JP6668741B2 - Hot rolled bar - Google Patents

Description

  The present invention relates to a hot-rolled rod or wire.

  The steel for machine structural use is used for machine parts such as industrial machines, construction machines, bearing parts, and transport machines typified by automobiles. Generally, steel for machine structural use is roughly worked by hot forging and then cut to finish into a machine part having a predetermined shape. In this mechanical part, in particular, a part requiring high strength has been conventionally carburized in a carburizing gas atmosphere at about 930 ° C.

  Since AlN is relatively stable at the above carburizing temperature of about 930 ° C., the conventional case hardening steel contains appropriate amounts of Al and N, and this Al and N are precipitated as AlN before carburizing. By using the austenitic pinning particles, the occurrence of coarsening has been suppressed.

  On the other hand, in order to shorten the processing time, recently, carburizing at a high temperature exceeding 1000 ° C. has been studied. This is because, for example, when carburizing is performed at 1050 ° C., the processing time can be reduced to ３ to ４ as compared with the case of carburizing at 930 ° C.

  In order to solve the above problems, various high-temperature carburizing steels utilizing a pinning action of fine AlN or Nb (CN) have been proposed.

  Patent Literature 1 proposes a steel which is excellent in crystal grain coarsening characteristics by controlling the number distribution of carbides, nitrides and carbonitrides containing Al, Ti and Nb, and further excellent in low cycle characteristics.

  Patent Document 2 proposes a steel having excellent crystal grain coarsening properties by controlling the number distribution of carbides, nitrides, and carbonitrides containing Ti, Nb, and Mo.

  Patent Literature 3 proposes a steel having excellent crystal grain coarsening characteristics and excellent rolling fatigue life by controlling the number distribution of carbides, nitrides, and carbonitrides containing Al and Ti.

Japanese Patent No. 4807949 Japanese Patent No. 5350181 Japanese Patent No. 4502929

However, Patent Document 1 does not mention the machinability, and when a large amount of Ti-based precipitates is precipitated on the material after hot working, the material may be too hard and the machinability may be reduced. .
Further, Patent Document 2 does not mention the machinability similarly to Patent Document 1, and when a large amount of Ti-based precipitates precipitates in the material after hot working, the material becomes too hard, May decrease. Furthermore, it does not mention the core hardness after carburizing and quenching, and may be inferior to the low cycle fatigue characteristics that require the core hardness.
Further, Patent Document 3 does not mention the machinability similarly to Patent Document 1, and when a large amount of Ti-based precipitates precipitates in the material after hot working, the material becomes too hard, May decrease. This steel is considered for the rolling fatigue life and is not considered for improving the low cycle fatigue characteristics.

  Further, in the steels proposed in Patent Literatures 1 to 3, when carburizing treatment is performed at a high temperature, a part of pinning particles (such as AlN) is dissolved and coarsened in a matrix (base), and the pinning effect is reduced. Because of the decrease, coarsening may occur.

  The present invention has been made in view of the above situation, and has as its object to provide a hot-rolled rod or wire having excellent crystal grain coarsening prevention characteristics, and also excellent machinability and low cycle fatigue characteristics.

  The present inventors have obtained the following findings as a result of research and study on case hardening steel.

(A) It has been clarified that the addition of a very small amount of Bi can suppress solid solution and coarsening of pinned particles (such as AlN) during carburization, and can suppress generation of coarse particles.

(B) Cutting is a breaking phenomenon that separates chips, and one of the key points in promoting this is embrittlement of the matrix. By finely dispersing the sulfide, breakage is facilitated, and the chip disposability is improved. In other words, when the sulfide is largely dispersed in a small number, the interval between the sulfides which is the starting point of chip separation becomes long, and as a result, the chip tends to become long.

(C) Sulfides in steel materials are often crystallized before solidification (in molten steel) or during solidification, and the size of sulfides is greatly affected by the cooling rate during solidification. In addition, the solidified structure of the continuous cast slab usually has a dendrite form, and this dendrite is formed due to diffusion of a solute element in the solidification process, and the solute element is concentrated in the dendritic tree. I do. Mn is concentrated in the inter-tree area, and MnS is crystallized between the trees.

(D) In order to finely disperse sulfide, it is necessary to shorten the interval between dendrite trees. Research on the primary arm spacing of dendrite has been conventionally performed, and according to the following non-patent document, it can be expressed by equation (A).

λ∝ (D × σ × ΔT) ^0.25 ... (A)

Here, λ: primary arm interval (μm) of the dendrite, D: diffusion coefficient (m ² / s), σ: solid-liquid interface energy (J / m ² ), ΔT: solidification temperature range (° C.).

  Non-patent document: W. Kurz and D.J. Fisher, "Fundamentals of Solidification", Trans Tech Publications Ltd., (Switzerland), 1998, p. 256

  From this equation (A), it can be seen that the primary arm interval λ of the dendrite depends on the solid-liquid interface energy σ, and if this σ can be reduced, λ will decrease. If λ can be reduced, MnS size crystallized between dendrite trees can be reduced.

  In addition, MnS includes not only pure MnS but also mainly MnS, and inclusions in which sulfides such as Fe, Ca, Ti, Zr, Mg, and REM coexist with MnS by solid solution or bonding. , An element other than S, such as MnTe, forms a compound with Mn and forms a compound with MnS to form a solid solution / bond and coexists, or includes the above-mentioned inclusions precipitated with an oxide as a nucleus; In the chemical formula, Mn sulfide-based inclusions that can be expressed as (Mn, X) (S, Y) (here, X: an element other than Mn and a sulfide-forming element other than Y: S, which binds to Mn) are collectively referred to. It is something to say.

  The present invention has been completed based on the above findings, and the gist of the present invention is a hot-rolled rod or wire shown in the following (1) and (2).

(1) In mass%,
C: 0.13 to 0.40%,
Si: 0.01 to 1.5%,
Mn: 1.0 to 2.0%,
S: 0.015 to less than 0.040%,
Cr: 0.01 to 1.6%,
Al: 0.010-0.045% and N: 0.010-0.025%,
Bi: 0.0001 to 0.0100%
And the balance consists of Fe and impurities,
Further, P and O are
P: 0.05% or less and O: 0.0005 to 0.0040%,
The contents of Mn and S satisfy the following formula (1),
A hot-rolled rod or wire having a sulfide having an equivalent circle diameter of 1 μm or more and less than 2 μm in a cross section parallel to a rolling direction of a steel material having an existing density of 300 pieces / mm ² or more.
[Mn] ≧ 8.4 × [S] +0.9 (1)

(2) Further, in mass%,
Nb: 0.05% or less under,
Mo: 1.0% or less,
Ni: 1.0% or less,
Ti: 0.01% or less,
Cu: 0.40% or less,
V: 0.30% or less,
B: 0.02% or less and Mg: 0.0035% or less, the hot-rolled rod or wire according to (1), which contains one or more kinds selected from the group consisting of:

  The hot-rolled rod or wire according to the present invention can pin crystal grain boundaries by finely dispersing pinning particles (such as AlN). Further, by suppressing solid solution and coarsening of the pinned particles during carburizing, it is possible to suppress coarsening of crystal grains even when carburizing is performed at a higher temperature and for a shorter time than in the conventional case. Further, by making MnS finer, the machinability is better than that of conventional steel, the hardenability is improved, and the core hardness is improved, so that the low cycle fatigue characteristics are excellent.

  Specifically, when the hot-rolled rod or wire according to the present invention is used, carburizing is performed at a higher temperature than before, after hot forging and cutting, and carburizing is completed in a shorter time than before. The performance as a gear or a bearing can be imparted to a carburized part.

Hereinafter, the present invention will be described in detail.
When the hot-rolled rod or wire of the present invention is processed into a component shape such as a gear, hot forging and cutting are performed after rolling a continuously cast slab and before surface hardening treatment such as carburizing and quenching. At that time, MnS is extremely effective for cutting. That is, MnS in the hot-rolled rod or wire that is a work material suppresses a tool change due to wear of a cutting tool, and has an effect of extending a so-called tool life.

  Therefore, in order to enhance the machinability, it is desirable to generate MnS in the steel. On the other hand, in hot rolling or hot forging, coarse MnS is often stretched. Therefore, in order to suppress the coarsening of MnS, it is desirable to reduce the solid-liquid interface energy in the molten steel and refine the dendrite structure. The dendrite structure greatly affects the size of the sulfide. The dendrite structure becomes finer and the size of MnS becomes smaller.

  To stably and effectively finely disperse MnS, a trace amount of Bi is added to reduce the solid-liquid interface energy in the molten steel. Since the solid-liquid interfacial energy is reduced, the dendrite structure becomes fine, and the sulfide crystallized therefrom becomes fine.

  In addition, the required cooling rate for obtaining the martensite structure cannot be obtained at the quenching stage during the carburizing process, and an incompletely quenched structure including a bainite structure is locally generated, and as a result, the toughness of the core decreases. There is fear.

  Therefore, the hardness of the core after carburization can be improved by adding Mn, which is a quenching property improving element, to improve the quenching property and to suppress the generation of an incomplete quenching structure including a bainite structure. .

  Hereinafter, the content of each component element in the hot-rolled rod or wire of the present embodiment will be described. Here, “%” for the components is% by mass.

C: 0.13 to 0.40%
Carbon (C) increases the tensile strength and fatigue strength of steel. On the other hand, if the content of C is too large, the cold forgeability of the steel decreases, and the machinability also decreases. Therefore, the content of C is 0.13 to 0.40%. The preferred C content is 0.14 to 0.28%, and more preferably 0.15 to 0.25%.

Si: 0.01 to 1.5%
Silicon (Si) forms a solid solution in ferrite in steel and increases the tensile strength of steel. On the other hand, if the content of Si is too large, the cold forgeability of steel decreases. Therefore, the content of Si is 0.01 to 1.5%. The preferred Si content is 0.15 to 0.70%, and more preferably 0.20 to 0.35%.

Mn: 1.0-2.0%
Manganese (Mn) has a deoxidizing effect and reduces oxide inclusions. Further, the hardenability during carburizing and quenching is remarkably enhanced, and the hardness of the core after carburizing is increased. Mn further combines with sulfur (S) in the steel to form MnS and enhances the machinability of the steel. On the other hand, if the content of Mn is too high, coarse MnS is generated, and the fatigue strength is reduced. Therefore, the content of Mn is 1.0 to 2.0%. The preferred Mn content is 1.20 to 1.70%, and more preferably 1.30 to 1.60%.

S: 0.015% or more and less than 0.040% Sulfur (S) combines with Mn in steel to form Mn sulfide and enhances machinability of steel. On the other hand, if S is excessively contained, the fatigue strength of steel decreases. Therefore, the content of S is 0.015% or more and less than 0.040%. A preferable S content is 0.020% or more and less than 0.030%.

Cr: 0.01 to 1.60%
Chromium (Cr) enhances the hardenability and tensile strength of steel. The mechanical part manufactured by the hot-rolled rod or wire according to the present embodiment may harden the surface of steel by carburizing or induction hardening. Cr enhances the hardenability of the steel and increases the surface hardness of the steel after carburizing or induction hardening. On the other hand, if the content of Cr is too large, the cold forgeability and fatigue strength of the steel decrease. Therefore, the content of Cr is 0.01 to 1.60%. When increasing the hardenability and tensile strength of steel, a preferable Cr content is 0.03 to 1.50%, and more preferably 0.10 to 1.20%.

Al: 0.010-0.045%
Al has a deoxidizing effect and is easily combined with N to form AlN, and is an element effective in preventing austenite grain coarsening during carburizing heating. However, if the Al content is less than 0.010%, the austenite grains cannot be stably prevented from being coarsened, and if the Al content is large, the fatigue strength is reduced. On the other hand, when the content of Al exceeds 0.045%, AlN becomes coarse, and does not contribute to suppression of coarsening of crystal grains. Therefore, the content of Al is set to 0.010 to 0.045%. A preferred lower limit of the Al content is 0.020%, and a preferred upper limit is 0.035%.

N: 0.010-0.025%
Nitrogen (N) is added for the purpose of refining crystal grains during carburization by precipitation of AlN and Nb (CN) and for suppressing the coarsening of crystal grains, but if less than 0.010%, the effect is insufficient. is there. On the other hand, if it exceeds 0.025%, the effect is saturated. Excessive addition of N causes the steel to be embrittled, causing cracks and flaws during casting and rolling. For the above reasons, the content needs to be within the range of 0.010 to 0.025%. A preferred range is from 0.013 to 0.020%.

Bi: 0.0001 to 0.0100%
Bi is an important element in the present invention. By containing a trace amount of Bi, the sulfide is finely dispersed with the refinement of the solidification structure of steel. Further, by adding a trace amount of Bi, it is possible to suppress the growth and coarsening of a precipitate such as AlN during carburizing, which suppresses coarsening of crystal grains. In order to obtain the effect of refining Mn sulfide, the Bi content needs to be 0.0001% or more. However, if the Bi content exceeds 0.0100%, the effect of refining the dendrite structure is saturated, and the hot workability of the steel is deteriorated, so that hot rolling becomes difficult. For these reasons, in the present invention, the Bi content is set to 0.0001 to 0.0100%. In order to further improve the machinability, the content of Bi is preferably set to 0.0010% or more.

P: 0.05% or less Phosphorus (P) is an impurity. P lowers cold forgeability and hot workability of steel. Therefore, it is preferable that the content of P is small. The content of P is 0.05% or less. The preferred P content is 0.035% or less, and more preferably 0.020% or less.

O (oxygen): 0.0005 to 0.0040%
O combines with Al to form hard oxide-based inclusions. When a large amount of oxide-based inclusions is present in steel, it becomes a precipitation site for AlN and Nb (CN), and AlN and Nb (CN) precipitate coarsely during hot rolling and suppress coarsening of crystal grains during carburizing. become unable. Therefore, it is desirable to reduce the O content as much as possible. For the above reasons, it is necessary to limit the content to 0.0040% or less. In bearing components and rolling components, oxide inclusions are the starting point of rolling fatigue fracture, so that the lower the O content, the longer the rolling life. Therefore, in the bearing parts and the rolling parts, the content of O is set to 0. It is desirable to limit it to 0012% or less.

[About selected elements]
The hot-rolled rod or rod according to the present embodiment may further contain one or more selected from the group consisting of Nb, Mo, Ni, Ti, Cu, V, B and Mg. Nb, Mo, Ni, Ti, Cu, V, B and Mg all increase the fatigue strength of steel.

Nb: 0.05% or less under <br/> niobium (Nb), when the carburizing heating, C in steel, combines with N to form a Nb (CN), grain refinement, and grain Is an element that is effective in suppressing coarsening. If the Nb content exceeds 0.05%, the hardness of the material increases, the workability is deteriorated, and the precipitate of Nb (CN) becomes coarse, which contributes to the suppression of coarsening of crystal grains. Disappears. For the above reasons, the content must be within the range of 0.05% or less. The preferred range is 0.04% or less. The lower limit of the Nb content is preferably 0.001% or more.

Mo: 1.0% or less Molybdenum (Mo) enhances the hardenability of steel and increases the fatigue strength of steel. Mo suppresses the incompletely quenched layer in the carburizing treatment. The above effect can be obtained if Mo is contained even a little. On the other hand, if the Mo content is too large, the machinability of the steel decreases. In addition, the cost of producing the steel also increases. Therefore, the content of Mo is 1.0% or less. When the content of Mo is 0.02% or more, the above-described effect is remarkably obtained. The preferred Mo content is 0.05 to 0.50%, and more preferably 0.10 to 0.30%.

Ni: 1.0% or less Nickel (Ni) has an effect of improving the hardenability and is an element effective for further increasing the fatigue strength. Therefore, nickel (Ni) may be contained as necessary. However, when the content of Ni exceeds 1.0%, not only the effect of increasing the fatigue strength due to the improvement in hardenability is saturated, but also the deformation resistance is increased and the cold forgeability is significantly reduced. Therefore, the content of Ni when contained is set to 1.0% or less. When it is contained, the amount of Ni is preferably 0.8% or less. Furthermore, in order to stably obtain the effect of enhancing the fatigue strength by improving the hardenability of Ni, the amount of Ni when it is contained is preferably 0.1% or more.

Ti: 0.01% or less Titanium (Ti) combines with N in steel to form TiN. The precipitate of TiN is coarse and does not contribute to the refinement of the crystal grains during carburization and the suppression of the coarsening of the crystal grains. Rather, if TiN is present, it becomes a precipitation site for AlN and Nb (CN), and AlN and Nb (CN) precipitate coarsely during hot rolling, making it impossible to suppress the coarsening of crystal grains during carburizing. Therefore, it is desirable to reduce the amount of Ti as much as possible. For the above reasons, it is necessary to limit the content of Ti to 0.01% or less.

Cu: 0.40% or less Copper (Cu) is an element that enhances the hardenability of steel. Since the addition of a large amount causes deterioration of the surface properties of the steel material and an increase in the alloy cost, the upper limit is set to 0.40%.

V: 0.30% or less Vanadium (V) forms carbides in the steel and increases the fatigue strength of the steel. Vanadium carbide precipitates in ferrite and increases the strength of the steel core (parts other than the surface layer). The above effect can be obtained if V is contained even a little. On the other hand, if the content of V is too large, the cold forgeability and the fatigue strength of the steel decrease. Therefore, the content of V is 0.30% or less. When the content of V is 0.03% or more, the above-described effects are remarkably obtained. The preferred V content is 0.04 to 0.20%, and more preferably 0.05 to 0.10%.

B: 0.02% or less Boron (B) enhances the hardenability of steel and increases the fatigue strength of steel. The above effect can be obtained if B is contained even a little. When the content of B exceeds 0.02%, the effect is saturated. Therefore, the content of B is 0.02% or less. When the content of B is 0.0005% or more, the above-described effects are remarkably obtained. The preferable content of B is 0.001 to 0.012%, and more preferably 0.0020 to 0.010%.

Mg: 0.0035% or less Magnesium (Mg) deoxidizes steel and refines oxides in steel, like Al. As the oxides in the steel become finer, the probability of starting from a coarse oxide as a fracture starting point decreases, and the fatigue strength of the steel increases. The above effect can be obtained if Mg is contained even a little. On the other hand, if the content of Mg is too large, the above effect is saturated and the machinability of the steel is reduced. Therefore, the content of Mg is 0.0035% or less. When the content of Mg is 0.0001% or more, the above-described effects are remarkably obtained. The preferred Mg content is 0.0003 to 0.0030%, and more preferably 0.0005 to 0.0025%.

  As described above, the hot-rolled rod or wire of the present embodiment contains the basic element described above, and is selected from the chemical composition including the balance of Fe and unavoidable impurities, or the basic element described above and the selected element described above. And at least one of them, and has a chemical composition consisting of the balance of Fe and inevitable impurities.

[Dendrite organization]
The solidification structure of the continuously cast slab usually has a dendrite shape. MnS in steel is often crystallized before solidification (in molten steel) or during solidification, and is greatly affected by the dendrite primary arm spacing. That is, if the interval between the primary arms of the dendrite is small, the sulfide crystallized between the trees becomes small. In the hot-rolled rod or wire according to the present embodiment, it is desirable that the dendrite primary arm interval in the slab stage is less than 600 μm.

[MnS]
Since MnS is useful for improving machinability, it is necessary to ensure its number density. The machinability improves as the S content increases. When MnS having an equivalent circle diameter of 1 μm or more and less than 2 μm is present in steel at a density of 300 pieces / mm ² or more, tool wear is suppressed. In addition, whether the inclusions are MnS may be confirmed by energy dispersive X-ray analysis attached to a scanning electron microscope. The equivalent circle diameter of MnS is the diameter of a circle having an area equal to the area of MnS, and can be determined by image analysis. Similarly, the number density of MnS is determined by image analysis. In addition, the fact that the inclusion is a sulfide may be confirmed by energy dispersive X-ray analysis attached to a scanning electron microscope.

  In the present invention, a large steel part made of the hot-rolled rod or wire of the present invention is reduced in incompletely quenched structure after carburizing treatment, and has an excellent core hardness by increasing the martensite area ratio. Therefore, it is important to provide high hardenability. It is necessary that the contents of Mn and S in the above-mentioned chemical components expressed by mass% satisfy the following formula (1).

  [Mn] ≧ 8.4 × [S] +0.9 (1)

  As described above, Mn combines with S in steel to form MnS and enhances the machinability of steel. That is, in order to further enhance the hardenability than the conventional steel, it is necessary to satisfy the formula (1) as a supplement of Mn consumed as MnS. If the formula (1) is not satisfied, Mn required for hardenability is insufficient, and the hardness of the core after carburizing cannot be improved.

[Production method]
A method for manufacturing a hot-rolled rod or wire according to an embodiment of the present invention will be described. The method for producing a hot-rolled rod or wire according to the present embodiment is a method for continuously casting a slab having the above chemical components and having a primary dendrite arm spacing of less than 600 μm within a range of 15 mm from the surface layer. Is manufactured by hot working and further annealing. Hot working may include hot rolling.

[Continuous casting process]
A steel slab that satisfies the above chemical composition and formula (1) is manufactured by a continuous casting method. An ingot (steel ingot) may be formed by the ingot making method. As the casting conditions, for example, using a 220 × 220 mm square mold, the superheat of molten steel in the tundish is set to 10 to 50 ° C., and the casting speed is set to 1.0 to 1.5 m / min.

  Further, when casting molten steel having the above chemical composition in order to make the above-mentioned dendrite primary arm interval less than 600 μm, the temperature range from the liquidus temperature to the solidus temperature at a depth of 15 mm from the slab surface when casting molten steel. Is desirably set to an average cooling rate of 100 ° C./min or more and 500 ° C./min or less. If the average cooling rate is less than 100 ° C./min, it is difficult to make the primary dendrite arm spacing at a depth of 15 mm from the slab surface less than 600 μm, and there is a possibility that MnS cannot be finely dispersed. On the other hand, if it exceeds 500 ° C./min, MnS crystallized from between dendrite trees becomes too fine, and there is a possibility that machinability may be reduced.

  The temperature range from the liquidus temperature to the solidus temperature is the temperature range from the start of solidification to the end of solidification. Therefore, the average cooling temperature in this temperature range means the average solidification rate of the slab. The above average cooling rate can be achieved by, for example, controlling the size of the mold section, the casting speed, and the like to appropriate values, or increasing the amount of cooling water used for water cooling immediately after casting. This is applicable to both the continuous casting method and the ingot making method.

The cooling rate at a depth of 15 mm is obtained by etching the cross section of the obtained slab with picric acid, and at each of positions at a depth of 15 mm from the slab surface, the dendrite secondary arm interval λ at a pitch of 5 mm in the casting direction. ₂ (μm) was measured at 100 points, and the cooling rate A (° C./sec) in the temperature range from the liquidus temperature to the solidus temperature of the slab was calculated from the value based on the following equation, and arithmetically averaged. Average.

λ ₂ = 710 × A− ^0.39

  For example, a plurality of slabs with different casting conditions are manufactured, the cooling rate of each slab is determined by the above equation, and the optimum casting condition may be determined from the obtained cooling rate.

[Hot rolling]
Next, the cast slab or ingot is hot-rolled such as slab rolling to produce a billet (steel slab). Further, the billet is hot-rolled to obtain a hot-rolled steel bar or wire rod or wire rod of the present embodiment. The reduction ratio in hot working is not particularly limited.

  In the hot rolling, for example, after a billet is heated at a heating temperature of 1150 to 1300 ° C for 1.5 hours or more, the hot rolling is performed at a finishing temperature of 900 to 1100 ° C. After the finish rolling, cooling is performed in the atmosphere at a cooling rate of 0.1 to 1.0 ° C./sec at a cooling temperature of 800 to 500 ° C. Note that a preferable range is 0.7 ° C./sec or less. After the finish rolling, it may be cooled to room temperature under the condition that the cooling rate is equal to or lower than the above-mentioned cooling, but in order to increase the productivity, at the time when the temperature reaches 500 ° C., air cooling is performed. It is preferable to cool by appropriate means such as mist cooling and water cooling. In addition, the above-mentioned heating temperature and heating time mean the average temperature in a furnace, and furnace time, respectively. The finishing temperature of the hot rolling means the surface temperature of the rod or wire at the exit of the last stand of the rolling mill having a plurality of stands. The cooling rate after the finish rolling refers to the cooling rate on the surface of the rod or rod.

  Further, when the hot-rolled bar or wire is processed into a machine part or the like, for example, the manufactured bar or wire (hot-rolled bar or wire) is hot-forged to produce a rough intermediate product. The refining process may be performed on the intermediate product. Further, the intermediate product is machined to form the intermediate product into a predetermined shape. The machining is, for example, cutting or drilling. Thereafter, a surface hardening treatment such as a carburizing treatment or a carbonitriding treatment is performed.

  By cutting the intermediate product after the surface hardening treatment into a predetermined shape by machining, a mechanical component made of a hot-rolled rod or wire is manufactured.

  Steels A to AB having the chemical compositions shown in Table 1 were melted in a 270-ton converter, and were continuously cast using a continuous casting machine to produce slabs of 220 × 220 mm square. The reduction was applied during the solidification of the continuous casting.

  In continuous casting of a slab, the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a depth of 15 mm from the surface of the slab is changed by changing the cooling water amount of the mold. Was.

  Steels A to O shown in Table 1 are steels having a chemical composition specified in the present invention. Steels P to Z are comparative steels whose chemical compositions deviate from the conditions specified in the present invention. The underline of the numerical value in Table 1 indicates that it is out of the range of the hot-rolled rod or wire according to the present embodiment.

  Using the slab obtained by continuous casting as a raw material, hot forging was performed, and a trial production of a steel bar was performed. In this example, the cast piece was once cooled to room temperature in order to collect a test piece for dendrite structure observation.

  Thereafter, the slab of each mark was heated at 1250-1300 ° C. for 2 hours. The hot slab is subjected to hot rolling at a final processing temperature of 900 to 1100 ° C., and then cooled at a cooling rate of 0.7 ° C./sec or less at a temperature of 800 to 500 ° C. At the stage when the temperature reached 500 ° C., a plurality of round bars having a diameter of 55 mm were manufactured. The presence or absence of surface cracks in each steel material after the heat treatment was visually determined. In this way, hot-rolled rod wires of Test Nos. 1 to 28 were manufactured.

[Coagulation structure observation method]
For the solidified structure, the cross section of the above slab was etched with picric acid, and the dendrite primary arm interval was measured at a point of 15 mm in the depth direction from the slab surface at a pitch of 5 mm in the pouring direction, and 100 points of the dendrite primary arm interval were measured. Was.

[Microstructure observation method]
The microstructure of the round bar was observed. The D / 4 position of the round bar was cut parallel to the axial direction, and a test piece for microstructure observation was collected. The cut surface of the test piece was polished and corroded with a nital etchant. After the corrosion, the microstructure at the center of the cut surface was observed with a 400 × optical microscope. The microstructure of the round bar of each mark was a ferrite / pearlite structure.

  Further, a Vickers hardness test specified in JIS Z2244 was performed using the test piece for microstructure observation. As a result of measuring the hardness at five points, the Vickers hardness of the round bar of each mark was in the range of Hv130 to 200, and the round bar of each mark had the same hardness.

[MnS observation method]
The metal structure of the steel was observed with an optical microscope, and MnS was determined from the contrast in the structure. The inclusions were identified using a scanning electron microscope and an energy dispersive X-ray spectrometer (EDS). The specimen for MnS observation was cut in parallel to the axial direction at the D / 4 position of the round bar, which is the same position as the specimen for micro observation, and the cross section including the longitudinal direction of the specimen was 10 mm long x 10 mm wide. Were prepared, and a predetermined position of each of the polished test pieces was photographed at a magnification of 100 with an optical microscope to prepare an image of an inspection reference area (region) of 0.9 mm ² for 10 visual fields. . The MnS observation visual field in the present invention is 9 mm ² . The particle size distribution of MnS having an equivalent circle diameter of 1 μm or more in the observation visual field (image) was detected. These dimensions (diameters) were converted to equivalent circle diameters indicating the diameter of a circle having the same area as the area of MnS. The number density of MnS was calculated from the detected MnS particle size distribution.

[Machinability test]
The machinability of the round bar of each steel number was evaluated.

  Specifically, the outer peripheral portion of a test material (round bar) having a diameter of 55 mm and a length of 500 mm was turned using an NC lathe under the following conditions, and the machinability was investigated.

  The chip controllability was evaluated by the following method. Chips discharged during 10 seconds during the machinability test were collected. The length of the collected chips was examined, and ten chips were selected in order from the longest one. The total weight of the selected 10 chips was defined as "chip weight". If the total number of chips was less than 10 as a result of long chips, the total weight of the collected chips was measured, and the value converted to the number of chips was defined as "chip weight". For example, when the total number of chips is 7 and the total weight is 12 g, the chip weight was calculated as 12 g × 10/7.

  If the chip weight of each mark was 15 g or less, it was determined that the chip disposability was high. When the chip weight exceeded 15 g, the chip disposition was evaluated as low.

<Used chip>
Base material: Carbide P20 grade
Coating: None.
<Turning conditions>
Circumferential speed: 250m / min,
Feed: 0.35 mm / rev,
Cut: 1.0 mm,
Lubrication: Uses water-soluble cutting oil.

[Coarse grain generation temperature evaluation test]
Carburizing simulation was performed on the intermediate product after machining. The conditions of the carburizing simulation are heating to 910 to 1090 ° C. for 5 hours and water cooling. Thereafter, the cut surface was polished and corroded, and the old austenite grain size was observed to determine the coarse grain generation temperature (crystal grain coarsening temperature). Since high-temperature carburization is usually performed at 1000 to 1050 ° C., those having a coarse grain generation temperature of 1000 ° C. or less were judged to be inferior in crystal grain coarsening characteristics. The measurement of the prior austenite grain size was carried out in accordance with JIS G 0051. Observation was performed at about 400 times at about 10 visual fields. If at least one coarse grain having a grain size number of 5 or less was present, it was determined that coarse grains were generated.

[Fatigue property evaluation]
Fatigue property evaluation was performed using the remaining material of the round bar having a diameter of 55 mm after hot working. Specifically, a four-point bending fatigue test piece of a square bar of 13 mm × 13 mm × 100 mm was sampled and carburized and quenched. In this carburizing and quenching, the test piece was heated to 900 ° C. in an atmosphere having a carbon potential of 0.8%, held for 5 hours, and quenched into an oil having a temperature of 130 ° C. Further, the test piece was kept at 150 ° C. for 2 hours and tempered. Further, a four-point bending fatigue test was performed using a hydraulic servo fatigue tester. The core hardness of the four-point bending fatigue test piece was examined using a Vickers hardness tester.

  The 500-time breaking strength of Test No. 16 was defined as 1, and evaluated by the relative value to the 500-time breaking strength of each test number. When the relative value was 1.2 or more, it was determined that the low cycle fatigue characteristics were excellent.

  Referring to Tables 1 and 2, the chemical composition of steels A to O is within the range of the chemical composition of the hot-rolled rod and wire according to the present embodiment, and has satisfied the number density of precipitates and MnS. . As a result, Steels A to O had excellent crystal grain coarsening suppression properties and machinability.

  Steel P is a steel specified in JIS SCr420 and did not contain Bi. Further, the number density of MnS defined in the present invention was not satisfied. For this reason, the machinability was low. Specifically, the chip weight exceeded 15 g. Furthermore, during the carburizing simulation, the coarsening of the precipitate occurred, and the coarsening suppressing property was inferior.

  Steel Q is an example in which the N content is below the range specified in the present invention. For this reason, the coarsening suppression characteristics were poor. Specifically, the number of fine precipitates was small, and the coarsening suppression characteristics were poor.

  Steel R is an example in which the content of N exceeds the range specified in the present invention. For this reason, coarse precipitates were present and fine dispersion of the precipitates was hindered, resulting in poor coarsening suppression characteristics.

  Steel S is an example in which the content of Al is below the range specified in the present invention. For this reason, the coarsening suppression characteristics were poor. Specifically, the number of fine precipitates was small, and the coarsening suppression characteristics were poor.

  Steel T is an example in which the content of Al exceeds the range specified in the present invention. For this reason, coarse precipitates were present and fine dispersion of the precipitates was hindered, resulting in poor coarsening suppression characteristics.

  Steel U is an example in which the content of Ti exceeds the range specified in the present invention. For this reason, coarse precipitates were present and fine dispersion of the precipitates was hindered, resulting in poor coarsening suppression characteristics.

  Steel V is an example in which the O content exceeds the range specified in the present invention. For this reason, coarse precipitates were present and fine dispersion of the precipitates was hindered, resulting in poor coarsening suppression characteristics.

  Steel W is an example in which Bi was contained but the S content was below the range specified in the present invention. Therefore, the number density of MnS defined in the present invention was not satisfied. Therefore, the machinability was poor.

  Steel Y is an example in which the content of Nb exceeds the range specified in the present invention. Since coarse Nb (CN) was present and the fine dispersion of AlN and Nb (CN) was hindered, the coarsening suppression characteristics were poor.

  Steel Z is an example in which the content of Bi exceeds the range specified in the present invention. For this reason, the hot workability was reduced and cracks occurred.

  Steel AA is an example in which the content of Mn is below the range specified in the present invention. For this reason, despite the addition of Bi, coarse MnS was present, and the low cycle fatigue life was inferior.

  The embodiment of the present invention has been described above, but the above-described embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

Claims (2)

In mass%,
C: 0.13 to 0.40%,
Si: 0.01 to 1.5%,
Mn: 1.0 to 2.0%,
S: 0.015 to less than 0.040%,
Cr: 0.01 to 1.6%,
Al: 0.010-0.045% and N: 0.010-0.025%,
Bi: 0.0001 to 0.0100%
And the balance consists of Fe and impurities,
Further, P and O are
P: 0.05% or less and O: 0.0005 to 0.0040%,
The contents of Mn and S satisfy the following formula (1),
A hot-rolled rod or wire having a sulfide having an equivalent circle diameter of 1 μm or more and less than 2 μm in a cross section parallel to a rolling direction of a steel material having an existing density of 300 pieces / mm ² or more.
[Mn] ≧ 8.4 × [S] +0.9 (1) Furthermore, in mass%,
Nb: 0.05% or less under,
Mo: 1.0% or less,
Ni: 1.0% or less,
Ti: 0.01% or less,
Cu: 0.40% or less,
V: 0.30% or less,
2. The hot-rolled rod or wire according to claim 1, comprising one or more selected from the group consisting of B: 0.02% or less and Mg: 0.0035% or less.