.

Si: 0.60% to 1.20%

Since Si strengthens ferrite through solid solution strengthening and degrades the ductility and toughness of the steel, the fracture splitability of the steel is improved. In order to obtain this effect, it is necessary that the lower limit of the Si content is set to 0.60%. On the other hand, when Si is contained in the steel excessively, the frequency of chipping generation on fractured surfaces is increased, and thus the upper limit of the Si content is set to 1.20%. The lower limit of the Si content may be set to 0.70%, 0.75%, or 0.80%. The upper limit of the Si content may be set to 1.00%, 0.90%, or 0.85%.

Mn: 0.60% to 1.00%

Since Mn strengthens ferrite through solid solution strengthening and degrades the ductility and toughness of the steel, the fracture splitability of the steel is improved. In addition, Mn bonds to S and thus forms a Mn sulfide. During fracturing and splitting a steel product made of the steel according to the present embodiment, since a crack is propagated along the stretched Mn sulfide in the rolling direction, the Mn sulfide has an effect for preventing a position displacement at the time of fitting the fractured surfaces by increasing unevenness on fractured surfaces. On the other hand, when Mn is contained in the steel excessively, ferrite becomes too hard so that the frequency of chipping generation on fractured surfaces is increased. In view of the above, the range of the Mn content is 0.60% to 1.00%. The lower limit of the Mn content may be set to 0.70%, 0.80%, or 0.82%. The upper limit of the Mn content may be set to 0.90%, 0.87%, or 0.85%.

P: 0.040% to 0.060%

P degrades the ductility and toughness of ferrite and pearlite, and the steel is embrittled. Generally, P is considered as an impurity element which is not preferable to be contained. In steel, which becomes a material for parts, manufactured by manufacturing method not including fracturing and splitting, in order to prevent the parts embrittlement, it is common that the P content is set to approximately 0.020% or less. However, in the steel according to the present embodiment which intends to improve the fracture splitability, P has an effect for improving the fracture splitability and is thus advantageous. Therefore, in the steel according to the present embodiment, it is necessary that the P content is set to 0.040% or more that is significantly more than the range of P whose amount is usually included in the steel as an impurity. However, when the P content is contained in the steel excessively, crystal grain boundaries are embrittled, and chipping generation on fractured surfaces is likely to occur. In view of the above, the range of the P content is 0.040% to 0.060%. The lower limit of the P content may be set to 0.042%, 0.045%, or 0.048%. The upper limit of the P content may be set to 0.058%, 0.055%, or 0.050%.

S: 0.060% to 0.100%

S bonds to Mn and thus forms a Mn sulfide. During fracturing and splitting a steel product made of the steel according to the present embodiment, since a crack is propagated along the stretched Mn sulfide in the rolling direction, S has an effect for preventing a position displacement at the time of fitting the fractured surfaces by increasing unevenness on fractured surfaces. In order to obtain the above effect, it is necessary that the lower limit of the S content is set to 0.060%. On the other hand, when S is contained in the steel excessively, the plastic deformation amount in the vicinity of fractured surfaces during fracturing and splitting is increased, and there is a case where the fracture splitability is degraded. Additionally, in a case where S is contained in the steel excessively, there is a case where chipping on fractured surfaces is promoted. For the above reasons, the range of the S content is set to 0.060% to 0.100%. The lower limit of the S content may be set to 0.070%, 0.075%, or 0.080%. The upper limit of the S content may be set to 0.090%, 0.088%, or 0.085%.

Cr: 0.05% to 0.20%

Similar to Mn, since Cr strengthens ferrite through solid solution strengthening and degrades the ductility and toughness of the steel, the fracture splitability of the steel is improved. However, when Cr is contained in the steel excessively, the lamellar spacing of pearlite becomes small so that the ductility and toughness of pearlite become higher. Therefore, the fracture splitability of the steel is degraded. Furthermore, when Cr is contained in the steel excessively, a bainite structure is likely to be generated, and a decrease in the yield strength due to a decrease in the yield ratio or a significant degradation in the fracture splitability is occurred. Therefore, the range of the Cr content is 0.05% to 0.20%. The lower limit of the Cr content may be set to 0.07%, 0.09%, or 0.10%. The upper limit of the Cr content may be set to 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. In a case where the steel contains a small amount of Bi, since a solid solution of Bi embrittles ferrite and the ductility and toughness of the steel is degraded, the fracture splitability of the steel is improved. What should be noted is that the ferrite embrittlement effect of Bi develops at an extremely small amount of Bi. According to the knowledge that the present inventors found, in order to obtain the above effect, it is necessary that the Bi content is set to 0.0001% or more. The above fact that the small amount of Bi improves the fracture splitability of steel has not yet been reported. In addition, in a case where ferrite is embrittled using Bi, no increase in the chipping generation amount has been confirmed. Although the reason therefore is not clear, since the Bi content is extremely small, it is assumed that the influence of Bi segregation is small enough to be negligible.
However, when the Bi content is more than 0.0050%, the ferrite embrittlement effect of Bi is saturated, and the yield strength is decreased. For the above reasons, in the steel according to the present embodiment, the Bi content is set to 0.0001% to 0.0050%. The lower limit of the Bi content may be set to 0.0025%, 0.0028%, or 0.0030%. The upper limit of the Bi content may be set to 0.0045%, 0.0042%, or 0.0040%.

N: 0.0020 to 0.0150%

When V or Ti is contained in the steel, N forms a nitride or a carbonitride thereof, and the rest of N is present in the steel with a state of a solid solution. Since a solid solution of N (that is, N with a state of a solid solution in the steel) embrittles ferrite and the ductility and toughness of the steel is degraded, the fracture splitability of the steel is improved. In order to obtain the above effect, the lower limit of the N content is set to 0.0020%. When N is contained in the steel excessively, there is a case where the hot ductility is degraded and cracks or defects are likely to be generated during hot working. Therefore, the upper limit of the N content is set to 0.0150%. The lower limit of the N content may be set to 0.0050%, 0.0070%, or 0.0080%. The upper limit of the N content may be set to 0.0100%, 0.0095%, or 0.0090%.

V: 0% to 0.010%

Since V forms a carbide or a carbonitride and causes precipitation strengthening in ferrite, V has an effect for decreasing the deformation amount during fracturing and splitting by degrading the ductility and toughness of ferrite. Therefore, according to the related art, there is a case where V is added to steel requiring favorable fracture splitability. However, in order to obtain the above effect using V, it is necessary that the V content is set to approximately 0.10% or more. In a case where approximately 0.10% or more of V is contained in the steel, since the segregation of V is caused, the ductility and toughness of ferrite is excessively degraded in regions having a high concentration of V. Therefore, the chipping generation is likely to occur at the time of fracturing and splitting the steel. That is, although V can decrease the deformation amount during fracturing and splitting, V increases the chipping amount during fracturing and splitting.
The steel according to the present embodiment contains the above small amount of Bi and thus does not require V in order to improve the fracture splitability. Therefore, the lower limit of the V content is 0%. In order to decrease the chipping generation amount, V is preferably not contained in the steel. However, in a case where the steel according to the present embodiment is manufactured using scraps as a material, there is a concern that V may be incorporated. In this case, 0.010% or less of V is allowed since the above amount of V does not increase the chipping generation amount. The upper limit of the V content may be set to 0.007%, 0.005%, 0.004%, or 0.002%. When scraps are not used as a material of the steel, the V content which is incorporated into the steel as an impurity is generally 0.010% or less. In addition, in the technical field to which the steel according to the present embodiment belongs, generally, 0.010% or less of V is considered as an impurity having no substantial influence on the properties of the steel. In Mill test reports, since 0.010% or less of V is regarded that the V content is 0%, there is a case where V is not disclosed.

Al: 0% to 0.0050%

Since the steel according to the present embodiment can exhibit the effects without containing Al, the lower limit of the Al content is 0%. In addition, 0.0050% or more of Al forms an Al oxide in the steel, and there is a case where this Al oxide impairs the machinability of the steel. For the above reason, the upper limit of the Al content in the steel according to the present embodiment is set to 0.0050%. The upper limit of the Al content may be set to 0.0040%, 0.0010%, or 0.0008%. In addition, in the technical field to which the steel according to the present embodiment belongs, generally, 0.0050% or less of Al is considered as an impurity having no substantial influence on the properties of the steel. Therefore, in Mill test reports, since 0.0050% or less of Al is regarded that the Al content is 0%, there is a case where Al is not disclosed.

Ti: 0% to 0.020%

Similar to V, since Ti forms a nitride and causes precipitation strengthening in ferrite, Ti has an effect for decreasing the deformation amount during fracturing and splitting by degrading the ductility and toughness of ferrite. However, similar to V, there is a concern that Ti may increase the chipping amount during fracturing and splitting.
The steel according to the present embodiment contains the above small amount of Bi and thus does not require Ti in order to improve the fracture splitability. Therefore, the lower limit of the Ti content is 0%. In order to decrease the chipping generation amount, Ti is preferably not contained in the steel. However, in a case where the steel according to the present embodiment is manufactured using scraps as a material, there is a concern that Ti may be incorporated. In this case, 0.020% or less of Ti is allowed since the above amount of Ti does not increase the chipping generation amount. The upper limit of the Ti content may be set to 0.010%, 0.005%, or 0.002%. When scraps are not used as a material of the steel, the Ti content which is incorporated into the steel as an impurity is generally 0.020% or less. In addition, in the technical field to which the steel according to the present embodiment belongs, generally, 0.020% or less of Ti is considered as an impurity having no substantial influence on the properties of the steel. In Mill test reports, since 0.020% or less of Ti is regarded that the Ti content is 0%, there is a case where Ti is not disclosed.

Ca: 0% to 0.0050%, Zr: 0% to 0.0050%, and Mg: 0% to 0.0050%

Since the steel according to the present embodiment can exhibit the effects without containing Ca, Zr, and Mg, the lower limits of the Ca content, the Zr content and the Mg content is 0% respectively. However, all of Ca, Zr, and Mg form an oxide, serve as the crystallization nucleus of MnS, and have an effect for uniformly and finely dispersing MnS. During fracturing and splitting a steel product made of the steel according to the present embodiment, since a crack is propagated along the stretched MnS in the rolling direction, as the Mn sulfide becomes larger, and unevenness on fractured surfaces becomes larger. On the other hand, the ductility and the toughness become higher, and the fracture splitability become poor. When MnS is finely dispersed, MnS is likely to be propagated in the crack propagation direction, and an effect for improving the fracture splitability can be obtained. In order to obtain the above effects, the steel according to the present embodiment may contain one or more elements 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. On the other hand, when each of the Ca content, the Zr content or the Mg content is more than 0.0050%, the hot workability of the steel is deteriorated, and thus it is difficult to perform hot rolling on the steel. For the above reasons, the upper limit of each of the Ca content, the Zr content, and the Mg content is set to 0.0050%.
The remainder of the chemical compositions of the steel according to the present embodiment includes iron and impurities. The impurities refer to elements which are incorporated due to raw materials such as minerals or scraps or a variety of causes in manufacturing steps during industrially manufacturing steel, and are allowed as long as the elements do not adversely affect the steel according to the present embodiment.
The metallographic structure of the steel according to the present embodiment is a so-called ferrite/pearlite structure which is substantially consisting of ferrite and pearlite and, in some cases, slightly includes an inclusion or the like. This structure can be obtained by controlling the chemical compositions of the steel to the above ranges. Therefore, although it is not necessary to clearly limit the metallographic structure of the steel according to the present embodiment, for example, the metallographic structure of the steel according to the present embodiment may be specified as a metallographic structure including ferrite and pearlite of 99 area% or more in total.
In materials of the related art, ferrite is embrittled by adding V, so that the fracture splitability is improved. On the other hand, in the above forging steels free of quenching and tempering, ferrite is embrittled through an effect that a small amount of Bi is added without adding V, so that the fracture splitability is improved.
The application of the above steel according to the present embodiment is not particularly limited. However, since the steel according to the present embodiment has favorable fracture splitability, it is preferably used as a material for mechanical parts (fracture split parts) obtained by a manufacturing method including a step of fracturing and splitting, and particularly preferably used as a material for connecting rods in automotive engines. In a fracture split connecting rod 1 as a steel product made of the forging steels free of quenching and tempering of the present embodiment, new processing of abutting surfaces or positioning pins are not required, and it is possible to significantly simplify manufacturing steps.
FIG. 1 is an exploded perspective view showing an example of a fracture split connecting rod made of the steel according to the present invention.
The fracture split connecting rod 1 illustrated in FIG. 1 is configured with a rod-attached semi-circular arc shaped upper side halved body 2 and a semi-circular arc shaped lower side halved body 3 which are separated up and down. On both end sides of a semi-circular arc portion 2A of the upper side halved body 2, screw holes 5 having a thread groove for the fixation to the lower side halved body 3 are formed respectively, and, on both end sides of a semi-circular arc portion 3A of the lower side halved body 3, insertion holes 6 for the fixation to the upper side halved body 2 are formed respectively.
The semi-circular arc portion 2A of the upper side halved body 2 and the semi-circular arc portion 3A of the lower side halved body 3 are brought together, bonding bolts 7 are inserted into the insertion holes 6 and the screw holes 5 on both end sides of the upper side halved body and the lower side halved body, and the bonding bolts, the insertion holes, and the thread holds are screwed together, thereby configuring a circular big end portion 8. On the top end side of a rod portion 2B of the upper side halved body 2, a circular small end portion 9 is formed.
The fracture split connecting rod 1 having the structure illustrated in FIG. 1 is combined into an internal combustion engine such as an automotive engine in order to convert the reciprocal motion of a piston in the internal combustion engine to a rotary motion, the small end portion 9 is connected to the piston not illustrated, and the big end portion 8 is connected to a connecting rod journal (not illustrated) of the internal combustion engine.
The semi-circular arc portion 2A of the upper side halved body 2 and the semi-circular arc portion 3A of the lower side halved body 3 in the fracture split connecting rod 1 are formed by embrittling and fracturing a portion which is originally one circular parts. As an example, a notch is provided in a portion of a hot-forged product, and the hot-forged product is fractured and split in a brittle manner from the notch as a starting point, thereby forming an abutting surface 2a of the semi-circular arc portion 2A of the upper side halved body 2 and an abutting surface 3a of the semi-circular arc portion 3A of the lower side halved body 3. These abutting surfaces 2a and 3a are formed by fracturing and splitting the steel according to the present embodiment having favorable fracture splitability, and thus abutting with a favorable positioning accuracy becomes possible.
In the fracture split connecting rod 1 having the above structure, new processing of the abutting surfaces or positioning pins are not required, and the manufacturing steps are significantly simplified.
Hereinafter, the present invention will be described in detail using examples. However, these examples are intended to describe the technical meaning and effects of the present invention and do not limit the scope of the present invention.

[Examples]

Ingot steel melted by converter having a chemical composition shown in Table 1 below was manufactured by continuously casting and was subjected to soaking and a blooming step as necessary, thereby producing a 162 mm x 162 mm rolled material. Next, the rolled material was hot rolled, thereby forming a steel bar shape having a diameter of 45 mm. Underlined values in Table 1 are values outside the scope of the present invention. In addition, the symbol "-" in Table 1 indicates that the element indicated by the symbol was not added in the manufacturing stage and the amount of the element was equal to or less than a level at which the element is generally considered as an impurity. However, the V content, the Al content and Ti content in Examples 1 to 23 and Comparative Examples a to h were small amounts in which the elements are considered as impurities in the technical field to which the present invention belongs according to the technical common sense, detailed measurements were performed in order to confirm the action effect of the present invention, and the values are shown in Table 1.
Next, in order to investigate the fracture splitability, the mechanical properties, and the structure, a test piece corresponding to a forged connecting rod was produced by hot forging the steel bar. Specifically, the steel bar having a diameter of 45 mm was heated to 1,150°C to 1,280°C, was forged perpendicularly to the longitudinal direction of the steel bar to a thickness of 20 mm, and was cooled to room temperature by air blast cooling using an air blasting cooler. A JIS No. 4 test piece and a Charpy impact test piece were processed from the cooled forged material. A 45-degree V notch having a depth of 2 mm, a tip curvature of 0.25 mm was processed in the Charpy impact test piece.
The fracture splitability is considered to be favorable in a case where the fracture form on fractured surface is brittle, the deformation amount in the vicinity of fracture surfaces caused by fracturing and splitting is small, and the chipping generation amount during fracturing and splitting is small. In steel having a low Charpy impact value, it is common that the suppression of the deformation amount and the improvement in the ratio of brittle fractured surfaces are achieved. Therefore, as an index for evaluating the fracture form on fractured surfaces and the deformation amount in the vicinity of fracture surfaces, the present inventors used the Charpy impact value. A Charpy impact test was repeatedly carried out on the Charpy impact test piece five times at room temperature on the basis of JIS Z 2242, and the average value of the five obtained values was considered as the Charpy impact value of the test piece. Steel having a Charpy impact value of 9 J/cm² or less was determined as steel in which the suppression of the deformation amount and the improvement in the ratio of brittle fracture surfaces were achieved.
In addition, the method for measuring the chipping generation amount was as described below. A test piece for evaluating the fracture splitability, which was a 80 mm x 80 mm and had a thickness of 18 mm, had a hole having a diameter of 50 mm in the central portion, and had 45-degree V notches having a depth of 1 mm and a tip curvature of 0.5 mm at two locations which lied at ±90 degrees with respect to the longitudinal direction of the steel bar which was the material before forging was produced. Furthermore, a through hole having a diameter of 8 mm was formed as a bolt hole in the test piece for evaluating the fracture splitability so that the central line of the through hole was located at a place 8 mm apart from a side surface on the notch-processed side. This test piece for evaluating the fracture splitability was fractured using a fracture split property evaluation tester. The tester for fracture split property evaluation was configured with sectional dies and a weight-drop tester. The sectional dies had a shape in which a column having a diameter of 46.5 mm formed on a rectangular steel material was divided into two pieces along the central line, one piece was fixed, and the other piece was movable on a rail. A wedge hole was processed on the mating surfaces of the two semi-circular columns. At the time of a fracture test, the hole having a diameter of 50 mm in the test piece was fitted into the column having a diameter of 46.5 mm of the sectional dies, and a wedge was put into the wedge hole and installed over a falling weight. The falling weight had a mass of 200 kg and was configured to drop along a guide. When the falling weight dropped, the wedge stroke into the test piece, and the test piece was tensile-fractured into two pieces. In addition, the periphery of the test piece was fixed so as to press the sectional dies so as to prevent the test piece from flying away from the sectional dies during fracturing. In the present test, the test piece was fractured at a weight dropping height of 100 mm. An operation in which the fractured surfaces obtained from the above test were abutted together, the fractured steel was fastened with a bolt at a torque of 20 N·m, and then the fractured surfaces were separated from each other by loosening the bolt was repeated 10 times. The total weight of fragments dropped by the above operation was defined as the chipping generation amount of the steel. Steel having a chipping generation amount of less than 1.00 mg was determined that the chipping generation amount was suppressed.
A tensile test was carried out on the JIS No. 4 tensile test piece according to JIS Z 2241 at normal temperature and a rate of 20 mm/min. Steel having a yield ratio of 0.75 or more was determined as a specimen having a favorable yield ratio.

Furthermore, a 10 mm x 10 mm sample was cut out from the same portion as in the Charpy impact test piece or the tensile test piece, nital etching was carried out, and the structure was observed.

[Table 1]

Test No.	Type	Chemical composition (mass%, the remainder is iron and impurities)
Test No.	Type	C	Si Mn		P	S	Cr	Bi	N	V	Al	Ti	Ca	Zr	Mg
1		0.10	0.81	0.85	0.051	0.084	0.09	0.0032	0.0082	0.002	0.0010	0.004	-	-	-
2		0.25	0.82	0.87	0.048	0.081	0.08	0.0030	0.0091	0.001	0.0012	0.008	-	-	-
3		0.22	0.61	0.87	0.053	0.075	0.10	0.0028	0.0074	0.002	0.0008	0.009	-	-	-
4		0.23	1.18	0.83	0.054	0.079	0.09	0.0025	0.0086	0.001	0.0017	0.006	-	-	-
5		0.20	0.85	0.60	0.048	0.083	0.11	0.0034	0.0078	0.005	0.0021	0.006	-	-	-
6		0.20	0.79	0.98	0.045	0.084	0.13	0.0039	0.0086	0.002	0.0012	0.009	-	-	-
7		0.21	0.85	0.92	0.042	0.080	0.11	0.0037	0.0081	0.001	0.0011	0.002	-	-	-
8		0.23	0.85	0.74	0.058	0.078	0.12	0.0034	0.0079	0.002	0.0016	0.006	-	-	-
9		0.19	0.87	0.86	0.057	0.062	0.09	0.0029	0.0086	0.003	0.0007	0.005	-	-	-
10		0.20	0.81	0.84	0.056	0.098	0.08	0.0027	0.0097	0.002	0.0009	0.002	-	-	-
11	Invention Example	0.21	0.80	0.84	0.054	0.083	0.05	0.0034	0.0082	0.002	0.0012	0.011	-	-	-
12		0.21	0.79	0.87	0.055	0.084	0.19	0.0032	0.0080	0.001	0.0014	0.008	-	-	-
13		0.19	0.83	0.83	0.056	0.076	0.11	0.0002	0.0076	0.002	0.0010	0.005	-	-	-
14		0.19	0.81	0.84	0.057	0.080	0.10	0.0049	0.0084	0.001	0.0008	0.012	-	-	-
15		0.19	0.81	0.82	0.054	0.078	0.10	0.0028	0.0022	0.002	0.0016	0.008	-	-	-
16		0.20	0.80	0.80	0.053	0.086	0.13	0.0035	0.0148	0.003	0.0010	0.007	-	-	-
17		0.22	0.78	0.86	0.048	0.082	0.09	0.0030	0.0097	0.002	0.0008	0.005	0	-	-
18		0.21	0.86	0.87	0.049	0.080	0.08	0.0027	0.0086	0.002	0.0006	0.005	-	0.003	0.002
19		0.22	0.79	0.84	0.045	0.076	0.11	0.0001	0.0078	0.002	0.0020	0.012	0	-	-
20		0.21	0.77	0.84	0.057	0.077	0.11	0.0029	0.0082	0.009	0.0013	0.007	0.003	-	-
21		0.20	0.82	0.83	0.054	0.083	0.11	0.0031	0.0078	0.002	0.0049	0.007	0.001	0.004	-
22		0.20	0.79	0.87	0.048	0.081	0.09	0.0035	0.0080	0.002	0.0017	0.019	0.002	-	0.002
23		0.19	0.80	0.86	0.050	0.081	0.13	0.0027	0.0084	0.004	0.0014	0.008	0.002	0.001	0.001
a	Comparative Example	0.08	0.81	0.88	0.050	0.075	0.09	0.0029	0.0084	0.002	0.0011	0.004	-	-	-
b		0.21	0.58	0.84	0.046	0.084	0.10	0.0027	0.0078	0.003	0.0018	0.006	0	0.002	-
c		0.21	0.83	0.57	0.049	0.087	0.11	0.0032	0.0089	0.001	0.0021	0.008	-	-	-
d		0.19	0.84	0.80	0.039	0.078	0.11	0.0037	0.0093	0.002	0.0008	0.002	0	-	0.002
e		0.22	0.81	0.83	0.053	0.076	0.22	0.0035	0.0099	0.002	0.0013	0.011	-	-	-
f		0.20	0.81	0.84	0.051	0.086	0.13	-	0.0087	0.002	0.0015	0.018	-	-	-
g		0.21	0.80	0.87	0.044	0.075	0.10	0.0053	0.0095	0.003	0.0021	0.005	-	-	-
h		0.19	0.81	0.85	0.055	0.082	0.12	0.0031	0.0081	0.011	0.0009	0.007	0.004	-	-

* Underlined values are outside the scope of the present invention.

Table 2 shows the test results. In all of Invention Examples of Steel Nos. 1 to 23, the chemical compositions of the steel were in the specified range of the present invention, and thus it was possible to set the Charpy impact value to 9 J/cm² or less, and furthermore, the chipping generation amount was also suppressed. That is, Steel Nos. 1 to 23 had favorable fracture splitability. Furthermore, Steel Nos. 1 to 23 had a high yield ratio and were thus available as a material for mechanical parts requiring a high buckling strength.
In contrast, in Comparative Example a, the C content was low, and thus the tensile strength was low, and the Charpy impact value was high.
In Comparative Examples b to d, the Si content, the Mn content or the P content was low, and thus the ferrite embrittlement effect was weak, and the Charpy impact value was high.
In Comparative Example e, the Cr content was high, and, in addition to a ferrite/pearlite structure, a bainite structure was generated in a portion, and thus the Charpy impact value was high, and furthermore, the yield ratio was impaired.
In Comparative Example f, the steel did not contain Bi, and thus the ferrite embrittlement effect was weak, and the Charpy impact value was high.
In Comparative Example g, the steel contained Bi, and thus the ferrite embrittlement effect was obtained, and the Charpy impact value was low. However, the Bi content was high, and thus the yield strength and the yield ratio were low.

In Comparative Example h, the V content was high, and thus the segregation of V was caused, the toughness of ferrite excessively degraded in a region having a high concentration of V, and the chipping generation amount at the time of fracturing and splitting the steel was increased.

[Table 2]

Test No.	Type	Charpy impact value (J/cm²)	Tensile strength (MPa)	Yield strength (MPa)	Yield ratio	Chipping generation amount (mg)
1		9	784	614	0.78	0.99
2		5	842	648	0.77	0.82
3		7	824	642	0.78	0.75
4		4	831	634	0.76	0.91
5		8	814	641	0.79	0.68
6		4	830	644	0.78	0.93
7		7	817	619	0.76	0.84
8		6	824	651	0.79	0.73
9		7	819	635	0.78	0.85
10		8	823	651	0.79	0.77
11	Invention Example	7	817	647	0.79	0.81
12		5	804	620	0.77	0.68
13		9	801	632	0.79	0.66
14		4	824	648	0.79	0.74
15		7	816	633	0.78	0.83
16		8	826	642	0.78	0.76
17		6	803	629	0.78	0.72
18		6	810	633	0.78	0.88
19		9	805	624	0.78	0.73
20		7	809	632	0.78	0.70
21		5	823	627	0.76	0.87
22		6	811	629	0.78	0.93
23		6	821	635	0.77	0.72
a	Comparative Example	20	756	594	0.79	0.03
b		15	799	623	0.78	0.23
c		13	804	621	077	0.45
d		12	800	632	0.79	0.38
e		21	863	553	0.64	0.13
f		26	833	645	0.77	0.06
g		7	812	521	0.64	0.74
h		6	818	622	0.76	1.56

[Industrial Applicability]

The steel according to the present invention can achieves both the degradation in the toughness and the suppression of the chipping generation amount, furthermore, has an excellent yield ratio. Therefore, when being used as forging steel free of quenching and tempering for hot forging which is a material for mechanical parts that are obtained using a manufacturing method including a fracturing and splitting step, the steel according to the present invention can be used to manufacture mechanical parts having a high buckling strength and is capable of significantly improving the economic efficiency at the time of manufacturing parts.

[Brief Description of the Reference Symbols]

1: FRACTURE SPLIT CONNECTING ROD (STEEL PRODUCT)
2: UPPER SIDE HALVED BODY
2A: SEMI-CIRCULAR ARC PORTION
2a: ABUTTING SURFACE
2B: ROD PORTION
3: LOWER SIDE HALVED BODY
3A: SEMI-CIRCULAR ARC PORTION
3a: ABUTTING SURFACE
5: SCREW HOLE
6: INSERTION HOLE
7: BONDING BOLT
8: BIG END PORTION
9: SMALL END PORTION

Claims

A steel comprising, 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% to 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%;

Mg: 0% to 0.0050%; and

a remainder including Fe and impurities.
The steel according to claim 1 comprising, in unit mass%:
one or more of:
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 or 2 comprising, in unit mass%:
N: 0.0020% to 0.0090%.
The steel according to any one of claims 1 to 3 comprising, in unit mass%:
Al: 0% to 0.0008%.
The steel according to any one of claims 1 to 4 comprising, in unit mass%:
V: 0% to 0.004%.