JP5811282B2 - Round steel for cold forging - Google Patents

Description

  The present invention relates to a round steel material, and more particularly to a round steel material for cold forging.

  Structural steel is used as a material for machine structural parts such as automobile parts, industrial machine parts, and construction machine parts. Carbon steel for machine structure and alloy steel for machine structure are used for the structural steel.

  In order to manufacture parts from these steel materials, conventionally, a hot forging process and a cutting process have been mainly performed. However, in recent years, for the purpose of improving productivity, production of parts by a cold forging process is being considered instead of these processes.

  However, the degree of cold forging is generally large. Therefore, it is a problem to suppress the occurrence of cracks in the steel during cold forging, in other words, to improve the cold forgeability of the steel.

  In the case of cold forging carbon steel for machine structure and alloy steel for machine structure, the spheroidization rate of carbide is usually obtained by subjecting hot-rolled steel to soft annealing (hereinafter referred to as spheroidizing annealing). To increase. Thereby, the hardness of steel materials falls and high cold forgeability is obtained. However, even steel materials that have been subjected to spheroidizing annealing may crack during cold forging.

  Japanese Patent Application Laid-Open No. 2001-240940 (Patent Document 1), Japanese Patent Application Laid-Open No. 2001-11575 (Patent Document 2) and Japanese Patent Application Laid-Open No. 2011-214130 are steel materials for cold forging having improved cold forgeability after spheroidizing annealing. This is proposed in a gazette (Patent Document 3).

  The chemical composition of the bar wire for cold forging disclosed in Patent Document 1 is mass%, C: 0.1 to 0.6%, Si: 0.01 to 0.5%, Mn: 0.2 to 1.7%, S: 0.01 to 0.15%, Al: 0.015 to 0.05%, N: 0.003 to 0.025%, if necessary, Ni: 3. 5% or less, Cr: 2% or less, Mo: 1% or less, Nb: 0.005 to 0.1%, V: 0.03 to 0.3%, Te: 0.02% or less, Ca: 0.0. 02% or less, Zr: 0.01% or less, Mg: 0.035% or less, Y: 0.1% or less, and rare earth elements: 0.15% or less, including P: 0.035 %, O: limited to 0.003% or less, and consists of the balance Fe and inevitable impurities. In the above bar wire, the area ratio of ferrite in the region from the surface to the depth of the bar wire radius × 0.15 is 10% or less, and the balance is substantially one or two of martensite, bainite and pearlite. It consists of the above. Further, the average hardness in the region where the depth is from the rod wire radius x 0.5 to the center is softer than 20 HV compared to the average hardness of the surface layer (region from the surface to the depth of the rod wire radius x 0.15).

The chemical composition of the steel bar and the steel wire for machine structure disclosed in Patent Document 2 is mass%, C: 0.1 to 0.5%, Si: 0.01 to 0.15%, Mn: 0.2 -1.7%, Al: 0.0005-0.05%, Ti: 0.005-0.07%, B: 0.0003-0.007%, N: 0.002-0.02% Containing, if necessary, 0.003 to 0.15% S, and / or Cr that is 0.8% or less and the total amount with Mn is 0.3 to 1.3%, P: 0.02% or less, O: 0.003% or less, and the balance consists of Fe and inevitable impurities. The microstructure of the steel bar and steel wire is composed of ferrite and spherical carbide, the ferrite grain size is 8 or more, and the number of spherical carbide per 1 mm ^{2 of} unit area is 1.5 × 10 5 depending on the amount of C. ⁶ pieces × C% or less.

The chemical composition of the induction-quenched rolled steel disclosed in Patent Document 3 is mass%, C: 0.38 to 0.55%, Si: 1.0% or less, Mn: 0.20 to 2.0%. P: 0.020% or less, S: 0.10% or less, Cr: 0.10 to 2.0%, Al: 0.10% or less and N: 0.004 to 0.03%, As needed, Cu: 1.0% or less, Ni: 3.0% or less, Mo: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, and V: 0.30 % Or less, and the balance consists of Fe and impurities, fn1 = C + (1/10) Si + (1/5) Mn + (5/22) Cr + 1.65V− (5/7) S ( However, C, Si, Mn, Cr, V, and S in the formula represent the content in mass% of each element.) The value of 1.20 or less . The microstructure of the rolled steel material is composed of ferrite, lamellar pearlite and spherical cementite, and the average crystal grain size of ferrite is 10 μm or less, and the area ratio of the lamellar pearlite in the lamella pearlite having a lamellar spacing of 200 nm or less is 20 to 20%. 50%, and the number of spherical cementite is 4 × 10 ⁵ pieces / mm ² or more.

JP 2001-240940 A JP 2001-11575 A JP 2011-214130 A

  In patent document 1, in order to improve the ductility after spheroidizing annealing, the surface layer of the steel material after hot rolling is made into a uniform fine structure such as a structure mainly composed of tempered martensite or a structure mainly composed of bainite. More specifically, the steel layer is rapidly cooled to a temperature range greatly below the Ms point so that the steel surface layer region has a structure mainly composed of tempered martensite, or cooling and recuperation are repeated a plurality of times to form the surface layer region. The organization is mainly bainite. In this case, since volume change due to transformation occurs in the steel material, when severe dimensional accuracy and straightness are required, drawing may be necessary before spheroidizing annealing.

In Patent Document 2, a steel material having a surface temperature of A _r3 point to A _r3 point + 150 ° C. is rolled. In Patent Document 2, it is described that when a steel material having a surface temperature of less than _Ar3 is rolled, when rolling in a so-called two-phase region is performed, fine ferrite and pearlite cannot be obtained, which is not preferable. However, when rolling is performed in the temperature range of A _r3 point to A _r3 point + 150 ° C., fine ferrite may not be obtained, and the percentage of pearlite in the steel may increase. Therefore, the cold forgeability of the steel material after spheroidizing annealing may be low.

  The rolled steel material disclosed in Patent Document 3 is suitable for use as a material for parts such as rack bars that require bending strength and impact properties after induction hardening. However, in this rolled steel material, the ratio of the lamellar pearlite to the entire microstructure of the lamellar pearlite having a lamellar interval of 200 nm or less is as large as 20 to 50%. For this reason, even if the rolled steel material is annealed into a spheroidizing shape, the rolled steel material is not necessarily sufficiently softened, and the excellent cold forgeability required for the steel material for cold forging may not be obtained.

  An object of the present invention is to provide a round steel material for cold forging excellent in cold forgeability after spheroidizing annealing.

The round steel material for cold forging according to the present embodiment is mass%, C: 0.15 to 0.60%, Si: 0.01 to 0.5%, Mn: 0.1 to 2.0%, P : 0.035% or less, S: 0.050% or less, Al: 0.050% or less, Cr: 0.02-0.5%, N: 0.003-0.030%, Cu: 0-0 0.5%, Ni: 0 to 0.3%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.0035%, Nb: 0 to 0.050%, and , Ti: 0 to 0.2%, with the balance having a chemical composition consisting of Fe and impurities. The microstructure of the round steel for cold forging is composed of ferrite, pearlite and spherical cementite, the average crystal grain size of ferrite is 8 μm or less, and the area ratio of pearlite having a lamellar spacing of 200 nm or less in the microstructure is 20%. Is less than. Further, in the microstructure in the region from the surface to the radius x 0.15 depth of the round steel material for cold forging, the pearlite having an average crystal grain size of ferrite of 4 μm or less and a lamellar spacing of 200 nm or less is a micro-structure of the above region. area percentage of the tissue is less than 10%, and the number of the spherical cementite 1.0 × 10 ⁵ cells / mm ² or more der Ri, 10 ° C. after maintaining 10 hours round steel for cold forging at 735 ° C. / The critical compression ratio exceeds 50% when cooled to room temperature at a cooling rate of h.

  The round steel for cold forging according to this embodiment is excellent in cold forgeability after spheroidizing annealing.

FIG. 1 is a schematic diagram of a pearlite colony. FIG. 2A is a plan view of a test piece used in the cold forgeability test of the example. FIG. 2B is a front view of the test piece shown in FIG. 2A.

  Hereinafter, the cold forging round steel material of this embodiment is demonstrated in detail. In the following description, “%” notation of the content of each element means “mass%”.

  The present inventors have conducted various studies to solve the above problems. As a result, the present inventors found the following items (A) to (C).

  (A) Cold forgeability is enhanced by increasing the spheroidization rate of the steel material after spheroidizing annealing. If the microstructure before spheroidizing annealing is a mixed structure of ferrite, pearlite and spherical cementite and the average crystal grain size of ferrite in the microstructure is 10 μm or less, the diffusion distance of C in steel during spheroidizing annealing is short. Become. Therefore, during spheroidizing annealing, cementite in pearlite is easily spheroidized, and the spheroidization ratio (ratio of the number of spherical cementite to the number of cementite in steel) is increased.

  (B) In the above microstructure, if the ratio of pearlite having a lamellar spacing of 200 nm or less (hereinafter referred to as fine pearlite) is large, softening after spheroidizing annealing may be insufficient. When the area ratio of the fine pearlite in the microstructure is less than 20%, the steel material after spheroidizing annealing is sufficiently softened, and the cold forgeability of the steel material is enhanced.

(C) Cracks during cold forging occur from the surface layer of the steel material. In the case of a round steel material, if the spheroidization ratio in at least a region from the surface to a radius x 0.15 depth (hereinafter referred to as a surface layer region) increases, cold forging cracks are unlikely to occur in the surface layer. In the microstructure of the surface layer region, the average crystal grain size of ferrite is 5 μm or less, the area ratio of the surface layer region of fine pearlite in the microstructure is less than 10%, and the number of spherical cementite is 1.0 × 10 ⁵ If it is / mm ² or more, the spheroidization rate of the surface layer region increases, and the cold forgeability further increases.

The round steel material for cold forging of this embodiment completed based on the knowledge of said (A)-(C) is the mass%, C: 0.15-0.60%, Si: 0.01-0 0.5%, Mn: 0.1-2.0%, P: 0.035% or less, S: 0.050% or less, Al: 0.050% or less, Cr: 0.02-0.5%, N: 0.003 to 0.030%, Cu: 0 to 0.5%, Ni: 0 to 0.3%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 -0.0035%, Nb: 0-0.050%, and Ti: 0-0.2% are contained, and the remainder has a chemical composition which consists of Fe and an impurity. The microstructure of the round steel for cold forging is composed of ferrite, pearlite and spherical cementite, the average crystal grain size of ferrite is 8 μm or less, and the area ratio of pearlite having a lamellar spacing of 200 nm or less in the microstructure is 20%. Is less than. Furthermore, in the round steel material for cold forging, in the microstructure in the region from the surface to the radius × 0.15 depth, the pearlite having an average crystal grain size of ferrite of 4 μm or less and a lamellar spacing of 200 nm or less is in the region. area percentage of the microstructure is less than 10%, and the number of the spherical cementite 1.0 × 10 ⁵ cells / mm ² or more der is, after holding for 10 hours the round steel for cold forging at 735 ° C. 10 The critical compression ratio exceeds 50% when cooled to room temperature at a cooling rate of ° C / h.

  The above-mentioned round steel material for cold forging is Cu: 0.05-0.5%, Ni: 0.05-0.3%, Mo: 0.05-0.3%, V: 0.05-0. You may contain 1 type (s) or 2 or more types selected from the group which consists of 3% and B: 0.0005-0.0035%.

  The round steel for cold forging may contain one or two selected from the group consisting of Nb: 0.005 to 0.050% and Ti: 0.005 to 0.2%. .

  Hereinafter, the round steel material for cold forging by this embodiment is explained in full detail.

[Chemical composition]
The chemical composition of the round steel material for cold forging according to the present embodiment contains the following elements.

C: 0.15-0.60%
Carbon (C) increases the strength of the steel material. If the C content is too low, the effect cannot be obtained. On the other hand, if the C content is too high, the area ratio of fine pearlite in the microstructure increases, and the cold forgeability after spheroidizing annealing decreases. Therefore, the C content is 0.15 to 0.60%. The minimum with preferable content of C is 0.20%, More preferably, it is 0.30%, More preferably, it is 0.35%. The upper limit with preferable C content is 0.58%, More preferably, it is 0.55%, More preferably, it is 0.53%.

Si: 0.01 to 0.5%
Silicon (Si) deoxidizes the steel during melting. If the Si content is too low, this effect cannot be obtained. On the other hand, Si solidifies and strengthens ferrite. Therefore, if Si content is too high, the hardness of the steel material after spheroidizing annealing will become high too much, and cold forgeability will fall. Therefore, the Si content is 0.01 to 0.5%. The minimum with preferable Si content is 0.05%, More preferably, it is 0.08%, More preferably, it is 0.10%. The upper limit with preferable Si content is 0.45%, More preferably, it is 0.40%.

Mn: 0.1 to 2.0%
Manganese (Mn) increases the strength of the final product (machine structural component) manufactured from the cold forged round steel material. If the Mn content is too low, the strength of the final product will be insufficient. On the other hand, if the Mn content is too high, the hardness of the steel material after spheroidizing annealing will not be sufficiently low. Therefore, the Mn content is 0.1 to 2.0%. The minimum with preferable Mn content is 0.2%, More preferably, it is 0.3%. The upper limit with preferable Mn content is 1.8%, More preferably, it is 1.6%, More preferably, it is 1.4%.

P: 0.035% or less Phosphorus (P) is an impurity. P tends to segregate in steel and causes local ductility reduction. Therefore, a lower P content is preferable. The P content is 0.035% or less. P content is preferably 0.030% or less, more preferably 0.025% or less.

S: 0.050% or less Sulfur (S) is inevitably contained in steel. When S is contained, there is an effect of improving machinability. However, if the S content is too high, coarse sulfides are produced in the steel. Coarse sulfides cause cracks during cold forging. Therefore, the S content is 0.050% or less. The preferable S content is 0.045% or less. When improving machinability, the preferable S content is 0.015% or more.

Al: 0.050% or less Aluminum (Al) is inevitably contained in steel. Al deoxidizes steel. However, if the Al content is too high, coarse inclusions are generated in the steel, and cracks during cold forging are likely to occur. Therefore, the Al content is 0.050% or less. The preferable Al content is 0.045% or less. When enhancing the deoxidation effect, the preferable Al content is 0.015% or more. In this specification, Al content means content of acid-soluble Al (sol.Al).

Cr: 0.02-0.5%
Chromium (Cr) stabilizes spherical cementite. If the Cr content is too low, the effect cannot be obtained. On the other hand, if the Cr content is too high, the hardness of the steel material after spheroidizing annealing will not be sufficiently low. Therefore, the Cr content is 0.02 to 0.5%. The minimum with preferable Cr content is 0.03%, More preferably, it is 0.05%, More preferably, it is 0.07%. The upper limit with preferable Cr content is 0.45%, More preferably, it is 0.40%, More preferably, it is 0.35%.

N: 0.003-0.030%
Nitrogen (N) produces nitrides and refines the crystal grains. If the N content is too low, this effect cannot be obtained. On the other hand, if the N content is too high, the above effect is saturated and the manufacturing cost is also increased. Therefore, the N content is 0.003 to 0.030%. The minimum with preferable N content is 0.004%, More preferably, it is 0.005%. The upper limit with preferable N content is 0.022%, More preferably, it is 0.020%, More preferably, it is 0.018%.

  When the round steel material for cold forging of this embodiment contains B mentioned later, if B couple | bonds with N, B cannot exhibit the effect which improves the hardenability of steel materials. In this case, it is necessary to contain a large amount of Ti. Therefore, when it contains B, the one where N content is low is preferable. The upper limit with preferable N content in this case is 0.010%, More preferably, it is 0.008%.

  The balance of the chemical composition of the round steel for cold forging of this embodiment is composed of Fe and impurities. In this specification, an impurity means the thing mixed from the ore as a raw material, a scrap, or a manufacturing environment, etc. when manufacturing steel materials industrially.

  The round steel for cold forging of this embodiment may further contain one or more selected from the group consisting of Cu, Ni, Mo, V, and B instead of a part of Fe. All of these elements increase the strength of machine structural parts manufactured from cold forged round steel.

Cu: 0 to 0.5%
Copper (Cu) is an optional element and may not be contained. Cu increases the strength of machine structural parts by solid solution strengthening. However, if the Cu content is too high, the hot workability decreases. Therefore, the Cu content is 0 to 0.5%. The minimum with preferable Cu content for acquiring the said effect more effectively is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Cu content is 0.4%, More preferably, it is 0.3%.

Ni: 0 to 0.3%
Nickel (Ni) is an optional element and may not be contained. Ni increases the strength of machine structural parts by solid solution strengthening. However, if the Ni content is too high, the economy is impaired. Therefore, the Ni content is 0 to 0.3%. The minimum with preferable Ni content for acquiring the said effect more effectively is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Ni content is 0.25%, More preferably, it is 0.2%.

Mo: 0 to 0.3%
Molybdenum (Mo) is an optional element and may not be contained. Mo increases the strength of machine structural parts by solid solution strengthening. However, if the Mo content is too high, the effect is saturated and the economy is impaired. Therefore, the Mo content is 0 to 0.3%. The minimum with preferable Mo content for acquiring the said effect more effectively is 0.05%, More preferably, it is 0.1%. The upper limit with preferable Mo content is 0.25%, More preferably, it is 0.20%.

V: 0 to 0.3%
Vanadium (V) is an optional element and may not be contained. V increases the strength of mechanical structural parts by precipitation strengthening. However, if the V content is too high, the hardness of the steel material becomes too high and the cold forgeability decreases. Therefore, the V content is 0 to 0.3%. The minimum with preferable V content for acquiring the said effect more effectively is 0.05%, More preferably, it is 0.1%. The upper limit with preferable V content is 0.25%, More preferably, it is 0.20%.

B: 0 to 0.0035%
Boron (B) is an optional element and may not be contained. B increases the hardenability of the steel material and increases the strength of the final product (machine structural component) manufactured from the steel material. However, if the B content is too high, the effect is saturated and the production cost is increased. Therefore, the B content is 0 to 0.0035%. The minimum with preferable B content for improving the said effect is 0.0005%, More preferably, it is 0.0010%. The upper limit with preferable B content is 0.0030%.

  As above-mentioned, the 1 type, or 2 or more types selected from the group which consists of Cu, Ni, Mo, V, and B may be contained in the round steel material for cold forging of this embodiment. The total content of these elements is preferably 1.40% or less, and more preferably 0.80% or less.

  The round steel for cold forging of the present embodiment may further contain one or two selected from the group consisting of Nb and Ti instead of a part of Fe. All of these elements form carbonitrides to refine crystal grains.

Nb: 0 to 0.050%
Niobium (Nb) is an optional element and may not be contained. Nb forms carbonitride and refines crystal grains. Due to the refinement of crystal grains, the cold forgeability of steel is increased. However, if the Nb content is too high, the carbonitride becomes coarse. Coarse carbonitride becomes a starting point of cracking during cold forging. Therefore, the Nb content is 0 to 0.050. The minimum with preferable Nb content for improving the said effect more is 0.005%, More preferably, it is 0.010%. The upper limit with preferable Nb content is 0.035%, More preferably, it is 0.030%.

Ti: 0 to 0.2%
Titanium (Ti) is an optional element and may not be contained. Ti forms carbonitrides and refines crystal grains. When the round steel material for cold forging of this embodiment contains B, Ti couple | bonds with N, forms a nitride, and suppresses that B couple | bonds with N. Therefore, B can be dissolved in steel to enhance the hardenability of the steel as described above. However, if the Ti content is too high, the carbonitrides become coarse and the toughness of the steel material decreases. Therefore, the Ti content is 0 to 0.2%. The minimum with preferable Ti content for improving the said effect more is 0.005%, More preferably, it is 0.010%. The upper limit with preferable Ti content is 0.18%, More preferably, it is 0.15%.

  As described above, Ti suppresses the bonding of B with N. Therefore, when B is contained, Ti is preferably also contained.

[Microstructure]
The microstructure of the round steel material for cold forging of the present embodiment having the above-described chemical composition is composed of ferrite, pearlite, and spherical cementite. In this microstructure, the average crystal grain size of ferrite is 10 μm or less, and the area ratio of pearlite (fine pearlite) having a lamellar spacing of 200 nm or less in the pearlite is less than 20%.

Furthermore, in the microstructure in the region (surface region) from the surface to the radius x 0.15 depth in the round steel material, the average crystal grain size of ferrite is 5 μm or less, and fine pearlite occupies the microstructure in the surface region. The area ratio is less than 10%. Furthermore, the number of spherical cementite in the microstructure of the surface layer region is 1.0 × 10 ⁵ pieces / mm ² or more.

  The round steel for cold forging of this embodiment has the above microstructure. Therefore, in the cold forging performed after spheroidizing annealing, generation | occurrence | production of the crack in the surface layer of steel materials is suppressed, and cold forgeability improves. Hereinafter, (1) the microstructure in the entire steel material and (2) the microstructure in the surface layer region of the steel material will be described in detail.

[Microstructure of the entire steel material]
As described above, the microstructure of the steel material is a mixed structure made of ferrite, pearlite, and spherical cementites. Therefore, the hardness of the microstructure is low compared to martensite and bainite.

[Average ferrite grain size in the microstructure of the entire steel]
Even in the mixed structure as described above, if the average crystal grain size of ferrite exceeds 10 μm, the diffusion distance of C during spheroidizing annealing becomes long. In this case, the cementite in the pearlite is difficult to spheroidize during spheroidizing annealing.

  In the present embodiment, the average crystal grain size of ferrite in the microstructure is 10 μm or less. Therefore, the diffusion distance of C is short, and cementite tends to be spheroidized during spheroidizing annealing.

[Area ratio in the microstructure of fine pearlite]
Even if it is the above-mentioned mixed structure and the ferrite grains are fine, if the area ratio of the pearlite in the pearlite (fine pearlite) with a lamellar spacing of 200 nm or less is large, the spheroidizing annealing may be performed. Steel is difficult to soften. In the present embodiment, the area ratio of the fine pearlite in the microstructure is less than 20%. Therefore, the cold forgeability of the steel material after spheroidizing annealing is enhanced.

  The lamella interval is obtained by the following method. Of pearlite, a region having the same lamella orientation (direction of cementite extension) is defined as a pearlite colony. FIG. 1 shows an example of a pearlite colony. The pearlite colony 1 includes a plurality of cementites 2 and a plurality of ferrites 3. The cementite 2 and the ferrite 3 are alternately arranged in a lamellar shape (layer shape). Within the pearlite colony, the plurality of cementites 2 are arranged substantially in parallel.

In the pearlite colony, the lamella spacing is determined at any three locations. For example, referring to FIG. 1, line segment L <b> 1 is drawn in a direction perpendicular to the extending direction of cementite 2 at measurement point P <b> 1. At this time, both end points P _L1 and P _L1 of the line segment L1 are arranged at the center of the width of each of the pair of cementite 2 closest to the boundary 10 of the pearlite colony 1 at the measurement location P1. The length of the line segment L1 and the number N of cementite crossing the line segment L1 are obtained, and the lamella interval (nm) at the measurement point P1 is obtained by the following equation.
Lamella interval at measurement point P1 = L1 / (N-1)

  In short, lamella spacing means the distance between adjacent cementites. At the measurement location P1, the number N of cementite that intersects the line segment L1 is “4”.

  Similarly, a line segment L2 is drawn at the measurement point P2. At this time, both end points of the line segment L2 are respectively arranged at the center of the width of each of the pair of cementite 2 closest to the boundary 10 of the pearlite colony 1 at the measurement location P2. The cementite number N at this time is “5”. Based on the above formula, the lamella interval at the measurement point P2 is obtained. Similarly, the lamella interval of the measurement location P3 is also obtained.

  The average of the lamella intervals determined at the measurement points P1 to P3 is defined as the “lamellar interval” (nm) of the pearlite colony 1. A pearlite colony having a lamella spacing of less than 200 μm is defined as “fine pearlite”.

[Microstructure in surface area]
Cracks during cold forging occur from the surface layer of the steel material. In the present embodiment, in order to further increase the spheroidization rate in the surface layer region after spheroidizing annealing, the average crystal grain size of ferrite, the area ratio of fine pearlite, and the number of spherical cementite in the microstructure of the surface layer region are defined as follows: To do.

[Average grain size of ferrite in the microstructure of surface layer]
If the average crystal grain size of ferrite in the microstructure of the surface layer region exceeds 5 μm, the cold forgeability in the surface layer region is lowered, and cracks may occur during cold forging. Therefore, the average crystal grain size of ferrite in the microstructure of the surface layer region is 5 μm or less.

[Area ratio in the microstructure in the surface area of fine pearlite]
If the area ratio of the fine pearlite in the surface layer region to the microstructure is 10% or more, the cold forgeability in the surface layer region is lowered, and cold forging cracks may occur. Therefore, the area ratio which occupies for the microstructure in the surface layer area | region of fine pearlite is less than 10%.

[Number of spherical cementite in the microstructure of surface layer]
The number of spherical cementite in the microstructure of the surface region is 1.0 × 10 ⁵ pieces / mm ² or more. In this case, during spheroidizing annealing, spherical cementite in the surface layer region becomes a nucleus, and spherical cementite is likely to be generated and grow. Therefore, the spheroidization rate of the surface layer region after spheroidizing annealing is further increased.

  The identification of the phase of the microstructure, the average crystal grain size of ferrite, the area ratio of fine pearlite, and the number of spherical cementite are obtained by the following methods.

[Identification of phase of microstructure]
The cross section of the round steel material (cross section perpendicular to the axial direction of the round steel material) is mirror-polished to obtain an observation surface. The mirror-polished observation surface is corroded with 3% nitric alcohol (nitral liquid) to reveal a microstructure. The revealed microstructure is observed with a scanning electron microscope (SEM).

  The radius of the observation surface of the round steel material is defined as R. Among the observation surfaces, a position having a radius R × 0.067 depth from the surface toward the center (hereinafter referred to as position Q1), a position having a radius R × 0.15 depth from the surface (hereinafter referred to as position Q2), A position having a radius R × 0.25 depth from the surface (referred to as position Q3), a position having a radius R × 0.5 from the surface (referred to as position Q4), and a center (referred to as position Q5) are specified. Microstructures are observed in a total of 15 visual fields, 3 at each specified position Q1 to Q5, and phases are identified. The area of each visual field is 25 μm × 20 μm. A captured image of each field of view is generated, and a phase is identified based on the captured image.

  About spherical cementite, the observation surface of the said round steel material is mirror-polished. After polishing, the observation surface is corroded with picric alcohol (picral liquid). Using the SEM of 5,000 times, a microscopic photographed image is generated for 15 fields of view in the same manner as the above-described phase identification. The major axis L and the minor axis W of each cementite in each visual field are measured by image processing using the captured image of each visual field. Among the plurality of observed cementites, cementite having an L / W of 2.0 or less is defined as spherical cementite.

[Average crystal grain size of ferrite]
The observation surface of the round steel material is mirror-polished. After polishing, the observation surface is corroded with 3% nitric acid alcohol (nitral liquid) to reveal a microstructure. Using the SEM of 5,000 times, a microscopic photographed image is generated for 15 fields of view in the same manner as the above-described phase identification. Image processing is performed using the photographed image, and the average crystal grain size of the ferrite in the 15 fields of view is determined based on the evaluation method based on the cutting method of ferrite crystal grains described in Appendix 2 of JIS G0551 (2005). The average of the obtained average crystal grain diameter of each visual field is defined as the average crystal grain diameter (μm) of ferrite in the entire microstructure.

  Furthermore, the average of the crystal grain diameter of ferrite in a total of six fields of view at the positions Q1 and Q2 is obtained and defined as the average crystal grain diameter (μm) of ferrite in the surface layer region.

[Area ratio of fine pearlite]
The area ratio of fine pearlite is measured by the following method. A pearlite colony is identified (sectioned) in each of the 15 fields of view (25 μm × 20 μm). For example, the pearlite colony is identified by image processing. In each perlite colony, the lamella spacing (nm) is determined by the method described above. And the pearlite colony whose lamella interval is 200 nm or less is specified as "fine pearlite". The area Af (μm ² ) of the specified fine pearlite is obtained, and the fine pearlite area ratio in each field of view is obtained based on the formula (1).
Fine pearlite area ratio (%) = Af / field of view × 100 (1)
Here, the visual field area is 25 × 20 = 500 (μm ² ). For example, the area Af can be obtained by using well-known image processing by marking the boundary 10 and the inside of the pearlite colony 1 in FIG.

  The average of the fine pearlite area ratio of each visual field obtained based on the formula (1) is defined as the area ratio (%) of the fine pearlite in the microstructure.

  Further, the average of the fine pearlite area ratios (total 6 fields of view) at the positions Q1 and Q2 obtained based on the formula (1) is defined as the area ratio (%) in the microstructure in the surface layer region of the fine pearlite.

[Number of spherical cementite]
The number of spherical cementite (L / W is 2.0 or less cementite) at positions Q1 and Q2 (total 6 fields of view) is counted. Based on the total number of spherical cementites in 6 fields of view, the number of spherical cementites per 1 mm ² area (pieces / mm ² ) is calculated. The number obtained is defined as the number of spherical cementites (pieces / mm ² ) in the microstructure in the surface layer region.

  A preferable average crystal grain size of ferrite in the entire microstructure of the round steel material of the present embodiment is 8 μm or less. A preferable average crystal grain size of ferrite in the microstructure of the surface layer region is 4 μm or less. The average grain size of ferrite in the entire round steel material and the microstructure of the surface layer region is preferably as small as possible. However, special processing conditions or equipment are required to form submicron-order crystal grains, and it is difficult to realize industrially. Therefore, in the average crystal grain size of ferrite in the microstructure of the whole round steel material and the average crystal grain size of ferrite in the microstructure of the surface layer region, the lower limit that can be industrially realized is 1 μm.

  A preferable area ratio of fine pearlite in the microstructure of the entire round steel material is less than 15%. A preferable area ratio of fine pearlite in the microstructure of the surface layer region to the microstructure of the surface layer region is 8% or less. In order to improve cold forgeability, these area ratios are preferably as small as possible, and may be 0%.

A preferable number of spherical cementite in the microstructure of the surface layer region is 2.0 × 10 ⁵ pieces / mm ² or more. The larger the number of spherical cementite, the better. However, the upper limit is substantially 1.0 × 10 ⁷ pieces / mm ² .

  In a round steel material with a microstructure of ferrite (pearlite, pearlite and spherical cementite), excellent cold forgeability after spheroidizing annealing if the number of spherical cementites in the microstructure of the surface layer satisfies the above-mentioned rules Is obtained. For this reason, the number of spherical cementite in the microstructure of the portion other than the surface layer region need not be specified.

[Production method]
An example of the manufacturing method of the cold forging round steel material of this embodiment is demonstrated.

  A material (for example, billet) having the above chemical composition is heated in a heating furnace. The heated raw material is extracted from the heating furnace, and hot rolled using a continuous rolling mill to produce a cold rolled forging steel. The continuous rolling mill includes a plurality of arranged rolling mills (stands). The cold forging round steel material is manufactured based on the all-continuous rolling method. The all-continuous rolling method means that the material extracted from the heating furnace is continuously rolled without stopping halfway until it leaves the final stand of the continuous rolling mill and becomes a round steel material for cold forging. Means the method. Hereinafter, manufacturing conditions in the all continuous rolling method will be described.

[Material heating temperature]
The material is heated so that the heating temperature of the material before hot rolling (that is, the surface temperature of the material) is 810 ° C. or lower. In this case, rolling in a two-phase region is performed. By carrying out rolling in the two-phase region, the ferrite grains in the round steel material after rolling can be made fine. On the other hand, if the heating temperature is too low, the load on the continuous rolling mill becomes excessive. Therefore, the minimum of the heating temperature of the preferable raw material before hot rolling is 670 degreeC.

[Total area reduction in all continuous rolling method]
The total area reduction in the all-continuous rolling method is made higher than 30%. The total area reduction rate (%) is defined by equation (2).
Total area reduction ratio = (cross-sectional area of material-cross-sectional area of round steel) / cross-sectional area of material x 100 (2)

Here, the cross-sectional area (mm ² ) of the material means the area of a cross section perpendicular to the central axis of the material. The cross-sectional area (mm ² ) of the round steel material means an area of a cross section perpendicular to the central axis of the round steel material manufactured by the all continuous rolling method.

  By making the total area reduction rate higher than 30%, the processing-induced precipitation of ferrite from austenite during processing is promoted. Furthermore, processing strain is introduced into the ferrite being processed, and the ferrite is refined by dynamic recrystallization. Furthermore, by introducing a large number of processing strains, the ferrite becomes finer during cooling described later.

[Surface temperature of round steel on the exit side of the final rolling mill]
The temperature of the round steel material immediately after rolling in the two-phase region, that is, the surface temperature of the round steel material on the final rolling mill exit side is set to Ac ₃ points or more. In this case, the processed structure is once reverse transformed. During hot rolling, the surface temperature of the material rises due to processing heat generation. By adjusting the cooling conditions during hot rolling, the surface temperature of the round steel material on the outlet side of the final rolling mill is set to _Ac3 point or higher. In this case, the structure of the round steel material once becomes an austenite single phase. Ferrite refined by dynamic recrystallization becomes fine austenite by reverse transformation.

[Cooling conditions immediately after rolling]
Within 5 seconds after rolling is completed, the round steel material is cooled to a temperature not exceeding 600 ° C. at ₃ points or less of Ar. Within 5 seconds the surface temperature of the round steel to below A _r3 point, tissue round steel is transformed again, fine ferrite is generated. Furthermore, by setting the cooling stop temperature to an _Ar3 point or lower and 600 ° C or higher, it is possible to suppress the formation of a hard structure such as bainite and martensite, and to suppress the generation of fine pearlite.

In the present embodiment, for example, the surface temperature of the round steel material is set to _{Ar 3} to 600 ° C. within 5 seconds by a water cooling device disposed on the exit side of the final rolling mill. When 5 seconds or more have elapsed after the completion of rolling, austenite produced by reverse transformation becomes coarse. When austenite becomes coarse, fine ferrite cannot be obtained even if the surface temperature of the round steel material is made _Ar3 or lower after that. The cooling time is not particularly limited as long as it is within 5 seconds. For example, the surface temperature of the round steel may be _{A r3} point to 600 ° C. for 3 seconds. After the surface temperature of the round steel material is set to _{Ar 3} to 600 ° C., cooling by the water cooling device is stopped.

As described above, the surface temperature of the steel material is cooled to a temperature not exceeding Ar ₃ point and not lower than 600 ° C. within 5 seconds after the end of rolling by the all-continuous rolling method, and then water cooling by the water cooling device is stopped. When the round steel material is further cooled to room temperature, a method that does not have such a high cooling rate that martensite and bainite are generated, for example, cooling may be performed.

  By the above manufacturing process, the round steel material for cold forging having the above-mentioned microstructure is manufactured. The manufactured round steel for cold forging is subjected to spheroidizing annealing and then cold forged into a final product (structural machine part or the like). Since the round steel material for cold forging of this embodiment is provided with the above-mentioned chemical composition and microstructure, it is excellent in cold forgeability after spheroidizing annealing.

  Square billets (cross section: 140 mm × 140 mm and length: 10 m) made of steels A to H having the chemical composition shown in Table 1 were prepared.

Referring to Table 1, the chemical compositions of steels A to E, G, and H were within the range of the chemical composition of the cold forging round steel material of the present embodiment. On the other hand, in the chemical composition of the steel F, the C content deviated from the range of the C content defined in the present embodiment. Table 1 shows the Ar ₃ point and Ac ₃ point of each steel.

  Square billets were heated under the production conditions shown in Table 2, and hot rolling was performed by an all-continuous rolling method to produce a cold forged round steel material having a diameter of 30 mm.

  In the “heating temperature” column of Table 2, the surface temperature (° C.) of the square billet (raw material) extracted from the heating furnace (before continuous rolling) is described. In the “post-rolling temperature” column, the surface temperature (° C.) of the round steel material on the outlet side of the final rolling mill (stand) among the continuous rolling mills is described. The “post-rolling temperature” was obtained by measuring with a radiation thermometer arranged on the exit side of the final rolling mill. In the “temperature after cooling” column, the surface temperature (° C.) of the round steel material 5 seconds after leaving the final rolling mill is described. The “temperature after cooling” was obtained by measuring the surface temperature of the round steel material with a radiation thermometer when 5 seconds passed.

  In any test number, the “total area reduction ratio” from the square billet (material) calculated by the equation (2) was 96%.

For test numbers 1 to 8, the water cooling conditions between the rolling mills (stands) in the continuous rolling mill are adjusted so that the surface temperature of the round steel material on the exit side of the final rolling mill is at least _{Ac 3} points. Adjusted. Furthermore, after the rolling by the final rolling mill is completed, the cooling rate is controlled by the amount of water using a water cooling device, and cooling is performed so that the surface temperature of the steel material is ₃ points or less of Ar and less than 600 ° C. within 5 seconds. Then, cooling by the water cooling device was stopped. After the cooling by the water cooling device was stopped, the round steel material was allowed to cool in the atmosphere.

  For Test No. 9 and Test No. 10, the round steel material after completion of continuous rolling was left to cool in the air as it was without being cooled by a water cooling device.

For test numbers 11 and 12, water cooling conditions between the stands were adjusted, and water cooling was performed after rolling. However, the post-rolling temperature of test number 10 was less than _Ac3 point. Test No. 11 had a temperature after water cooling of less than 600 ° C.

  The following tests were carried out on the manufactured round steel materials (steel bars) of each test number.

[Microstructure observation test]
A test piece having a length of 20 mm was cut out from each round steel material having a diameter of 30 mm. These test pieces were embedded in a resin and mirror-polished so that the cross section (cross section perpendicular to the central axis of the round steel material) was an observation surface. After polishing, it was corroded with 3% nitric acid alcohol (nitral liquid) to reveal a microstructure, and was observed using SEM. Specifically, position Q1 at a depth of 1 mm (radius × 0.067 depth) from the surface, position Q2 at a depth of 2.25 mm (radius × 0.15 depth) from the surface, and depth of 3.75 mm from the surface. A total of five tissues, a position Q3 of (radius × 0.25 depth), a position Q4 of 7.5 mm depth (radius × 0.5 depth) from the surface, and a position Q5 of the central portion (near the center), A total of 15 visual fields were observed, 3 visual fields per place, and the phases constituting the microstructure were identified by the method described above. As described above, the area of each visual field was 25 μm × 20 μm.

[Measurement of average crystal grain size of ferrite]
By the method described above, the average crystal grain size of ferrite in the microstructure of the entire round steel material of each test number and the average crystal grain size of ferrite in the microstructure of the surface layer region were measured.

[Measurement of fine pearlite area ratio and number of spherical cementite]
By the method described above, in each test number, the area ratio of fine pearlite in the microstructure of the whole round steel material and the microstructure of the surface layer region was obtained. Furthermore, the number of spherical cementites (pieces / mm ² ) in the microstructure of the surface layer region was determined by the method described above.

[Measurement of spheroidizing rate after spheroidizing annealing]
Spheroidizing annealing was performed on the round steel material of each test number. Specifically, each round steel material was held at 735 ° C. for 10 hours. Then, it cooled to normal temperature with the cooling rate of 10 degrees C / h.

  A test piece having a length of 20 mm was cut out from each round steel material after spheroidizing annealing. Of the surface of the test piece, the surface corresponding to the longitudinal section of the round steel material was embedded in a resin so as to be an observation surface, and mirror-polished.

After polishing, the sample was corroded with picric alcohol (picral solution), and a microscopic photographed image was generated for 15 fields of view using the SEM of 5000 times, similar to the above phase identification. As in the case of the microstructure observation test described above, the major axis L and the minor axis W of each cementite were individually measured using the captured images. Then, the ratio of the number of cementite (ie, spherical cementite) having L / W of 2.0 or less to the number of cementite in the photographed image (each field of view described later) was determined and used as the spheroidization rate (%).
Specifically, the observed position is a position Q1 having a depth of 1 mm (radius × 0.067 depth) from the surface, a position Q2 having a depth of 2.25 mm (radius × 0.15 depth) from the surface, and from the surface. A total of 5 positions: Q3 at a depth of 3.75 mm (radius × 0.25 depth), a position Q4 at a depth of 7.5 mm (radius × 0.5 depth) from the surface, and a position Q5 at the center (near the center). A total of 15 visual fields were observed with 3 visual fields per site. The area of each visual field was 25 μm × 20 μm.

  Of the spheroidization rates determined in each field of view, the average value of the spheroidization rates in the six fields of view at positions Q1 and Q2 was defined as the surface layer spheroidization rate (%) after spheroidizing annealing. The average value of the spheroidizing ratio in nine visual fields at positions Q3 to Q5 was defined as the internal spheroidizing ratio (%) after spheroidizing annealing.

[Cold forgeability test]
A test piece shown in FIGS. 2A and 2B was produced from each round steel material after the spheroidizing annealing treatment. FIG. 2A is a plan view of the test piece, and FIG. 2B is a front view of the test piece. 2A and 2B, the diameter D1 of the test piece was 29 mm, and the length L4 was 44 mm. A cutout portion extending in the axial direction was formed on the outer peripheral surface of the test piece. The notch angle A1 of the notch was 30 °, and the corner radius R1 of the groove bottom portion of the notch was 0.15 mm. The depth D2 of the notch was 0.8 mm.

  A compression test was performed in the cold (room temperature) using the test piece and the press. In the compression test, the specimen was first compressed to 15% in the axial direction. Thereafter, the test piece was unloaded each time 1.5 to 2.5% axial compression was applied to the test piece, and the test piece was observed for cracking. The compression, unloading and observation were repeated until cracking occurred. When a fine crack (length 0.5-1.0 mm) was first observed using the naked eye or a simple magnifier, it was determined that a crack had occurred. Five test pieces were prepared for each test number, and the above compression test was performed on the five test pieces. The average value of the compression ratios of the five test pieces when cracking occurred was defined as “limit compression ratio”. When the critical compression ratio exceeded 50%, it was evaluated that the cold forgeability was excellent.

[Test results]
Table 2 shows the test results. In Table 2, “F” in the “Phase” column of the “Total microstructure” column indicates ferrite, “LP” indicates lamellar pearlite, and “SC” indicates spherical cementite. In the “crystal grain size” column, the ferrite average crystal grain size (μm) in the microstructure of the entire round steel material in each test number is described. In the “fine LP ratio” column, the area ratio (%) of the fine pearlite in the entire microstructure is described.

In Table 2, the “crystal grain size” column in the “microstructure in the surface region” column lists the ferrite average crystal grain size (μm) in the surface layer region for each test number. In the “fine LP ratio” column, an area ratio (%) of the surface area of the fine pearlite in the microstructure is described. In the “SC number” column, the number of spherical cementite (pieces / mm ² ) in the microstructure of the surface layer region is described.

  In the “after spheroidizing annealing” column in Table 2, the surface spheroidization rate (%), internal spheroidization rate (%), and critical compression rate (%) of each test number are described.

  “A” in the “Evaluation” column in Table 2 means that the cold forgeability is evaluated to be excellent, and “NA” means that the cold forgeability is evaluated to be low. “−” In the number column of spherical cementite of test number 9 and test number 10 indicates that the phase is “F + LP” and spherical cementite is not present.

With reference to Table 2, the chemical composition of the steel materials of the test numbers 1-7 was appropriate, and manufacturing conditions (total area reduction rate, heating temperature, temperature after rolling, temperature after cooling) were also appropriate. Therefore, the microstructure of the round steel materials of test numbers 1 to 7 is composed of ferrite, pearlite, and spherical cementite, the average crystal grain size of ferrite in the entire microstructure of the round steel material is 10 μm or less, and the fine LP rate is also less than 20%. Met. Furthermore, the average crystal grain size of ferrite in the microstructure of the surface region of test numbers 1 to 7 is 5 μm or less, the fine LP rate is less than 10%, and the number of spherical cementite is 1.0 × 10 ⁵ / mm ² or more. Therefore, the surface spheroidization rate after spheroidizing annealing was as high as 80% or more, and the internal spheroidization rate was as high as 70% or more. As a result, the limit compressibility of the round steel materials of test numbers 1 to 7 exceeded 50%, and excellent cold forgeability was exhibited.

  On the other hand, in test number 8, the C content of the steel material was too high. Therefore, the fine LP rate in the microstructure of the surface layer region was 10% or more. As a result, the critical compression ratio was 50% or less.

  In Test No. 9, although the chemical composition of the steel material was appropriate, the heating temperature was too high and the temperature after cooling was too high. Therefore, spherical cementite did not exist in the microstructure of the round steel material. Furthermore, the ferrite in the whole round steel material and the microstructure of the surface layer region was not refined, and the average crystal grain size of the ferrite was too large. Therefore, the surface spheroidization rate and the internal spheroidization rate after spheroidizing annealing were low, and the critical compression rate was 50% or less.

  In test number 10, although the chemical composition of the steel material was appropriate, the temperature after cooling was too high. Therefore, spherical cementite did not exist in the microstructure of the round steel material, and the ferrite was also coarse. Therefore, the critical compression rate was 50% or less.

  In test number 11, although the chemical composition of the steel material was appropriate, the temperature after rolling was too low. Therefore, the fine LP rate in the whole round steel material and the microstructure of the surface layer region was too high. Therefore, the surface spheroidization rate and the internal spheroidization rate after spheroidizing annealing were low, and the critical compression rate was 50% or less.

  In test number 12, although the chemical composition of the steel material was appropriate, the temperature after cooling was too low. Therefore, the fine LP rate in the whole round steel material and the microstructure of the surface layer region was too high. Therefore, the surface spheroidization rate and the internal spheroidization rate after spheroidizing annealing were low, and the critical compression rate was 50% or less.

  The round steel for cold forging of this embodiment has a high spheroidization rate and is excellent in cold forgeability after spheroidizing annealing. Therefore, it can be widely applied to applications requiring excellent cold forgeability. The round steel material for cold forging of the present embodiment is used as a material for machine structural parts such as automobile parts, industrial machine parts, construction machine parts, etc., which have been manufactured in the hot forging process and the cutting process so far. be able to. Particularly when used in such applications, the cold forged round steel material of the present embodiment can contribute to the near net shaping of parts.

Claims (3)

A round steel material for cold forging,
% By mass
C: 0.15-0.60%,
Si: 0.01 to 0.5%,
Mn: 0.1 to 2.0%,
P: 0.035% or less,
S: 0.050% or less,
Al: 0.050% or less,
Cr: 0.02 to 0.5%,
N: 0.003-0.030%,
Cu: 0 to 0.5%,
Ni: 0 to 0.3%,
Mo: 0 to 0.3%,
V: 0 to 0.3%
B: 0 to 0.0035%,
Nb: 0 to 0.050% and
Ti: 0 to 0.2%,
The balance has a chemical composition consisting of Fe and impurities,
The microstructure of the cold forged round steel material is composed of ferrite, pearlite and spherical cementite, the ferrite has an average crystal grain size of 8 μm or less, and the pearlite having a lamellar spacing of 200 nm or less is the microstructure of the pearlite. The area ratio is less than 20%,
In the microstructure in the region from the surface of the round steel material for cold forging to the radius x 0.15 depth, the average grain size of ferrite in the region is 4 μm or less, and the pearlite having a lamellar spacing in the region of 200 nm or less area percentage of microstructure of the region is less than 10%, the number of spherical cementite 1.0 × 10 ⁵ cells in the region / mm ² or more der is, the cold forging round steel at 735 ° C. A round steel material for cold forging, which has a limit compression ratio exceeding 50% when held for 10 hours and then cooled to room temperature at a cooling rate of 10 ° C./h . A round steel material for cold forging according to claim 1,
Cu: 0.05 to 0.5%,
Ni: 0.05-0.3%
Mo: 0.05-0.3%
V: 0.05-0.3% and
B: A round steel material for cold forging containing one or more selected from the group consisting of 0.0005 to 0.0035%. A cold forging round steel material according to claim 1 or claim 2,
Nb: 0.005 to 0.050% and
Ti: A round steel material for cold forging containing one or two selected from the group consisting of 0.005 to 0.2%.

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