JP6528898B2 - Non-tempered bar - Google Patents

Description

  The present invention relates to a steel bar, and more particularly to a steel bar used for a non-heat treated hot forging (hereinafter referred to as a non-heat treated steel bar).

  A connecting rod (hereinafter referred to as a connecting rod) used in an automobile engine or the like is an engine component that connects a piston and a crankshaft, and converts the reciprocating motion of the piston due to explosion into rotational motion of a crank.

  FIG. 1 is a front view of a conventional connecting rod. As shown in FIG. 1, the conventional connecting rod 1 includes a large end 100, a flange 200 and a small end 300. The large end 100 is disposed at one end of the ridge 200, and the small end 300 is disposed at the other end of the ridge 200. The large end 100 is coupled to the crankpin. The small end 300 is coupled to the piston.

  The conventional connecting rod 1 comprises two parts (cap 2 and rod 3). These parts are usually manufactured by hot forging. One end of the cap 2 and the rod 3 corresponds to the large end 100. The portions other than the one end portion of the rod 3 correspond to the collar portion 200 and the small end portion 300. The large end 100 and the small end 300 are formed by cutting. Therefore, the connecting rod 1 is required to have high machinability.

  The connecting rod 1 receives a load from peripheral members during engine operation. Recently, in order to save fuel consumption, downsizing of the connecting rod 1 and improvement of in-cylinder pressure in the cylinder are required. Therefore, the connecting rod 1 is required to have an excellent yield strength that can cope with the explosion load transmitted from the piston even if the brim portion 200 is narrowed. Furthermore, since the connecting rod is subjected to repeated compressive load and tensile load, excellent fatigue strength is also required.

  Moreover, in recent years, from the viewpoint of energy saving and cost reduction, non-refining connecting rods in which the tempering treatment (quenching and tempering) is omitted have begun to be adopted. Therefore, there is a need for a non-heat-treated steel which can provide sufficient yield strength, fatigue strength and machinability without heat treatment after hot forging.

  By the way, in the conventional connecting rod 1, the cap 2 and the rod 3 are separately manufactured as described above. Therefore, a knock pin processing step is performed to position the cap 2 and the rod 3. Furthermore, a cutting process is performed on the mating surface of the cap 2 and the rod 3. Therefore, cracking connecting rods that can omit these steps are beginning to spread.

  In the cracking con rod, after the connecting rod is integrally molded, a jig is inserted into the hole of the large end 100, stress is applied to break the large end, and two parts (corresponding to the cap 2 and the rod 3) To divide. Then, the two parts separated when attaching to the crankshaft are combined. If the fracture surface of the large end portion 100 is a brittle fracture surface without deformation, the fracture surfaces of the cap 2 and the rod 3 can be aligned and connected by a bolt. Therefore, in this case, the knock pin processing step and the cutting step are omitted. As a result, the manufacturing cost is reduced.

  However, when mass-producing cracking con rods, bainite may partially occur in the hot forged product (cracking con rods) in the hot forging step due to temperature variations in the heating furnace and processing heat generation. In this case, the cracking property is reduced. Specifically, since the toughness of bainite is high, if bainite is present in the hot forged product, a ductile fracture surface is likely to occur on the fractured surface after cracking. When a ductile fracture surface occurs, the large end is plastically deformed. Therefore, even if the fractured surfaces are aligned, the internal diameter D of the large end portion 100 in FIG. 1 is deviated from the desired numerical value. As a result, one-side contact may occur at the crank connection portion (large end portion), which may cause vibration or noise during traveling of the vehicle.

  Japanese Patent Application Laid-Open Nos. 2004-277817 (Patent Document 1), 2011-195862 (Patent Document 2), and International Publication 2009/107282 (Patent Document 3) propose steels having high cracking properties. .

The high strength untempered steel for fracture separation disclosed in Patent Document 1 has C: 0.2 to 0.6%, Si: 0.1 to 2%, Mn: 0.1 to 1. 5%, S: 0.03 to 0.2%, P: 0.02 to 0.15%, Cu: 0.03 to 1%, Ni: 0.03 to 1%, Cr: 0.05 to 1 %, V: 0.02 to 0.4%, Ti: 0.01 to 0.8%, s-Al: 0.005 to 0.045%, N: 0.008 to 0.035%, the balance inevitable And the structure is a ferrite pearlite structure. The maximum diameter of TiN inclusions in the steel is 5 μm or more, and the amount thereof is 5 pieces / mm ² or more in number density. In this document, it is described that the above-mentioned TiN can form appropriate unevenness on the fracture surface and can suppress the side slip on the bonding surface.

  The untempered steel for hot forging disclosed in Patent Document 2 has C: 0.35 to 0.55%, Si: 0.15 to 0.40%, Mn: 0.50 to 1 in mass%. 00%, P: 0. 100% or less, S: 0.040 to 0.100%, Cr: 1.00% or less, V: 0.20 to 0.50%, Ca: 0.0005 to 0.0100 %, N: 0.0150% or less, the balance being Fe and unavoidable impurities, 2Mn + 5Mo + Cr ≦ 3.1, C + Si / 5 + Mn / 10 + 10P + 5V ≧ 1.8, Ceq = C + Si / 7 + Mn / 5 + Cr / 9 + V is 0.90 to 1.10. Furthermore, the hardness is HV330 or more, the yield ratio is 0.73 or more, and the structure is a ferrite pearlite structure having bainite of 10% or less. In this document, it is described that the formation of bainite is suppressed by satisfying 2Mn + 5Mo + Cr ≦ 3.1, and the excellent cracking property is obtained by satisfying C + Si / 5 + Mn / 10 + 10P + 5V ≧ 1.8.

  C. More than 0.35% to 0.60%, Si: 0.50 to 2.50%, Mn: 0.20% by mass, as the hot forging non-heat treated steel disclosed in Patent Document 3 ~ 2.00%, P: 0.010-0. 150%, S: 0.040-0. 150%, V: 0.10-0. 50%, Zr: 0.0005-0.0050%, Ca: 0.0005 to 0.0050%, N: 0.0020 to 0.0200%, Al: limited to less than 0.010%, the balance substantially consisting of Fe and unavoidable impurities, width The ratio of the number of MnS inclusions of 1 μm or more to the total MnS inclusions is 10% or less (including 0%), and the average aspect ratio of the MnS inclusions is 10 or less. The bainite structure fraction is 3% or less (including 0%), and the remaining structure is a ferrite pearlite structure. Furthermore, in this document, it is described that the break separation property is enhanced by finely dispersing a large amount of MnS-based inclusions.

JP, 2004-277817, A JP, 2011-195862, A International Publication No. 2009/107282

  However, in Patent Document 1, when bainite is formed in the hot forged product, a ductile fracture surface may occur on the fracture surface, and the inner diameter of the large end may be deformed to reduce the cracking property.

  In Patent Document 2, the formation of bainite in a hot forging product is allowed to some extent. However, in the case of the steel of Patent Document 2, a ductile fracture surface may occur on the fracture surface, and the cracking property may be reduced.

  Patent Document 3 assumes that the microstructure of the hot forged product mainly consists of ferrite and pearlite. Therefore, when bainite is formed in the hot forged product, the cracking property may be reduced.

  An object of the present invention is to provide a non-heat-treated steel bar having high machinability, yield strength and fatigue strength, and further having excellent cracking properties even if bainite is formed after hot forging.

The non-heat-treated steel bar according to the present embodiment is, by mass%, C: 0.39 to 0.55%, Si: 0.10 to 1.00%, Mn: 0.50 to 1.50%, P: 0 .010 to 0.100%, S: 0.040 to 0.130%, Cr: 0.05 to 0.50%, V: 0.05 to 0.40%, Ti: 0.15 to 0.25 %, Al: 0.005 to 0.050%, N: 0.002 to 0.020%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, Mo: 0 to 0.10% , Pb: 0 to 0.30%, Te: 0 to 0.3000%, Ca: 0 to 0.0100%, and Bi: 0 to 0.3000%, with the balance being Fe and impurities, The number density of TiN having a chemical composition satisfying the formula (1) and having a circle equivalent diameter of 20 μm or more in steel is 0.3 to 4.0 pieces / mm ² .
0.60 ≦ C + 0.2 Mn + 0.25 Cr + 0.75 V + 0.81 Mo ≦ 1.00 (1)
Here, the content (mass%) of the corresponding element is substituted into the element symbol in the formula (1).

  The non-heat-treated steel bar according to the present embodiment has high machinability, yield strength and fatigue strength, and furthermore, excellent cracking properties can be obtained even if bainite is formed after hot forging.

FIG. 1 is a front view of a conventional connecting rod. FIG. 2A is a plan view of a test piece used in the cracking evaluation test in the example. FIG. 2B is a cross-sectional view of the test piece shown in FIG. 2A. FIG. 2C is a plan view of a test piece showing a state in which the test piece of FIG. 2A is broken and separated. FIG. 2D is a plan view of a test piece showing a state in which the test piece of FIG. 2C is bolted.

  The present inventors investigated and examined the strength (yield strength and fatigue strength), machinability and cracking properties after hot forging of non-heat treated bar steel. As a result, the present inventors obtained the following findings.

  (1) Yield strength and fatigue strength, and machinability are contradictory mechanical properties. If the chemical composition can be properly adjusted, it is possible to achieve both of these mechanical properties.

  It defines as fn1 = C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo. fn1 is an index of the yield strength and shows a positive correlation with the yield strength. If fn1 is less than 0.60, the yield strength of the steel is too low. If fn1 is higher than 1.00, the tensile strength of the steel becomes too high, and the machinability of the steel decreases. If fn1 is 0.60 to 1.00, excellent yield strength and machinability can be obtained.

  (2) Even if bainite is formed in the microstructure of the hot forged product, excellent cracking properties can be obtained while maintaining hot workability by setting the number density of coarse TiN within the appropriate range. .

  In the solidification process of molten steel by continuous casting, Ti forms Ti nitride (TiN), Ti sulfide and Ti carbosulfide. Among these, TiN remains without solid solution even in the heating process before hot forging. Therefore, such TiN remains in the hot forging product. The remaining TiN becomes a starting point of fracture at multiple points during cracking, and a sharp initial crack is generated at the interface between TiN and the matrix. Since the tip of a sharp crack is in a state of strong plastic constraint, brittle fracture is likely to occur. A brittle fracture surface is obtained by bonding a brittlely developing crack from an initial crack with a crack generated from adjacent TiN. Therefore, even in the microstructure containing high toughness bainite, by generating the above-mentioned initial crack with TiN, brittle crack growth occurs, the fracture surface becomes a brittle fracture surface, and the ductile fracture surface is suppressed. As a result, excellent cracking properties can be obtained.

In order to obtain such an effect, it is preferable to have a large size of TiN. Specifically, if the number density of TiN (hereinafter referred to as coarse TiN) having a circle equivalent diameter of 20 μm or more is less than 0.3 piece / mm ² , sufficient cracking properties can not be obtained. On the other hand, if the number density of the coarse TiN exceeds 4.0 pieces / mm ² , although excellent cracking properties can be obtained, the hot workability is lowered. If the number density of coarse TiN is 0.3 to 4.0 pieces / mm ² , excellent cracking can be obtained while maintaining hot workability even if bainite is generated by hot forging.

The non-tempered bar steel of the present embodiment completed according to the above findings contains, by mass%, C: 0.39 to 0.55%, Si: 0.10 to 1.00%, Mn: 0.50 to 1.. 50%, P: 0.010-0.100%, S: 0.040-0.130%, Cr: 0.05-0.50%, V: 0.05-0.40%, Ti: 0 .15 to 0.25%, Al: 0.005 to 0.050%, N: 0.002 to 0.020%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, Mo: 0 to 0.10%, Pb: 0 to 0.30%, Te: 0 to 0.3000%, Ca: 0 to 0.0100%, and Bi: 0 to 0.3000%, with the balance being Fe and impurities, having a chemical composition satisfying the formula (1), the number density of TiN having 20μm or more circle-equivalent diameter in the steel 0.3 to 4.0 pieces / mm ² A.
0.60 ≦ C + 0.2 Mn + 0.25 Cr + 0.75 V + 0.81 Mo ≦ 1.00 (1)
Here, the content (mass%) of the corresponding element is substituted into the element symbol in the formula (1).

  The chemical composition is one or two selected from the group consisting of Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, and Mo: 0.01 to 0.10%. It may contain more than species. The above chemical composition is Pb: 0.05 to 0.30%, Te: 0.0003 to 0.3000%, Ca: 0.0003 to 0.0100%, and Bi: 0.0003 to 0.3000% It may contain one or more selected from the group consisting of

  Hereinafter, the non-heat treated bar steel of the present embodiment will be described in detail. “%” Of the content of each element means “mass%”.

[Chemical composition]
The chemical composition of the non-heat treated bar steel according to the present embodiment contains the following elements.

C: 0.39 to 0.55%
Carbon (C) enhances the yield strength and fatigue strength of the steel. If the C content is too low, this effect can not be obtained. On the other hand, if the C content is too high, the machinability is reduced. Therefore, the C content is 0.39 to 0.55%. The preferable lower limit of the C content is 0.40%, more preferably 0.41%, and still more preferably 0.42%. The upper limit of the C content is preferably 0.54%, more preferably 0.53%, still more preferably 0.52%.

Si: 0.10 to 1.00%
Silicon (Si) deoxidizes the steel. Si further dissolves in the steel to increase the fatigue strength of the steel. If the Si content is too low, these effects can not be obtained. On the other hand, if the Si content is too high, the above effect is saturated. If the Si content is too high, furthermore, the hot workability of the steel is reduced and the production cost of the steel bar is also increased. Therefore, the Si content is 0.10 to 1.00%. The lower limit of the Si content is preferably 0.11%, more preferably 0.12%, and still more preferably 0.15%. The upper limit of the Si content is preferably 0.99%, more preferably 0.95%, still more preferably 0.90%, and still more preferably 0.89%.

Mn: 0.50 to 1.50%
Manganese (Mn) deoxidizes the steel. Mn further enhances the yield strength and fatigue strength of the steel. If the Mn content is too low, these effects can not be obtained. On the other hand, if the Mn content is too high, the hot workability of the steel is reduced. Therefore, the Mn content is 0.50 to 1.50%. The preferable lower limit of the Mn content is 0.51%, more preferably 0.55%, and still more preferably 0.60%. The upper limit of the Mn content is preferably 1.49%, more preferably 1.45%, and still more preferably 1.40%.

P: 0.010 to 0.100%
Phosphorus (P) segregates at grain boundaries to embrittle the steel. Therefore, the fractured surface of the cracking connecting rod after fracture splitting becomes brittle. As a result, the amount of internal diameter deformation of the large end of the cracking connecting rod after breaking and splitting becomes small. If the P content is too low, this effect can not be obtained. On the other hand, if the P content is too high, the hot workability of the steel is reduced. Therefore, the P content is 0.010 to 0.100%. The lower limit of the P content is preferably 0.011%, more preferably 0.015%, and still more preferably 0.020%. The upper limit of the P content is preferably 0.090%, more preferably 0.080%, and still more preferably 0.070%.

S: 0.040 to 0.130%
Sulfur (S) combines with Mn and Ti to form sulfides and enhances the machinability of the steel. If the S content is too low, this effect can not be obtained. On the other hand, if the S content is too high, the hot workability of the steel is reduced. Therefore, the S content is 0.040 to 0.130%. The preferable lower limit of the S content is 0.041%, more preferably 0.045%, and still more preferably 0.050%. The upper limit of the S content is preferably 0.129%, more preferably 0.125%, and still more preferably 0.120%.

Cr: 0.05 to 0.50%
Chromium (Cr) enhances the yield and fatigue strength of the steel. If the Cr content is too low, this effect can not be obtained. On the other hand, if the Cr content is too high, the hardness of the steel increases and the machinability decreases. Furthermore, if the Cr content is too high, the manufacturing cost will be high. Therefore, the Cr content is 0.05 to 0.50%. The lower limit of the Cr content is preferably 0.10%, more preferably 0.12%, and still more preferably 0.15%. The upper limit of the Cr content is preferably 0.49%, more preferably It is 0.45%, more preferably 0.40%.

V: 0.05 to 0.40%
Vanadium (V) precipitates as carbides in ferrite in the cooling process after hot forging, and enhances the yield strength and fatigue strength of the steel. V further enhances the cracking properties of the steel by being contained together with Ti. If the V content is too low, these effects can not be obtained. On the other hand, if the V content is too high, not only the production cost of steel becomes extremely high, but also the machinability decreases. Therefore, the V content is 0.05 to 0.40%. The preferable lower limit of the V content is 0.06%, more preferably 0.07%, and still more preferably 0.10%. The upper limit of the V content is preferably 0.39%, more preferably 0.35%, and still more preferably 0.32%.

Ti: 0.15 to 0.25%
Titanium (Ti) forms TiN in the solidification process of continuous casting to enhance cracking. More specifically, in the solidification process of molten steel by continuous casting, Ti forms TiN, Ti sulfide and Ti carbosulfide. The TiN formed at this time does not form a solid solution even in the subsequent heating step before hot forging, and improves the cracking property by satisfying the size and number density described later.

  Furthermore, Ti precipitates as carbides in the ferrite together with V in the cooling process after hot forging, and enhances the fatigue strength of the steel. Ti further forms sulfides or carbon sulfides to enhance the machinability of the steel. More specifically, when the non-heat treated bar is heated prior to hot forging, Ti sulfide in the steel and part of Ti in Ti carbosulfide form a solid solution. Furthermore, when the steel material is allowed to cool to the atmosphere after hot forging, part of Ti remains in solid solution until ferrite transformation starts. Then, when ferrite transformation is started, solid solution Ti precipitates as carbides together with V in the ferrite to increase the fatigue strength of the steel. Furthermore, Ti sulfide and carbon sulfide remaining in the steel without solid solution increase the machinability of the steel.

  If the Ti content is too low, these effects can not be obtained. On the other hand, if the Ti content is too high, the hot workability is reduced. Therefore, the Ti content is 0.15 to 0.25%. The preferred lower limit of the Ti content is more than 0.15%, and more preferably 0.16%. The upper limit of the Ti content is preferably 0.24%, more preferably 0.22%.

Al: 0.005 to 0.050%
Aluminum (Al) deoxidizes the steel. If the Al content is too low, this effect can not be obtained. On the other hand, if the Al content is too high, the above effect is saturated. If the Al content is too high, the hot workability of the steel is further reduced, and the production cost of the steel material is also increased. Therefore, the Al content is 0.005 to 0.050%. The preferred lower limit of the Al content is 0.020%. The preferred upper limit of the Al content is 0.040%. In the non-tempered bar steel of the present embodiment, the Al content means acid-soluble Al (so-called "sol. Al").

N: 0.002 to 0.020%
Nitrogen (N) combines with Ti to form TiN and enhances cracking. If the N content is too low, this effect can not be obtained. On the other hand, if the N content is too high, the hot workability of the steel is reduced. Therefore, the N content is 0.002 to 0.020%. The preferable lower limit of the N content is 0.003%, more preferably 0.004%, and still more preferably 0.005%. The upper limit of the N content is preferably 0.019%, more preferably 0.018%, and still more preferably 0.017%.

  The balance of the chemical composition of the non-heat treated steel bar according to the present embodiment consists of Fe and impurities. Here, the impurities are mixed from the ore as a raw material, scrap, or the production environment when industrially producing the non-heat treated bar steel, and are adversely affected to the non-heat treated bar steel of this embodiment. Means what is acceptable without giving.

  The chemical composition of the non-heat-treated steel bar according to the present embodiment may further contain one or more selected from the group consisting of Cu, Ni and Mo, instead of part of Fe. These elements are optional elements, and all increase the fatigue strength of the steel.

Cu: 0 to 0.40%
Copper (Cu) is an optional element and may not be contained. When it is contained, Cu dissolves in the steel to increase the fatigue strength of the steel. However, if the Cu content is too high, not only the production cost of the steel will be high, but the machinability will be reduced. Therefore, the Cu content is 0 to 0.40%. The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%, and still more preferably 0.10%. The upper limit of the Cu content is preferably 0.39%, more preferably 0.35%, and still more preferably 0.30%.

Ni: 0 to 0.30%
Nickel (Ni) is an optional element and may not be contained. When it is contained, Ni dissolves in the steel to increase the fatigue strength of the steel. However, if the Ni content is too high, not only the manufacturing cost increases, but also the ductility fractured surface is formed on the fractured surface after fracture separation, and the cracking property is lowered due to the increase of the toughness. Therefore, the Ni content is 0 to 0.30%. The preferable lower limit of the Ni content is 0.01%, more preferably 0.02%, and still more preferably 0.05%. The upper limit of the Ni content is preferably 0.29%, more preferably 0.28%, and still more preferably 0.25%.

Mo: 0 to 0.10%
Molybdenum (Mo) is an optional element and may not be contained. When it is contained, Mo forms carbides in the steel to enhance the yield strength and fatigue strength of the steel. However, if the Mo content is too high, the hardness of the steel increases and the machinability decreases. If the Mo content is too high, the production cost will be high. Therefore, the Mo content is 0 to 0.10%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.02%, and still more preferably 0.05%. The preferable upper limit of the Mo content is 0.09%, more preferably 0.08%, and still more preferably 0.07%.

  The chemical composition of the non-heat-treated steel bar according to the present embodiment may further contain one or more selected from the group consisting of Pb, Te, Ca and Bi, instead of part of Fe. These elements are optional elements and all enhance the machinability of the steel.

Pb: 0 to 0.30%
Lead (Pb) is an optional element and may not be contained. When contained, Pb enhances the machinability of the steel. However, if the Pb content is too high, the hot workability of the steel is reduced. Therefore, the Pb content is 0 to 0.30%. The preferable lower limit of the Pb content is 0.05%, and more preferably 0.10%. The upper limit of the Pb content is preferably 0.29%, more preferably 0.25%, and still more preferably 0.20%.

Te: 0 to 0.3000%
Tellurium (Te) is an optional element and may not be contained. When contained, Te enhances the machinability of the steel. However, if the Te content is too high, the hot workability of the steel is reduced. Therefore, the Te content is 0 to 0.3000%. The preferable lower limit of the Te content is 0.0003%, more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of the Te content is preferably 0.2900%, more preferably 0.2500%, and still more preferably 0.2000%.

Ca: 0 to 0.0100%
Calcium (Ca) is an optional element and may not be contained. When contained, Ca enhances the machinability of the steel. However, if the Ca content is too high, the hot workability of the steel is reduced. Therefore, the Ca content is 0 to 0.0100%. The preferable lower limit of the Ca content is 0.0003%, more preferably 0.0005%, and still more preferably 0.00100%. The upper limit of the Ca content is preferably 0.0090%, more preferably 0.0080%, and still more preferably 0.0050%.

Bi: 0 to 0.3000%
Bismuth (Bi) is an optional element and may not be contained. When contained, Bi enhances the machinability of the steel. However, if the Bi content is too high, the hot workability of the steel decreases. Therefore, the Bi content is 0 to 0.3000%. The lower limit of the Bi content is preferably 0.0003%, more preferably 0.0005%, and still more preferably 0.0010%. The preferable upper limit of the Bi content is 0.2900%, more preferably 0.2000%, and still more preferably 0.1000%.

[About formula (1)]
The chemical composition of the non-heat treated bar steel of the present embodiment further satisfies the formula (1).
0.60 ≦ C + 0.2 Mn + 0.25 Cr + 0.75 V + 0.81 Mo ≦ 1.00 (1)
Here, the content (mass%) of the corresponding element is substituted into the element symbol in the formula (1).

  If fn1 (= C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo) is less than 0.60, the yield strength of the steel is too low. If fn1 is higher than 1.00, the strength of the steel becomes too high, and the machinability of the steel decreases. If fn1 is 0.60 to 1.00, excellent yield strength and machinability can be obtained in the non-heat treated bar steel. The preferred lower limit of fn1 is 0.61, more preferably 0.63, and still more preferably 0.65. The preferred upper limit of fn1 is 0.99, more preferably 0.98, and still more preferably 0.95.

[Microstructure]
In the case of the above-mentioned chemical composition, the microstructure of the non-heat-treated steel bar mainly consists of ferrite and pearlite. Specifically, in the non-heat treated bar steel of the above chemical composition, the total area ratio of ferrite and pearlite in the microstructure is 65% or more. When the total area ratio of ferrite and pearlite is not 100%, the remainder of the matrix structure is bainite. The lower limit of the total area ratio of ferrite and pearlite is preferably 70%, more preferably 75%, still more preferably 80% or more, and most preferably 100%. The upper limit of the area ratio of bainite is preferably 30%, more preferably 25%, still more preferably 20%, and most preferably 0%.

  The area fraction of bainite in the microstructure can be measured by the following method. Ten samples are taken from any R / 2 part (the central part of a line segment (radius) connecting the central axis of the steel bar and the outer peripheral surface) of the non-heat treated steel bar. Of the samples collected, the surface perpendicular to the central axis of the non-heat treated bar is taken as the observation surface. After polishing the observation surface, etching is performed with 3% nitric alcohol (Nital corrosion solution). The etched viewing surface is observed with a 200 × optical microscope to produce a photographic image of any 5 fields of view.

In each view, each phase such as ferrite, perlite, bainite, etc. has different contrast for each phase. Therefore, each phase is identified based on the contrast. Among the identified phases, the area (μm ² ) of bainite in each field of view is determined. The ratio of the area of bainite in all the fields to the total area of all the fields (5 fields × 10) is defined as the area ratio (%) of bainite.

[Number density of coarse TiN]
In the non-heat treated bar steel according to the present embodiment, the number density of TiN (hereinafter referred to as coarse TiN) having a circle equivalent diameter of 20 μm or more is 0.3 to 4.0 / mm ² . Here, in the present specification, TiN means an inclusion having a total content of Ti and N in inclusions of 70% or more by mass.

  The non-heat treated bar steel of this embodiment is manufactured into a cracking connecting rod by hot forging. If the temperature of the steel during hot forging becomes higher than 1300 ° C due to the variation of the heating temperature during operation, the microstructure of the hot forging (cracking connecting rod) forms bainite together with ferrite and pearlite There is a case. In this case, in the above chemical composition, the area ratio of bainite that can be produced is, for example, 5 to 30%.

  Bainite has high toughness compared to ferrite and pearlite. Therefore, when the large end of the cracking connecting rod is broken to manufacture two parts (cap and rod), the broken portion is plastically deformed and a ductile fracture surface is generated on the broken surface. That is, the cracking property is reduced.

Even if bainite is generated in the microstructure by hot forging, the non-heat-treated bar steel of the present embodiment maintains excellent cracking properties by setting the number density of the coarse TiN within an appropriate range. If the number density of the coarse TiN is less than 0.3 pieces / mm ² , sufficient cracking properties can not be obtained. On the other hand, if the number density of the coarse TiN exceeds 4.0 pieces / mm ² , although excellent cracking properties can be obtained, the hot workability is lowered. If the number density of coarse TiN is 0.3 to 4.0 pieces / mm ² , even if hot forging is carried out under the conditions for forming bainite, excellent cracking property is maintained while maintaining hot workability. can get.

The preferable lower limit of the number density of coarse TiN for further enhancing the cracking property is 0.4 pieces / mm ² , more preferably 0.5 pieces / mm ² . The preferable upper limit of the number density of coarse TiN for further enhancing the hot workability is 3.9 pieces / mm ² , more preferably 3.8 pieces / mm ² .

The number density of coarse TiN can be measured by the following method. Take a sample from the R / 2 part of the steel bar. Of the surfaces of the sample, the surface corresponding to the cross section (longitudinal cross section) including the axial direction of the bar is taken as the observation surface. It does not corrode the observation surface, but it observes with a 200x optical microscope as it is, and produces | generates a photographic image in arbitrary 100 visual fields. The total area of 100 views is 11.9 mm ² . The total content of Ti and N of inclusions and precipitates in each field of view is analyzed using an electron probe microanalyzer (EPMA) and identified as TiN in each field of view. The area of each identified TiN is determined using the photographic image of each field of view, and the equivalent circle diameter is calculated from the obtained area. TiN having a circle equivalent diameter of 20 μm or more is identified as coarse TiN, and the total number of coarse TiN is determined. The value obtained by dividing the total number of coarse TiN obtained by the total area of 100 views is defined as the number density (number / piece / mm ² ) of coarse TiN.

[Production method]
An example of the manufacturing method of the above-mentioned non-heat treated bar steel is demonstrated. The present manufacturing method includes a casting process and a hot rolling process.

[Casting process]
A molten steel satisfying the above-described chemical composition and the formula (1) is manufactured by a known method. A cast steel (slab or bloom) is manufactured by a continuous casting method using molten steel.

  In order to bring the number density of coarse TiN into the above range, in the casting process, continuous casting is performed so as to satisfy the following conditions.

Superheat degree ΔT: 30 to 50 ° C
The difference between the molten steel temperature and the TLL (liquidus temperature) in a tundish arranged on a continuous casting machine is defined as the degree of superheat ΔT (° C.). If ΔT is less than 30 ° C., the crystallization amount of TiN becomes insufficient. On the other hand, if the degree of superheat ΔT (° C.) exceeds 50 ° C., coarse TiN is excessively formed, and the number density of coarse TiN exceeds 4.0 pieces / mm ² . If the degree of superheat ΔT is 30 to 50 ° C., the amount of crystallization of coarse TiN can be made into an appropriate range while stabilizing the operation.

Cross section of cast slab: Side length 300 mm or more Casting speed Vc: 0.2 to 0.8 m / min
If the cooling rate in the solidification process of the slab is too fast, the crystallization and aggregation of TiN will be insufficient. On the other hand, if the cooling rate is too slow, TiN will agglomerate excessively and the number density of coarse TiN will exceed 4.0 pieces / mm ² .

If one side of the cross section (rectangle) of the cast slab is 300 mm or more and the casting speed Vc is 0.2 to 0.8 m / min, TiN crystallizes sufficiently and coheres, so coarse TiN The number density of is 0.3 pieces / mm ² or more.

  The specific water amount is not particularly limited, and may be a known specific water amount. Preferably, the specific water content is as low as possible without causing sagging of the slab. The preferred specific water content is, for example, 5 L / kg or less.

[Hot working process]
In the hot working process, hot working is performed on the slab produced in the above-described casting process to produce a bar. The hot working process includes, for example, a rough rolling process and a finish rolling process.

[Rough rolling process]
The billet or ingot is hot rolled to produce a billet. Hot rolling is performed, for example, using a mass rolling mill and a continuous rolling mill in which a plurality of stands are arranged in a row and each stand has a plurality of rolls.

[Finish rolling process]
A bar is manufactured using a billet. Specifically, the billet is heated in a heating furnace (heating step). After heating, the billet is hot-rolled (finish rolling) using a continuous rolling mill to produce a non-tempered bar steel (finish rolling step). Each step will be described below.

[Heating process]
In the heating step, preferably, the billet is heated at a heating temperature of 1000 to 1300 ° C. for 30 minutes or more. If the heating temperature is too low, TiN in the billet is less likely to agglomerate. Therefore, the fine TiN present in the billet is not agglomerated and is taken over even after hot rolling, and a large amount of fine Ti nitride is present in the bar. In this case, coarse TiN in the steel is reduced. On the other hand, if the heating temperature is too high, the Ti nitride excessively condenses during heating. If the heating temperature at the time of finish rolling is 1000 to 1300 ° C., the number density of coarse TiN is stably in an appropriate range (0.3 to 4.0 pieces / mm ² ) on the premise that the above-described casting condition is satisfied. ).

[Hot rolling process]
Using a finishing mill, the billet after heating is finish-rolled (hot-rolled) by a known method to produce a non-heat treated bar steel. The finish rolling mill has a plurality of stands arranged in a row, each stand having a plurality of rolls (roll group) arranged around a pass line. The rolls of each stand form a hole mold which is pressed as the billet passes through the hole mold to produce a steel bar.

The reduction of area in a continuous rolling mill is preferably 70% or more. Here, the reduction rate is defined by the following equation.
Area reduction ratio = (cross-sectional area of billet before finish rolling−cross-sectional area of non-heat treated bar steel after finish rolling) / cross-sectional area of billet before finish rolling × 100

  The above-mentioned non-heat treated bar steel is manufactured by the above manufacturing process.

[Method of manufacturing hot forged product]
As an example of a method of manufacturing a hot forged product using the above-described non-heat treated bar, a method of manufacturing a cracking connecting rod will be described. First, the steel material is heated in a high frequency induction heating furnace. In this case, the preferred heating temperature is 1000 to 1300 ° C., and the preferred heating time is 10 to 15 minutes. Since the heating time is short, the form of Ti nitride in the bar does not particularly change. Hot forging is performed on the heated steel bar to produce a cracking connecting rod. Preferably, the working degree at the time of hot forging is 0.22 or more. Here, the processing degree is the maximum value of the logarithmic strain generated in the portion excluding the burrs in the forging process.

  After hot forging, the cracking connecting rod is allowed to cool to room temperature. The large end of the connecting rod has a small cross-sectional area, so the cooling rate is fast. Therefore, the form of TiN does not particularly change during cooling. Carry out machining on the cracked connecting rod after cooling if necessary. The cracking con rod is manufactured by the above process.

[Microstructure of hot forged products]
The microstructure of the manufactured hot forging (cracking connecting rod) mainly consists of ferrite and pearlite. Preferably, the microstructure has a total area percentage of ferrite and perlite of 100%. However, if the heating temperature of the steel bar at the time of hot forging exceeds 1300 ° C., the microstructure of the produced cracking connecting rod may contain bainite.

  In the microstructure of the cracking connecting rod manufactured by hot forging using the above-described non-heat treated bar, the total area ratio of ferrite and pearlite is 65% or more. When the total area ratio of ferrite and pearlite is not 100%, the remainder of the matrix structure is bainite. The lower limit of the total area ratio of ferrite and pearlite is preferably 70%, more preferably 75%, still more preferably 80% or more, and most preferably 100%. The upper limit of the area ratio of bainite is preferably 30%, more preferably 25%, still more preferably 20%, and most preferably 0%. An example of the area ratio of bainite is 5 to 30%.

When the microstructure includes bainite, when the large end is broken and divided into two parts (cap and rod), the broken part is plastically deformed and a part of the broken surface tends to be a ductile fracture surface, and cracking property Is likely to decline. However, in the non-heat treated bar steel of the present embodiment, since the number density of coarse TiN in the steel is 0.3 to 4.0 pieces / mm ² , the fractured surface tends to be a brittle fractured surface and excellent cracking property It can be maintained.

  The area ratio of bainite in the microstructure in the hot forging can be measured by the following method. Ten samples are taken from any part of the hot forging. For each sample collected, the phase of the microstructure is identified and the area fraction of bainite is determined by the same method as the microstructure observation in the non-heat treated steel bar.

  In the above description, the cracking connecting rod has been described as an example of the method of manufacturing a forged product. However, the non-heat treated steel bar of the present embodiment is not limited to the cracking connecting rod application. The non-heat treated bar steel of this embodiment can be widely applied to forging applications.

  Moreover, the manufacturing method of non-heat treated bar steel is not limited to the said manufacturing method, if the number density of coarse TiN can be made into the said range.

  A molten steel having the chemical composition shown in Table 1 was produced.

  Referring to Table 1, the chemical compositions of test numbers E-1 to E-45, C-9, C-10, C-12 and C-13 were appropriate and satisfied the formula (1). On the other hand, for test numbers C-1 to C-8 and C-11, the content of any element in the chemical composition was inadequate or did not satisfy the formula (1). Moreover, the chemical composition of test number C-11 was in the range of the chemical composition of the steel of patent document 1.

  Molten steel of each test number was manufactured with a 70 ton converter. A slab was produced from molten steel by a continuous casting method using a continuous casting machine. The cross section of Bloom was 300 mm x 400 mm. In each test number, the molten steel temperature (° C.) in the tundish was measured, and the degree of superheat ΔT (° C.) which was the difference between the molten steel temperature and the TLL (liquidus temperature) was determined. Furthermore, in each test number, casting was performed at a casting speed Vc (m / min) shown in Table 2. In any of the test numbers, the specific water amount was 5 L / kg or less.

  The manufactured slab was hot-rolled to produce a billet. The billet was heated at 1150 ° C. for 35 minutes, and then finish rolling was performed using a finish rolling mill to produce a 40 mm diameter steel bar.

[Manufacturing of heat forgings]
The steel bar was cut in the direction perpendicular to the longitudinal direction, and a specimen having a diameter of 40 mm and a length of 100 mm was collected. The test piece was heated and held at 1300 ° C. for 5 minutes. Immediately after heating, 90% hot compression was performed in the axial direction to form a disk shape, and a hot forging simulation product (referred to as a heat forging simulation product) was manufactured. The molded heat forged model was allowed to cool in the air. The temperature of each test piece measured using a radiation thermometer at the time of compression was 1350 ° C.

[Evaluation test]
The following evaluation tests were carried out using the test material and the heat forge simulation product.

[Number density measurement of coarse TiN]
A sample was taken from R / 2 part of each test material. Among the surfaces of the sample, the surface corresponding to the cross section (longitudinal cross section) including the axial direction of the test material was used as the observation surface. The observation surface was not corroded and was observed as it was with a 200 × optical microscope, and a photographic image was generated with an arbitrary 100 fields of view. The total area of 100 views was 11.9 mm ² . TiN was specified by the method described above, and the number density (pieces / mm ² ) of coarse TiN was determined. The determined number density is shown in Table 2.

[Microstructure observation]
A microstructure observation test was performed using each heat forging simulation product. Specifically, a sample containing R / 2 part was taken out of the longitudinal section of the heat forging simulation product. The surface perpendicular to the central axis of the non-heat treated bar was taken as the observation surface. After the observation surface was polished, it was etched with 3% nitric alcohol (Nital corrosion solution). The etched observation surface was observed with a 200 × optical microscope, and the area ratio (%) of bainite was determined by the method described above. The area ratio of bainite determined is shown in Table 2.

[Hot workability evaluation]
Fifty heat forging replicas were manufactured for each test number. The presence or absence of a crack on the surface of the heat forged model after production was visually confirmed. Evaluation "A" is the case where the occurrence of a crack is 0 out of 50, evaluation "B" when it is one, and evaluation "C" when it is two or three. When there was an evaluation, it was evaluated as "x". In the case of the evaluations "A" to "C", it was judged that sufficient hot workability was obtained, and in the case of the evaluation "x", it was judged that the hot workability was low. The evaluation results are shown in Table 2.

[Cracking evaluation]
From each of the thermal forgings, a test piece 10 simulating the large end of the connecting rod shown in FIG. 2A was manufactured by machining. The length of one side of the test piece 10 was 80 mm, and the thickness was 10 mm. A hole (through hole) 11 was formed at the center of the test piece 10. The diameter of the hole 11 was 60 mm, and its center was coaxial with the center of the test piece 10. As shown in FIG. 2A, V-shaped notches M were machined at two places corresponding to the respective end points of the diameter among the periphery of the hole 11. The depth of the notch M was 1 mm, the tip R was 0.1 mm, and the opening angle was 60 °.

  The jig 12 was fitted into the hole 11. The jig 12 consisted of a pair of semi-disc-like members, and when combined two, it became a disc whose diameter corresponds to the inside diameter of the hole 11. At the center of the jig 12 was formed a hole 14 for driving the wedge 13 (see FIG. 2B).

  After the jig 12 was fitted in the hole 11, the wedge 13 was driven in and the test piece 10 was broken into two members 10A and 10B at room temperature (25 ° C.) (see FIG. 2C).

  Bolt holes were formed in the vicinity of both side surfaces of the members 10A and 10B, and the members 10A and 10B were fastened with a bolt 15 shown in FIG. 2D. Measure the diameter D0 of the hole 11 of the test piece 10 (see FIG. 2A) before breaking and separation and the diameter D1 (FIG. 2D) of the hole 11 of the test piece 10 after breaking and separating and after fastening the bolt 15 The difference between them was defined as an inner diameter deformation amount ΔD (= D1−D0, unit: μm).

  When the inside diameter deformation amount ΔD is 0 to 30 μm, the evaluation is “A”, 31 to 50 μm is an evaluation “B”, and 51 to 80 is an evaluation “C”. And when inside diameter deformation amount (DELTA) D is 81 micrometers or more, it was set as evaluation "x." In the case of evaluation "A"-"C", it was judged that cracking property was fully acquired. In the case of evaluation "x", it was judged that cracking property was low.

[Yield strength evaluation]
Two JIS 14A test pieces were collected from R / 2 part of each heat forging simulation product. The tensile test was carried out at room temperature (25 ° C.) in the atmosphere using the collected test pieces to determine the average yield strength YS (MPa) of the two strands.

  The case where the yield strength is 1000 to 801 MPa is evaluated as "A", the case of 800 to 601 MPa is evaluated as "B", and the case of 600 to 401 MPa is evaluated as "C". The case where the yield strength was 400 MPa or less was evaluated as “x”.

  In the case of evaluation "A"-"C", it was judged that sufficient yield strength was obtained. In the case of evaluation "x", it was judged that the yield strength was low.

[Fatigue strength evaluation]
A JIS 14A test piece was collected from R / 2 part of each heat forged simulation product. Using the collected test pieces, a sine wave phase 0 (MPa) double swing fatigue test was performed at room temperature (25 ° C.) in the air. The maximum stress that did not break after several 10 ⁷ cycles was taken as the fatigue strength (MPa). The frequency was 15 Hz.

  Evaluation “S” for fatigue strength of 500 to 451 MPa, evaluation “A” for 450 to 401 MPa, evaluation “B” for 400 to 351 MPa, and evaluation “C” for 350 to 301 MPa. The case where the fatigue strength was 300 MPa or less was evaluated as “x”.

  In the case of evaluation "S" and "A"-"C", it was judged that sufficient fatigue strength was obtained. In the case of the evaluation "x", it was determined that the fatigue strength was low.

[Machinability evaluation]
Five thermal forgings were prepared for each test number. The drill was drilled at any position on the five prepared thermal forgings, and the amount of tool wear was measured when a total of 50 holes were drilled. The drill diameter was 10 mm, and the rotation speed of the spindle was 1000 times / min.

  The case where the amount of tool wear was 0 to 10 μm was evaluated as “S”, the case of 11 to 30 μm as “A”, the case of 31 to 50 μm as “B”, and the case of 51 μm to 70 μm as “C”. The case where the amount of tool wear was 71 μm or more was evaluated as “x”.

  In the case of evaluation "S", "A"-"C", it was judged that sufficient machinability was obtained. In the case of evaluation "x", it was judged that the machinability is low.

[Evaluation results]
The evaluation results are shown in Table 2. Referring to Table 2, the chemical compositions of test numbers E-1 to E-45 were appropriate, and fn1 also satisfied the formula (1). Furthermore, the degree of superheat ΔT and the casting speed Vc were also appropriate. Therefore, the number density of coarse TiN was in the range of 0.3 to 4.0 pieces / mm 2. As a result, although the area ratio of bainite is 0 to 30%, excellent cracking properties were obtained. Furthermore, yield strength YS, fatigue strength, machinability and hot workability were also good.

  On the other hand, V content of test number C-1 was too high. Therefore, the strength was too high and the machinability was low.

  The V content of Test No. C-2 was too low. Therefore, the fatigue strength was low.

  The Ti content in Test No. C-3 was too high. Therefore, the hot workability was low.

  The Ti content in Test No. C-4 was too low. Therefore, the fatigue strength was low. Furthermore, the degree of superheat ΔT was too small. Therefore, the number density of coarse TiN was low. As a result, the cracking properties of the bainite-containing steel material were low.

  The N content of Test No. C-5 was too high. Therefore, the hot workability was low.

  The N content in Test No. C-6 was low, and the number density of coarse TiN was low. Therefore, the cracking property in the steel containing bainite was low.

  In test No. C-7, fn1 was too high. Therefore, the machinability was low.

  In test No. C-8, fn1 was too low. Therefore, the yield strength was low.

  In the test number C-9, although the chemical composition was appropriate and the formula (1) was satisfied, the degree of superheat ΔT was too large. Therefore, the number density of coarse TiN was too high. As a result, the hot workability was low.

  In the test number C-10, although the chemical composition was appropriate and the formula (1) was satisfied, the degree of superheat ΔT was too small. Therefore, the number density of coarse TiN was too low. As a result, the cracking properties of the bainite-containing steel material were low.

  The chemical composition of test number C-11 corresponded to Example 11 of patent document 1. In Test No. C-11, the C content and the Mn content were too low. Therefore, the fatigue strength was low. Furthermore, the N content was too high. Therefore, the hot workability was low. Furthermore, the degree of superheat ΔT was too small. Therefore, the number density of coarse TiN was too low. As a result, the cracking properties of the bainite-containing steel material were low.

  In Test No. C-12, the chemical composition was appropriate, and although the formula (1) was satisfied, the casting speed Vc was too low. Therefore, the number density of coarse TiN was too high. As a result, the hot workability was low.

  In the test number C-13, although the chemical composition was appropriate and the formula (1) was satisfied, the casting speed Vc was too high. Therefore, the number density of coarse TiN was too low. As a result, the cracking properties of the bainite-containing steel material were low.

  The embodiment of the present invention has been described above. However, the embodiments described above are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the scope of the invention.

Claims (3)

In mass%,
C: 0.39 to 0.55%,
Si: 0.10 to 1.00%,
Mn: 0.50 to 1.50%,
P: 0.010-0.100%,
S: 0.040 to 0.130%,
Cr: 0.05 to 0.50%,
V: 0.05 to 0.40%,
Ti: 0.15 to 0.25%,
Al: 0.005 to 0.050%,
N: 0.002 to 0.020%,
Cu: 0 to 0.40%,
Ni: 0 to 0.30%,
Mo: 0 to 0.10%,
Pb: 0 to 0.30%,
Te: 0 to 0.3000%,
Ca: 0 to 0.0100%, and
Bi: containing 0 to 0.3000%, the balance being Fe and impurities, and having a chemical composition satisfying the formula (1),
The number density of TiN having a circle equivalent diameter of more than 20μm in the steel is 0.3 to 4.0 pieces / mm ^2, non-heat treated steel bars.
0.60 ≦ C + 0.2 Mn + 0.25 Cr + 0.75 V + 0.81 Mo ≦ 1.00 (1)
Here, the content (mass%) of the corresponding element is substituted into the element symbol in the formula (1). The non-heat treated bar steel according to claim 1, wherein
The chemical composition is
Cu: 0.01 to 0.40%,
Ni: 0.01 to 0.30%, and
Mo: Non-tempered bar steel containing one or more selected from the group consisting of 0.01 to 0.10%. It is a non-heat treated bar steel according to claim 1 or claim 2,
The chemical composition is
Pb: 0.05 to 0.30%,
Te: 0.0003 to 0.3000%,
Ca: 0.0003 to 0.0100%, and
Bi: non-tempered bar steel containing one or more selected from the group consisting of 0.0003 to 0.3000%.

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