JP5397247B2 - Hot rolled steel bar or wire rod - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a hot-rolled steel bar or wire, and more specifically, is a hot-rolled steel bar or wire having excellent cold forgeability after spheroidizing annealing and excellent austenite grain coarsening prevention characteristics during carburizing or carbonitriding. About.

  Parts such as gears, pulleys and shafts of automobiles and industrial machines are often roughly formed by hot forging or cold forging.

  Cold forging has higher dimensional accuracy than hot forging. For this reason, there is a merit that the amount of cutting after forging can be reduced, and in recent years, there are an increasing number of parts that are roughly formed by cold forging.

  When rough forming is performed by cold forging, spheroidizing annealing is often performed in order to reduce deformation resistance in forging and to improve deformability. However, even when spheroidizing annealing is performed, there are still problems that deformation resistance is high and cracking is likely to occur.

  Furthermore, after cold forging, cutting is performed, and then the surface is hardened by carburizing quenching or carbonitriding quenching, but if the austenite grains before quenching are coarsened, the fatigue strength as a part may be reduced. Problems such as large deformation during quenching are likely to occur.

  Therefore, there is a need for a hot-rolled steel bar or wire that can stably suppress the coarsening of austenite grains while considering deformation resistance and deformability in cold forging. Proposed.

  In Patent Document 1, in steel wire rods and steel bars containing 0.2 to 0.6% of C, the pro-eutectoid ferrite fraction is 5 to 30% by area, and the balance is composed mainly of bainite, and The average value of the lath spacing of cementite in the bainite is 0.3 μm or more, and it is possible to achieve improvement in deformability and reduction in deformation resistance after spheroidizing treatment, and excellent cold forgeability. A “steel wire rod / bar having excellent cold forgeability after spheroidization and a method for producing the same” is disclosed.

  Patent Document 2 has a mixed structure containing ferrite, bainite, and pearlite, and by defining the area fraction of bainite to be 30% or more, carbide refinement when spheroidizing annealing is achieved and high deformability is achieved. It has disclosed “a steel wire rod and steel bar for case hardening excellent in cold forgeability after spheroidization”.

  In Patent Document 3, by regulating the precipitation amount of Nb (CN) and AlN after hot rolling and the bainite structure fraction to 30% or less, the generation of coarse grains in the high-temperature carburizing and quenching process and the outermost layer hardness are reduced. "High-temperature carburizing steel excellent in high-temperature carburizing characteristics and hot forged members for high-temperature carburizing" that can prevent the decrease in thickness and obtain the desired hardened layer depth are disclosed.

JP 2001-89830 A JP 2005-220377 A JP 2001-89830 A

  The technique proposed in Patent Document 1 described above is insufficient in reducing deformation resistance because the pro-eutectoid ferrite fraction is as low as 5 to 30%. Furthermore, no measures are taken against austenite coarsening.

  Since the technique proposed in Patent Document 2 only considers the area fraction of bainite for the structure, measures for reducing deformation resistance are insufficient. Furthermore, no consideration is given to the austenite grain coarsening.

  The technique proposed in Patent Document 3 does not consider deformation resistance and deformability during cold forging.

  The present invention has been made in view of the above situation, and its purpose is to realize excellent cold forgeability after spheroidizing annealing, that is, low deformation resistance and high deformability, and further, after cold forging. An object of the present invention is to provide a hot-rolled steel bar or wire that can stably prevent coarsening of austenite grains during carburizing or carbonitriding.

  In order to achieve the above object, the present inventors first conducted various investigations on the influence of the metal structure on the deformability and the deformation resistance when cold forging after spheroidizing annealing on a hot rolled steel bar or wire. Repeated research. As a result, the following findings (a) to (d) were obtained.

  (A) When the area ratio of bainite increases, the deformation resistance after spheroidizing annealing increases, but the deformability often increases.

  (B) When the area ratio of ferrite increases, the deformation resistance after spheroidizing annealing often decreases.

  (C) When the ferrite grain size is decreased, the deformation resistance after spheroidizing annealing is often increased.

  (D) When the ferrite grain size increases, the deformability after spheroidizing annealing often decreases.

From the above (a) to (d), the present inventors
(E) At the time of cold forging after spheroidizing annealing, in order to achieve both high deformability and low deformation resistance at a high level, the metal structure should satisfy all of the following <1> to <3>. The conclusion has been reached.

<1> The area ratio of the ferrite-bainite structure, which is a mixed structure of ferrite and bainite, is a specific value or more.
<2> The area ratio of bainite is not less than a specific value, and <3> the ferrite grain size is in a specific range.

  Next, the present inventors conducted various investigations and studies on the influence of precipitates on the coarsening of austenite grains in the carburizing or carbonitriding process for hot rolled steel bars or wires. As a result, the following findings (f) to (i) were obtained. In the following description, “carburization or carbonitriding” may be simply referred to as “carburization”.

  (F) Austenite grains are less likely to be coarsened during carburization when the amount of precipitation of AlN, Nb (CN) and AlN-Nb (CN) is smaller at the stage of hot rolled steel bars or wires that are hot rolled materials.

  (G) Coarse AlN, Nb (CN), and AlN-Nb (CN) are often produced in the slab after continuous casting with a large cross section, which is a general mass production process. When remaining in the hot-rolled material, even if the precipitation amount of AlN, Nb (CN) and AlN-Nb (CN) is small, austenite grains are likely to become coarse during carburizing heating.

  (H) In heating the slab and the steel slab obtained by performing the ingot rolling on the slab, since the temperature is increased from the surface side, it takes a long time for the temperature of the central portion to be equal to the surface. Therefore, in general, the amount of precipitation of AlN, Nb (CN) and AlN-Nb (CN), and coarse AlN, Nb (CN) and AlN-Nb in the center portion of the hot rolled material as compared with the surface layer portion. Since (CN) increases, austenite grain coarsening at the time of carburizing cannot always be prevented stably.

  (I) AlN, Nb (CN), and AlN-Nb (CN) precipitation amounts are generally determined by analyzing residues extracted electrolytically from the vicinity of the surface layer, and therefore, general extraction residue analysis The precipitation amount of AlN, Nb (CN) and AlN-Nb (CN) obtained by the above does not serve as an index for preventing austenite grain coarsening during carburizing heating near the center. In order to achieve prevention of coarsening of austenite grains at the time of carburizing heating in the vicinity of the center part, the precipitation amounts of AlN, Nb (CN) and AlN-Nb (CN) in the vicinity of the center part also need to be set to a predetermined amount or less.

  This invention is completed based on said knowledge, The summary exists in the hot-rolled steel bar or wire shown to following (1)-(3).

(1) hot rolled steel bar or wire,
% By mass
C: 0.1 to 0.3%
Si: 0.05 to 1.0%,
Mn: 0.4 to 2.0%,
S: 0.005 to 0.05%,
Cr: 0.5 to 2.0%,
Al: 0.01 to 0.06%,
N: 0.005-0.025% and Nb: 0.02-0.08%
And the balance consists of Fe and impurities,
P, Ti and O (oxygen) in the impurities are respectively
P: 0.025% or less,
Having a chemical composition of Ti: 0.003% or less and O (oxygen): 0.002% or less;
In the region from the surface of the steel bar or wire to 1/5 of the radius and the region from the center to 1/5 of the radius, the Al amount precipitated as AlN and AlN-Nb (CN) is 0.010% or less, Nb amount precipitated as Nb (CN) and AlN—Nb (CN) is 0.020% or less, and the total of AlN, Nb (CN) and AlN—Nb (CN) having a diameter of 100 nm or more The number density is 50/100 μm ² or less,
The area ratio of the ferrite bainite structure is 80% or more, the area ratio of bainite is 30 to 70%, and the ferrite average particle diameter is 15 to 40 μm.
Hot-rolled steel bar or wire rod characterized by that.

(2) Instead of a part of Fe, in mass%,
Cu: 0.5% or less,
Containing at least one selected from Ni: 1.5% or less and Mo: 0.8% or less,
The hot-rolled steel bar or wire described in (1) above.

(3) Instead of part of Fe, in mass%,
Pb: 0.3% or less,
Te: 0.08% or less,
Containing at least one selected from Ca: 0.01% or less and Bi: 0.3% or less,
The hot-rolled steel bar or wire according to (1) or (2) above.

  The “impurities” in the remaining “Fe and impurities” refer to those mixed from ore, scrap, or the environment as raw materials when industrially producing steel materials.

  “AlN—Nb (CN)” refers to a composite precipitate of AlN and Nb (CN).

  The “diameter” of AlN, Nb (CN), and AlN—Nb (CN) is an AlN, Nb (CN) when an extraction replica sample is prepared by a general method and observed using a transmission electron microscope. And the arithmetic average of the major axis and minor axis of each of AlN-Nb (CN).

  The “ferrite / bainite structure” refers to a mixed structure of ferrite and bainite.

  “Ferrite average particle diameter” is defined as follows. That is, in the cross section of a hot-rolled steel bar or wire, first, the area of each ferrite grain is obtained, the diameter of a circle that is equivalent to the area is obtained, and this is used as the apparent grain diameter of each ferrite grain. Next, the average value of the apparent particle diameters of all the ferrite grains whose areas were measured is defined as the apparent ferrite average particle diameter, and the average ferrite particle diameter obtained by multiplying the apparent ferrite average particle diameter by 1.12 is defined as the ferrite average particle diameter.

  The method for observing the ferrite grains is not particularly limited, but after cutting a section perpendicular to the longitudinal direction of the hot-rolled steel bar or wire and including the central portion, the specimen was mirror-polished and corroded with nital. Is preferably 400 times magnification, the field size is 250 μm × 250 μm, each of 10 fields is randomly observed, and the average ferrite particle diameter is determined from the photograph by the method described above.

  The hot-rolled steel bar or wire of the present invention is excellent in cold forgeability after spheroidizing annealing, and can stably prevent coarsening of austenite grains during carburizing or carbonitriding after cold forging. It can be suitably used as a raw material for parts such as gears, pulleys, and shafts that are roughly formed by cold forging.

It is a figure explaining the shape of the test piece cut out in order to investigate cold forgeability in an Example.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) Chemical composition C: 0.1 to 0.3%
C is an essential element for securing the core strength of the parts when carburized and quenched or carbonitrided. If the content is less than 0.1%, the above effect is insufficient. On the other hand, if the C content exceeds 0.3%, the machinability after cold forging is significantly reduced. Therefore, the content of C is set to 0.1 to 0.3%. The C content is preferably 0.18% or more and 0.25% or less.

Si: 0.05-1.0%
Si is an element that has a large effect of improving hardenability and temper softening resistance, and has an effect of improving fatigue strength. However, if the content is less than 0.05%, the above effects are insufficient. On the other hand, when the Si content exceeds 1.0%, not only the effect of increasing the fatigue strength is saturated, but also the machinability after cold forging is significantly reduced. Therefore, the Si content is set to 0.05 to 1.0%. When the Si content is 0.4% or more, the effect of improving the fatigue strength becomes remarkable. Therefore, the Si content is preferably 0.4% or more. Note that the Si content is preferably 0.8% or less.

Mn: 0.4 to 2.0%
Mn is an element that has a large effect of improving hardenability and temper softening resistance and has an effect of improving fatigue strength. However, if the content is less than 0.4%, the above effect is insufficient. On the other hand, when the content of Mn exceeds 2.0%, not only the effect of increasing the fatigue strength is saturated, but also the machinability after cold forging becomes remarkable. Therefore, the Mn content is set to 0.4 to 2.0%. In addition, it is preferable that content of Mn is 0.8% or more and 1.2% or less.

S: 0.005-0.05%
S combines with Mn to form MnS and improves machinability. However, if the content is less than 0.005%, it is difficult to obtain the above effect. On the other hand, when the S content increases, coarse MnS tends to be generated, and the fatigue strength tends to decrease. When the S content exceeds 0.05%, the fatigue strength decreases remarkably. Therefore, the content of S is set to 0.005 to 0.05%. In addition, it is preferable that content of S is 0.01% or more and 0.03% or less.

Cr: 0.5 to 2.0%
Cr is an element that has a large effect of improving hardenability and temper softening resistance and has an effect of improving fatigue strength. However, if the content is less than 0.5%, the above effect is insufficient. On the other hand, when the content of Cr exceeds 2.0%, not only the effect of increasing the fatigue strength is saturated, but also the machinability after cold forging becomes remarkable. Therefore, the Cr content is set to 0.5 to 2.0%. When the Cr content is 1.3% or more, the fatigue strength is significantly improved. Therefore, the Cr content is preferably 1.3% or more. Note that the Cr content is preferably 1.8% or less.

Al: 0.01 to 0.06%
Al is an element that has a deoxidizing action and at the same time easily binds to N to form AlN and has an effect of preventing coarsening of austenite grains during carburizing heating. Moreover, Al has the effect | action which suppresses the dynamic strain aging of cold forging and aims at reduction of a deformation resistance. However, when the Al content is less than 0.01%, even if the other requirements are satisfied, the target property of preventing coarsening of austenite grains and the effect of reducing deformation resistance cannot be obtained. On the other hand, when the Al content exceeds 0.06%, even if other requirements are satisfied, the target austenite grain coarsening preventing effect is not obtained and the toughness is reduced. Therefore, the Al content is set to 0.01 to 0.06%. Note that the Al content is preferably 0.02% or more and 0.04% or less.

N: 0.005 to 0.025%
N is an element that easily forms AlN, NbN, and TiN by combining with Al, Nb, and Ti. In the present invention, among these nitrides, AlN, Nb (CN) and NbN have an effect of preventing austenite grain coarsening during carburizing heating. However, when the content of N is less than 0.005%, even if other requirements are satisfied, the effect of preventing the coarsening of austenite grains targeted in the present invention cannot be obtained. On the other hand, if the N content exceeds 0.025%, stable mass production in the steelmaking process becomes difficult. Therefore, the N content is set to 0.005 to 0.025%. The N content is preferably 0.013% or more and 0.020% or less.

Nb: 0.02 to 0.08%
Nb is an element that is easily bonded to C and N to form NbC, NbN, and Nb (CN) and has an effect of preventing austenite grain coarsening during carburizing heating. However, if the content is less than 0.02%, the above effect is insufficient. On the other hand, if the Nb content exceeds 0.08%, the effect of preventing austenite grain coarsening is rather lowered. Therefore, the Nb content is set to 0.02 to 0.08%. The Nb content is preferably 0.02% or more and 0.04% or less.

  One of the chemical compositions of the hot-rolled steel bar or wire of the present invention is that, in addition to the above elements, the balance consists of Fe and impurities, and P, Ti, and O (oxygen) in the impurities are each P: 0.025% Hereinafter, Ti: 0.003% or less and O: 0.002% or less.

  Hereinafter, P, Ti, and O in the impurities will be described.

P: 0.025% or less P is an element that easily segregates at the grain boundary and easily embrittles the grain boundary. When it exceeds 0.025%, the fatigue strength is reduced. Therefore, the content of P in the impurities is set to 0.025% or less. In addition, it is preferable that content of P in an impurity shall be 0.015% or less.

Ti: 0.003% or less Ti combines with N to easily form hard and coarse TiN, and reduces fatigue strength. In particular, when the Ti content exceeds 0.003%, the fatigue strength is significantly reduced. Therefore, the Ti content in the impurities is set to 0.003% or less. In addition, although it is desirable to reduce Ti content as an impurity element as much as possible, considering the cost in the steel making process, it is preferable to make it 0.002% or less.

O (oxygen): 0.002% or less O is liable to form hard oxide inclusions by combining with Al and lower fatigue strength. In particular, when the O content exceeds 0.002%, the fatigue strength is significantly reduced. Therefore, the O content in the impurities is set to 0.002% or less. In addition, although it is desirable to reduce O content as an impurity element as much as possible, considering the cost in a steelmaking process, it is preferable to make it 0.001% or less.

  Another one of the chemical compositions of the hot-rolled steel bar or wire of the present invention contains one or more elements of Cu, Ni, Mo, Pb, Te, Ca and Bi instead of part of Fe. To do. Hereinafter, the effect of these arbitrary elements and the reason for limiting the content will be described.

  Cu, Ni, and Mo all have an effect of improving hardenability. For this reason, when it is desired to obtain greater hardenability, these elements may be contained. Hereinafter, Cu, Ni, and Mo will be described.

Cu: 0.5% or less Cu has an effect of improving hardenability and is an effective element for increasing the fatigue strength. Therefore, Cu may be contained as necessary. However, when the Cu content exceeds 0.5%, the hot ductility is lowered, and the hot workability is significantly lowered. Therefore, when Cu is included, the content of Cu is set to 0.5% or less. In addition, it is preferable that content of Cu in the case of making it contain is 0.3% or less.

  On the other hand, in order to reliably obtain the effect of increasing the fatigue strength by improving the hardenability of Cu described above, the content of Cu when contained is preferably 0.1% or more.

Ni: 1.5% or less Ni has an effect of improving the hardenability and is an element effective for increasing the fatigue strength. Therefore, Ni may be contained as necessary. However, when the Ni content exceeds 1.5%, the effect of increasing the fatigue strength by improving the hardenability is saturated. Further, the machinability after hot forging is significantly reduced, and the alloy cost is also increased. Therefore, the Ni content when contained is set to 1.5% or less. In addition, when Ni is contained, the content of Ni is preferably 0.8% or less.

  On the other hand, in order to surely obtain the effect of increasing the fatigue strength due to the improvement of the hardenability of Ni described above, the content of Ni when contained is preferably 0.1% or more.

Mo: 0.8% or less Mo has an effect of improving hardenability and is an effective element for increasing the fatigue strength. Therefore, Mo may be contained as necessary. However, if the Mo content exceeds 0.8%, the effect of increasing the fatigue strength is saturated. Further, the machinability after hot forging is significantly reduced, and the alloy cost is also increased. Therefore, the Mo content in the case of inclusion is set to 0.8% or less. In addition, it is preferable that content of Mo in the case of making it contain is 0.4% or less.

  On the other hand, in order to surely obtain the effect of increasing the fatigue strength by improving the hardenability of Mo described above, the Mo content in the case of inclusion is preferably 0.05% or more.

  Said Cu, Ni, and Mo can be contained only in any one of them, or 2 or more types of composites. The total content of these elements may be 2.8% or less, but is preferably 1.8% or less.

  Pb, Te, Ca, and Bi all have an effect of improving machinability. For this reason, when it is desired to obtain greater machinability, these elements may be contained. Hereinafter, Pb, Te, Ca, and Bi will be described.

Pb: 0.3% or less Pb has an effect of improving machinability. For this reason, when it is desired to further precisely cut and finish the inner surface of a part formed by cold working, it may be contained as necessary for the purpose of improving machinability. However, if the Pb content exceeds 0.3%, the fatigue characteristics are deteriorated. Therefore, the content of Pb when contained is set to 0.3% or less. In addition, when Pb is contained, the content of Pb is preferably 0.2% or less.

  On the other hand, in order to reliably obtain the effect of improving the machinability of Pb described above, the Pb content when contained is preferably 0.005% or more.

Te: 0.08% or less Te has an effect of improving machinability like Pb. For this reason, when it is desired to further precisely cut and finish the inner surface of a part formed by cold working, it may be contained as necessary for the purpose of improving machinability. However, when the Te content exceeds 0.08%, the hot workability is lowered. Therefore, the Te content when contained is set to 0.08% or less. When Te is contained, the content of Te is preferably 0.02% or less.

  On the other hand, in order to surely obtain the above-described effect of improving the machinability of Te, the Te content when contained is preferably 0.001% or more.

Ca: 0.01% or less Ca, like Pb and Te, has an effect of improving machinability. For this reason, like Pb and Te, when it is desired to further precisely finish the inner surface of a part formed by cold working, etc., it may be contained as necessary for the purpose of improving machinability. However, when the Ca content exceeds 0.01%, the hot workability is lowered. Therefore, the Ca content when contained is set to 0.01% or less. When Ca is included, the Ca content is preferably 0.004% or less.

  On the other hand, in order to reliably obtain the above-described effect of improving the machinability of Ca, the Ca content when contained is preferably 0.001% or more.

Bi: 0.3% or less Bi, like Pb, Te and Ca, has an effect of improving machinability. For this reason, like Pb, Te, and Ca, when it is desired to further precisely finish the inner surface of a part formed by cold working, it may be contained as necessary for the purpose of improving machinability. However, when the Bi content exceeds 0.3%, the hot workability is lowered. Therefore, the Bi content when contained is set to 0.01% or less. In addition, it is preferable that content of Bi in the case of making it contain is 0.2% or less.

  On the other hand, in order to reliably obtain the above-described effect of improving the machinability of Bi, the Bi content when contained is preferably 0.01% or more.

  Said Pb, Te, Ca, and Bi can be contained only in one of them, or 2 or more types of composites. The total content of these elements may be 0.69% or less, but is preferably 0.35% or less.

(B) Metal structure The metal structure at the stage of hot-rolled steel bar or wire rod in a hot-worked state is the cold forgeability when forming into a required part shape such as a gear after spheroidizing annealing, that is, deformability and Affects deformation resistance.

  For this reason, it is necessary to make the metal structure appropriate in the stage of hot rolled steel bar or wire, the area ratio of ferrite bainite structure in the metal structure is 80% or more, the area ratio of bainite is 30 to 70% and When the average ferrite particle size is 15 to 40 μm, excellent cold forgeability, that is, low deformation resistance and high deformability can be realized.

  The “phase” in the above-mentioned metal structure is, for example, a magnification of 400 with respect to a test piece that is perpendicular to the longitudinal direction of a hot-rolled steel bar or wire and includes the center, and then mirror-polished and corroded with nital. The size of the field of view can be identified by observing 10 fields at random with a field size of 250 μm × 250 μm. Moreover, image analysis by a normal method is performed for each of the above-described visual fields, and the area ratio of ferrite, the area ratio of bainite, and the ferrite average particle diameter can be obtained. The area ratio of the ferrite-bainite structure was obtained by adding the area ratio of ferrite and the area ratio of bainite.

  As described above, the method for calculating the average ferrite particle diameter is to obtain the area of each ferrite grain, obtain the diameter of a circle that is equivalent to the area, and use that as the apparent grain diameter of each ferrite grain. . Next, the average value of the apparent particle diameters of all the ferrite grains whose areas were measured is defined as the apparent ferrite average particle diameter, and the apparent ferrite average particle diameter is multiplied by 1.12.

  The area ratio of the ferrite bainite structure in the metal structure is preferably 90% or more, and may be 100%. Moreover, it is preferable that the area ratio of a bainite is 50% or more.

  If the area ratio of the ferrite bainite structure in the metal structure is 80% or more and the area ratio of bainite is 30 to 70%, the other phases need not be particularly limited.

  The average ferrite particle diameter is preferably 25 μm or less.

(C) Al amount precipitated as AlN, Nb (CN) and AlN-Nb (CN) in the region from the surface to 1/5 of the radius and from the center to 1/5 of the radius, respectively Nb amount and total number density of AlN, Nb (CN) and AlN-Nb (CN) with a diameter of 100 nm or more Since the slab and steel slab have a large cross-section, the center part has a predetermined temperature. It takes a long time. Therefore, when the slab and the steel slab are heated, the temperature of the central part is generally lower than that of the surface layer part, or the time during which the center part is kept at a predetermined temperature is short. Therefore, at the stage of hot-rolled steel bar or wire rod in a hot-worked state, the precipitation amount of AlN, Nb (CN) and AlN-Nb (CN) at the surface layer portion and the central portion, and AlN, Nb (CN) And the dispersion state of AlN—Nb (CN) are different, and there is also a difference in the coarsening of austenite grains during carburizing heating after cold forging.

However, the amount of Al deposited as AlN and AlN-Nb (CN) is 0 in the region from the surface of the hot rolled steel bar or wire to 1/5 of the radius and from the center to 1/5 of the radius. 0.010% or less, the amount of Nb precipitated as Nb (CN) and AlN—Nb (CN) is 0.020% or less, and AlN, Nb (CN) and AlN—Nb (diameter 100 nm or more) If the total number density of (CN) is 50/100 μm ² or less, the austenite grain coarsening during carburizing heating after cold forging can be suppressed in the entire region from the surface layer portion to the central portion.

Therefore, in the present invention, the amount of Al deposited as AlN and AlN-Nb (CN) in the region from the surface of the steel bar or wire to 1/5 of the radius and from the center to 1/5 of the radius. Is 0.010% or less, the amount of Nb precipitated as Nb (CN) and AlN-Nb (CN) is 0.020% or less, and AlN, Nb (CN) and AlN- It was defined that the total number density of Nb (CN) was 50/100 μm ² or less.

For the amount of Al deposited as AlN and AlN-Nb (CN) and the amount of Nb deposited as Nb (CN) and AlN-Nb (CN), for example, an appropriate test piece was taken and this test was performed. After the cross section of the piece is masked with a resin so as not to be electropolished, a so-called “10% AA electrolyte”, that is, 10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol, which is a general condition Obtained by performing extraction (electrolysis) using a solution at a current density of 250 to 350 A / m ² , filtering the extracted solution through a filter having a mesh size of 0.2 μm, and performing general chemical analysis on the filtrate. Can do.

For AlN, Nb (CN) and AlN-Nb (CN) having a diameter of 100 nm or more in the region from the surface to 1/5 of the radius and from the center to 1/5 of the radius, for example, to prepare an extraction replica sample in a conventional manner from the area, by using a transmission electron microscope, magnification 20000 times, in the area 10 [mu] m ² per one field by the 10 field observation randomly per area 100 [mu] m ² It can be determined as the number density.

  In the above two regions, the amount of Al precipitated as AlN and AlN-Nb (CN) is 0.008% or less, and the amount of Nb precipitated as Nb (CN) and AlN-Nb (CN). The amount is preferably 0.015% or less.

In each of the two regions, the total number density of AlN, Nb (CN) and AlN-Nb (CN) having a diameter of 100 nm or more is preferably 40/100 μm ² or less.

  The amount of Al deposited as AlN and AlN-Nb (CN), the amount of Nb deposited as Nb (CN) and AlN-Nb (CN), AlN, Nb (CN) and AlN-Nb ( CN) total number density (dispersed state) and the above-mentioned metal structure include the chemical composition of steel, the production conditions of slabs and steel slabs, segregation of constituent elements in the slabs and steel slabs, hot-rolled steel bars or wires This affects the hot working conditions and the cooling rate after hot working.

  Therefore, the amount of Al deposited as AlN and AlN-Nb (CN), the amount of Nb deposited as Nb (CN) and AlN-Nb (CN), AlN, Nb (CN) and AlN- As an example of a method for obtaining a dispersed state of Nb (CN) and a metal structure, 0.20 to 0.25% C, 0.4 to 0.8% Si, 0.5 to 0.8% A case where steel containing Mn and 1.0 to 1.5% Cr is used will be described. Of course, the method for producing a hot-rolled steel bar or wire according to the present invention is not limited to this.

・ Applying reduction to the slab during solidification,
-The slab is heated at a temperature of 1270 to 1300 ° C and heated for a heating time of 5 hours or more and then subjected to block rolling.
・ Cooling of steel slab after partial rolling is allowed to cool.
-Hot rolling the steel slab at a heating temperature of 1230 to 1280 ° C and a heating time of 1.5 hours or more,
-The hot rolling finish temperature is 950 to 1050 ° C, and after finish rolling, cooling to room temperature by cooling in the atmosphere (hereinafter simply referred to as "cooling"),
-The wrought ratio from steel slab to steel bar and wire (cross-sectional area of steel slab / cross-sectional area of steel bar and wire) is 8 or more.

  In this specification, “heating temperature” means the average value of the furnace temperature of the heating furnace, and “heating time” means the in-furnace time. The “finishing temperature” of hot working refers to the surface temperature of the steel bar and wire, and “room temperature”, which is the temperature to cool after finishing, also refers to the surface temperature of the steel bar and wire.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
Components of steel α and steel β having chemical compositions shown in Table 1 were adjusted using a 70-ton converter, and then continuous casting was performed to produce a 400 mm × 300 mm square slab (bloom), which was cooled to 600 ° C. Note that reduction was applied during the solidification of continuous casting. Both the steel α and the steel β are steels whose chemical compositions are within the range defined by the present invention.

  The slab produced in this manner was heated from the above 600 ° C. to 1280 ° C. and then rolled into a 180 mm × 180 mm square steel slab, which was allowed to cool to room temperature. Furthermore, after heating the above 180 mm × 180 mm square steel pieces, hot steel bar rolling was performed to obtain a steel bar having a diameter of 36 mm.

  In Table 2, as production conditions <1> to <8>, when finishing a steel plate having a diameter of 36 mm from a cast piece of 400 mm × 300 mm, the heating condition of the cast piece, the cooling condition after the block rolling, and the heating condition of the steel piece The details of the rolling finishing temperature of the steel bar rolling and the cooling conditions after rolling are shown.

上記のようにして得た直径３６ｍｍの各棒鋼について、金属組織における「相」の同定を行うとともに、フェライト・ベイナイト組織の面積率、ベイナイトの面積率およびフェライト平均粒径を求めた。

For each steel bar having a diameter of 36 mm obtained as described above, the “phase” in the metal structure was identified, and the area ratio of the ferrite-bainite structure, the area ratio of bainite, and the ferrite average particle diameter were determined.

  Specifically, after cutting a cross section perpendicular to the longitudinal direction and including the central portion from each steel bar, a test piece which was mirror-polished and corroded with nital was used with an optical microscope at a magnification of 400 times and a field of view. Observe at random 10 fields of view with a size of 250 μm × 250 μm to identify the “phase” in the metal structure, and perform image analysis by the usual method for each field of view, and the area ratio of ferrite and the area ratio of bainite The average ferrite particle diameter was determined. The area ratio of the ferrite-bainite structure was obtained by adding the area ratio of ferrite and the area ratio of bainite. The method for calculating the average ferrite particle diameter is as described above.

  About each steel bar of diameter 36mm obtained as mentioned above, it precipitates as AlN and AlN-Nb (CN) in the area | region from the surface to 1/5 of a radius, and the area | region from a center part to 1/5 of a radius. Al content, Nb (CN) and Nb amount precipitated as AlN-Nb (CN), and the total number density of AlN, Nb (CN) and AlN-Nb (CN) having a diameter of 100 nm or more, The following method was used for investigation.

Since a steel bar with a diameter of 36 mm has a scale on the surface, extraction residue analysis cannot be performed as it is. Test specimens were collected. About the cross section of this test piece, after masking with resin so as not to be electropolished, it is extracted (electrolysis) at a current density of 250 to 350 A / m ² using 10% AA-based electrolytic solution, which is a general condition. did. The extracted solution is filtered through a filter having a mesh size of 0.2 μm, and the filtrate is subjected to general chemical analysis. The amount of Al precipitated as AlN and AlN—Nb (CN), Nb (CN) and The amount of Nb precipitated as AlN-Nb (CN) was determined.

In a cross section of a steel bar with a diameter of 36 mm, an extraction replica sample was prepared by a general method from a region from the surface to a radius of 1/5 and from a center to a radius of 1/5, respectively. Using a microscope, each field of view was randomly observed at a magnification of 20,000 times and an area of 10 μm ² per field. First, for each of AlN, Nb (CN), and AlN-Nb (CN), the major axis and minor axis The arithmetic average diameter was determined, and then the total number density of AlN, Nb (CN), and AlN-Nb (CN) having a diameter of 100 nm or more per 100 μm ² area was determined.

  Furthermore, we conducted a test simulating cold forging after spheroidizing annealing to investigate cold forgeability, and conducted a test simulating carburization after cold forging to show the occurrence of coarsening of austenite grains as follows: It was investigated by the method shown.

  That is, after holding for 36 hours at 760 ° C. on a steel bar having a diameter of 36 mm produced according to each manufacturing condition symbol in Table 2, it was cooled to 660 ° C. at a cooling rate of 15 ° C./hour, and then allowed to cool to spheroidize. For the steel bar that was annealed and manufactured under the manufacturing condition symbol <1> in Table 2 for each steel, the normalizing that was kept at 920 ° C. for 1 hour and then allowed to cool to room temperature was replaced with the spheroidizing annealing. Were also made and investigated for cold forgeability.

  Furthermore, about the case where said spheroidizing annealing was given to the steel bar of diameter 36mm produced with each manufacturing condition symbol of Table 2, the coarsening generation | occurrence | production characteristic of the austenite grain at the time of carburizing after cold forging was investigated.

  The cold forgeability was first investigated for “deformation resistance” by the following method. That is, centering on the position of 18 mm from the surface of the steel bar subjected to the spheroidizing annealing and the steel bar subjected to normalization, the test piece shown in FIG. 1 was cut out, the end face was constrained, and from the height direction of the cylinder, The compression resistance of 60% of the height was performed, and the deformation resistance value at that time was measured. In addition, about each steel, when the deformation resistance value of the normalizing material is normalized as “100”, the deformation resistance value of the spheroidized annealing material is obtained, and when the value is 90 or less, It was evaluated as “good” because the deformation resistance was low. Among them, when the deformation resistance value was 85 or less, it was evaluated as “excellent” because the deformation resistance after spheroidizing annealing was particularly low. On the other hand, when the deformation resistance value exceeded 90, the deformation resistance after spheroidizing annealing was high, and it was evaluated as “impossible”.

  Next, “deformability” as cold forgeability was investigated by the following method. That is, five test pieces shown in FIG. 1 were produced around the position of 18 mm from the surface of the steel bar subjected to the spheroidizing annealing and the steel bar subjected to normalization. Next, using this test piece, the end face was constrained, and compression processing of 50% of the height was performed from the height direction of the cylinder, and then compression processing was performed by 2% of the initial height each time. The presence or absence of cracks was visually confirmed, and the compression ratio when cracks occurred in three of the five test specimens was defined as the limit compression ratio. In addition, about each steel, when the limit compressibility of the normalizing material is standardized as “100”, the limit compressibility of the spheroidized annealed material is obtained, and when the value is 110 or more, It was evaluated as “good” because of its high deformability. Among them, when the critical compression ratio was 115 or more, it was evaluated as “excellent” because the deformability after spheroidizing annealing was particularly high. On the other hand, when the critical compression ratio was less than 110, the deformability after spheroidizing annealing was low, and “impossible” was evaluated.

  The austenite grain coarsening characteristics when carburizing after cold forging were investigated by the following method. That is, for the steel bars having a diameter of 36 mm produced with the manufacturing condition symbols shown in Table 2, after the spheroidizing annealing described above, six test pieces shown in FIG. 1 are provided centering on the position of 18 mm from the surface of each steel bar. Cutting, first, 60% compression processing in the height direction was performed to simulate cold forging, and then 60% compression processing was applied in the height direction to simulate carburization. Was kept at 950 ° C., 970 ° C., 990 ° C., 1010 ° C., 1030 ° C. and 1050 ° C. for 300 minutes, and then cooled to room temperature by water cooling.

  After cutting a longitudinal section including the center of each test piece thus obtained, the cut surface was mirror-polished, corroded with a saturated aqueous solution of picric acid to which a surfactant was added, and then magnified using an optical microscope. Ten fields of view were randomly observed at a magnification of 100 to investigate the occurrence of coarsening of austenite grains. The size of each visual field in the above investigation is 1.0 mm × 1.0 mm.

  It was defined that coarsening of the austenite grains occurred when there were a total of two or more crystal grains having an austenite grain size number of 5 or less by the above-described optical microscope observation. When the austenite grains were not coarsened when held at a temperature of 1030 ° C. or lower for 300 minutes, it was evaluated as “excellent” because the austenite grain coarsening prevention effect was excellent. On the other hand, when austenite grains coarsened when held at a temperature of 1030 ° C. or lower for 300 minutes, it was evaluated as “impossible” because there was no effect of preventing austenite grain coarsening.

  Table 3 summarizes the results of the above surveys together with the manufacturing conditions of the steel bars. The manufacturing condition symbols in Table 3 correspond to the condition symbols described in Table 2 above.

表３から、化学組成が本発明で規定する範囲内にあり、しかも、表面から半径の１／５までの領域および中心部から半径の１／５までの領域において、ＡｌＮおよびＡｌＮ−Ｎｂ（ＣＮ）として析出しているＡｌ量、Ｎｂ（ＣＮ）およびＡｌＮ−Ｎｂ（ＣＮ）として析出しているＮｂ量、直径１００ｎｍ以上の、ＡｌＮ、Ｎｂ（ＣＮ）およびＡｌＮ−Ｎｂ（ＣＮ）の合計の個数密度、フェライト・ベイナイト組織の面積率、ベイナイトの面積率、ならびにフェライト平均粒径の全てが本発明で規定する条件を満たす本発明例の試験番号１および９の場合には、球状化焼鈍後の変形抵抗は低く、また、変形能は高いので冷間鍛造性に優れ、さらに、冷間鍛造後に優れたオーステナイト粒粗大化防止効果が得られていることが明らかである。

From Table 3, AlN and AlN—Nb (CN) are within the range defined by the present invention, and in the region from the surface to 1/5 of the radius and from the center to 1/5 of the radius. The amount of Al deposited, Nb (CN) and Nb deposited as AlN-Nb (CN), the total number of AlN, Nb (CN) and AlN-Nb (CN) having a diameter of 100 nm or more In the case of test numbers 1 and 9 of the present invention examples in which the density, the area ratio of the ferrite bainite structure, the area ratio of bainite, and the ferrite average particle diameter all satisfy the conditions specified in the present invention, Since the deformation resistance is low and the deformability is high, the cold forgeability is excellent, and it is clear that an excellent austenite grain coarsening prevention effect is obtained after cold forging.

  On the other hand, even if the chemical composition is within the range defined by the present invention, AlN and AlN-Nb (in the region from the surface to 1/5 of the radius and from the center to 1/5 of the radius CN), the amount of Al precipitated, Nb (CN), and the amount of Nb precipitated as AlN-Nb (CN), the total of AlN, Nb (CN) and AlN-Nb (CN) having a diameter of 100 nm or more. Test numbers 2 to 8 and 10 to 16 of comparative examples in which at least one of the number density, the area ratio of the ferrite bainite structure, the area ratio of bainite, and the ferrite average particle diameter deviates from the conditions defined in the present invention. In some cases, either one or both of the cold forgeability after spheroidizing annealing and the coarsening resistance characteristics of austenite grains when carburizing after cold forging are inferior.

(Example 2)
Components of steel a to m having the chemical composition shown in Table 4 were adjusted in a 70-ton converter, and then continuous casting was performed to produce a 400 mm × 300 mm square slab (bloom), which was cooled to 600 ° C. Note that reduction was applied during the solidification of continuous casting.

  Among the above steels, steels a to i are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steels j to m are steels of comparative examples whose chemical compositions deviate from the range defined in the present invention.

  The slab produced in this manner was heated from the above 600 ° C. to 1280 ° C. and then rolled into a 180 mm × 180 mm square steel slab, which was allowed to cool to room temperature. Furthermore, after heating the above 180 mm × 180 mm square steel pieces, hot steel bar rolling was performed to obtain a steel bar having a diameter of 36 mm.

  The manufacturing conditions for finishing a 400 mm × 300 mm slab into a steel bar with a diameter of 36 mm are any of the manufacturing condition symbols <1> and manufacturing condition symbols <2> to <8> described in Table 2 above for each steel. One of these two conditions was set.

  Table 5 shows the manufacturing conditions in which each steel was finished from a slab of 400 mm × 300 mm to a steel bar having a diameter of 36 mm, using the manufacturing condition symbols shown in Table 2.

上記のようにして得た直径３６ｍｍの各棒鋼について、前記の（実施例１）におけるのと同じ方法で各種の調査を実施した。

With respect to each steel bar having a diameter of 36 mm obtained as described above, various investigations were performed by the same method as in the above (Example 1).

  That is, first, the “phase” in the metal structure is identified, and the area ratio of the ferrite bainite structure, the area ratio of bainite, and the ferrite average particle diameter are obtained, and the region and center from the surface to 1/5 of the radius Amount of Al deposited as AlN and AlN-Nb (CN), Nb deposited as Nb (CN) and AlN-Nb (CN), and diameter in the region from the portion to 1/5 of the radius The total number density of AlN, Nb (CN) and AlN-Nb (CN) of 100 nm or more was investigated.

  Next, a test simulating cold forging after spheroidizing annealing was conducted to investigate "deformation resistance" and "deformability" as cold forgeability, and a test simulating carburization after cold forging was conducted. The state of coarsening of austenite grains was investigated.

  In evaluating the deformation resistance and deformability after spheroidizing annealing, and the austenite grain coarsening prevention property in a test simulating carburization, the same criteria as shown in the above (Example 1) were used.

  Table 5 shows the results of the above investigations together with the manufacturing condition symbols for the steel bars.

  From Table 5, the chemical composition is within the range defined by the present invention, and AlN and AlN-Nb (CN) in the region from the surface to 1/5 of the radius and from the center to 1/5 of the radius. The amount of Al deposited, Nb (CN) and Nb deposited as AlN-Nb (CN), the total number of AlN, Nb (CN) and AlN-Nb (CN) having a diameter of 100 nm or more Test Nos. 17, 19, 21, 23, 25, 27, 29 of the examples of the present invention satisfying all of the conditions, the density, the area ratio of the ferrite-bainite structure, the area ratio of bainite, and the ferrite average particle diameter satisfy the conditions defined in the present invention. 31 and 33, the deformation resistance after spheroidizing annealing is low, and the deformability is high, so the cold forgeability is excellent, and further, austenite grain coarsening is excellent after cold forging. It is clear that the stop effect is obtained.

  On the other hand, in the case of “Comparative Example” that does not satisfy all of the conditions specified in the present invention at the same time, the cold forgeability after spheroidizing annealing and the rough resistance of austenite grains when carburizing after cold forging. Either or both of the granulation properties are inferior.

  That is, even in the case of using steels a to i whose chemical composition is within the range defined in the present invention, in the region from the surface to 1/5 of the radius and from the center to the region of 1/5 of the radius. AlN precipitated as AlN and AlN-Nb (CN), Nb precipitated as Nb (CN) and AlN-Nb (CN), AlN, Nb (CN) and AlN- Test number 18 of the comparative example in which at least one of the total number density of Nb (CN), the area ratio of the ferrite bainite structure, the area ratio of bainite, and the average particle diameter of the ferrite is outside the conditions defined in the present invention , 20, 22, 24, 26, 28, 30, 32 and 34, cold forgeability after spheroidizing annealing and coarse resistance of austenite grains when carburizing after cold forging Either or both of the reduction characteristic is inferior.

  When using steels j to m whose chemical composition is outside the range defined in the present invention, AlN and AlN are used in the region from the surface to 1/5 of the radius and from the center to 1/5 of the radius. Al amount precipitated as -Nb (CN), Nb amount precipitated as Nb (CN) and AlN-Nb (CN), AlN, Nb (CN) and AlN-Nb (CN) having a diameter of 100 nm or more Regardless of whether or not the total number density, ferrite / bainite structure area ratio, bainite area ratio, and ferrite average grain size satisfy the conditions specified in the present invention, cold forgeability after spheroidizing annealing and Either one or both of the coarsening resistance characteristics of the austenite grains when carburizing after cold forging are inferior.

  The hot-rolled steel bar or wire of the present invention is excellent in cold forgeability after spheroidizing annealing, and can stably prevent coarsening of austenite grains during carburizing or carbonitriding after cold forging. It can be suitably used as a raw material for parts such as gears, pulleys, and shafts that are roughly formed by cold forging.

Claims (3)

Hot rolled steel bar or wire,
% By mass
C: 0.1 to 0.3%
Si: 0.05 to 1.0%,
Mn: 0.4 to 2.0%,
S: 0.005 to 0.05%,
Cr: 0.5 to 2.0%,
Al: 0.01 to 0.06%,
N: 0.005-0.025% and Nb: 0.02-0.08%
And the balance consists of Fe and impurities,
P, Ti and O (oxygen) in the impurities are respectively
P: 0.025% or less,
Having a chemical composition of Ti: 0.003% or less and O (oxygen): 0.002% or less;
In the region from the surface of the steel bar or wire to 1/5 of the radius and the region from the center to 1/5 of the radius, the Al amount precipitated as AlN and AlN-Nb (CN) is 0.010% or less, Nb amount precipitated as Nb (CN) and AlN—Nb (CN) is 0.020% or less, and the total of AlN, Nb (CN) and AlN—Nb (CN) having a diameter of 100 nm or more The number density is 50/100 μm ² or less,
The area ratio of the ferrite bainite structure is 80% or more, the area ratio of bainite is 30 to 70%, and the ferrite average particle diameter is 15 to 40 μm.
Hot-rolled steel bar or wire rod characterized by that. Instead of part of Fe, in mass%,
Cu: 0.5% or less,
Containing at least one selected from Ni: 1.5% or less and Mo: 0.8% or less,
The hot-rolled steel bar or wire according to claim 1. Instead of part of Fe, in mass%,
Pb: 0.3% or less,
Te: 0.08% or less,
Containing at least one selected from Ca: 0.01% or less and Bi: 0.3% or less,
The hot-rolled steel bar or wire according to claim 1 or 2.

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