KR101655006B1 - Steel wire rod or bar steel - Google Patents

Abstract

Wherein the chemical composition is 0.1 to 0.6% of C, 0.01 to 1.5% of Si, 0.05 to 2.5% of Mn, 0.015 to 0.3% of Al, 0.004 to 0.015% of N, And P and S in the impurity are 0.035% or less of P and 0.025% or less of S, and the surface layer or the bar steel from the surface to the depth of 15% Region is a ferrite having an average grain size of 1 to 15 占 퐉, a steel structure of spherical cementite having an average aspect ratio of 2 or less and an average grain size of 0.1 to 2 占 퐉 and a depth of 25% The internal region is a steel structure composed of ferrite having an average particle diameter of 15 to 40 占 퐉 and pearlite and / or spheroidizing cementite, and the surface roughness (Ra) in the circumferential direction of the surface after removal of the surface scale is 4 占 퐉 or less, A steel wire rod or rod having a depth of the grain boundary oxide layer of 30 탆 or less .

Description

{STEEL WIRE ROD OR BAR STEEL}

The present invention includes a steel wire rod or a bar steel coil (bar in coil) suitable for cold forging or the like. Hereinafter the same). The present application claims priority based on Japanese Patent Application No. 2012-131316 filed on June 8, 2012, the contents of which are incorporated herein by reference.

In recent years, there has been an increasing demand for application to cold forging of heavy carbon steels due to reduction or omission of machining, which is representative of cutting, from the improvement of productivity. However, since carbon steel has a higher deformation resistance and lacks deformability (ductility) as compared with low carbon steel or low alloy steel, there is a problem that breakage of the mold and breakage of the steel are likely to occur. Therefore, it is general to perform spheroidizing annealing to spheroidize cementite in order to reduce deformation resistance and improve deformability in a steel material to be provided for cold forging. Patent Document 1 discloses that, in the region from the surface to the bar wire radius of 0.15 in depth, the area ratio of the ferrite is 10% or less and the remaining amount is one or two of tempering martensite or tempering martensite and bainite and pearlite And the core portion is a ferrite pearlite, which is a rod for cold forging after spheroidizing annealing.

Patent Document 2 discloses a steel sheet having a ferrite decarburized layer having a depth of 0.01 to 0.5 mm in a steel surface layer portion and having a total decarburization region layer containing the ferrite decarburized layer in a range of 0.039 to 0.37 in terms of a steel material radius, Discloses a cold forging steel having excellent workability by forming a spheroidizing cementite structure.

Patent Document 3 discloses a cementite having a ferrite particle diameter of 2 to 5.5 m and a long diameter of 3 m or less and an aspect ratio of 3 or less expressed by a long diameter / To 70%, which is excellent in cold working.

Patent Document 4 discloses a high carbon steel plate having excellent cold workability and hardenability by specifying ferrite particle size, cementite particle size, cementite aspect ratio, and cementite density ratio.

Patent Document 5 discloses a method for producing a high carbon steel sheet excellent in moldability and hardenability, in which cementite having an average particle diameter of 1.1 μm or less and an average aspect ratio of 1.5 or less and ferrite particles having an average particle diameter of 2 μm or more are formed.

Patent Document 6 discloses that when an area ranging from 5 to 30% of the diameter of a line drawn from the surface is a surface layer, an average particle diameter of the surface layer is 5 占 퐉 or less and a depth of 0.3 to 0.4 mm from the outermost surface of the surface layer Wherein the outermost layer has an average grain size of 2 占 퐉 or more and an inside grain diameter of the inside of the surface layer is 10 占 퐉 or less.

Patent Document 7 discloses a ferrite having an average particle size of ferrite of 2 to 5.5 占 퐉, a long diameter of 3 占 퐉 or less and an aspect ratio expressed by a long diameter / short diameter of 3 or less Discloses a steel wire rod excellent in cold workability, wherein the cementite has a cementite content of 70% or more. However, only the steel structure of ferrite and cementite is specified, and the surface roughness and the grain boundary oxidation depth of the surface which greatly influence cracking in cold forging are not specified.

The methods disclosed in Patent Documents 1 to 7 relate to a technique for preventing breakage of a steel material, which is inherently problematic in cold working with a high degree of processing, but recently, improvement of the cold-rolled composition of a single layer has been required.

Japanese Patent No. 4435954 Japanese Patent No. 3167550 Japanese Patent Application Laid-Open No. 2000-192148 Japanese Patent No. 3468172 Japanese Patent No. 3577957 Japanese Patent Application Laid-Open No. 2000-119806 Japanese Patent No. 3527641

The present invention has been made in view of the above-described circumstances. It is an object of the present invention to provide a steel wire rod or rod steel which can prevent cracking of a steel material which is a factor of inhibiting cold shortening in processing with high degree of processing.

Means for Solving the Problems The present inventors have made extensive studies in order to solve the above problems. As a result, the present inventors found that it is useful to appropriately control the surface roughness of the steel material and the depth of the grain boundary oxide layer in addition to the steel material, the steel structure after the spheroidizing annealing, and the like in order to improve the deformability to prevent cracking of the steel during cold forging Respectively.

The present invention has been made based on the above-described novel knowledge, and the gist of the present invention is as follows.

(One)

The chemical composition, in% by mass,

C: 0.1 to 0.6%

Si: 0.01 to 1.5%

Mn: 0.05 to 2.5%

Al: 0.015 to 0.3%

N: 0.004 to 0.015%

Cr: 0 to 3.0%

Mo: 0 to 1.5%

Cu: 0 to 2.0%

Ni: 0 to 5.0%

B: 0 to 0.0035%,

Ca: 0 to 0.005%,

Zr: 0 to 0.005%,

Mg: 0 to 0.005%,

Rem: 0 to 0.015%,

Ti: 0 to 0.2%

Nb: 0 to 0.1%

V: 0 to 1.0%

W: 0 to 1.0%

Sb: 0 to 0.0150%,

Sn: 0 to 2.0%

Zn: 0 to 0.5%

Te: 0 to 0.2%

Bi: 0 to 0.5%

Pb: 0 to 0.5%

, The balance being composed of iron and impurities,

Wherein P and S in the impurity are,

P: not more than 0.035%

S: 0.025% or less,

And the surface layer region from the surface to the depth of 15% of the cross-sectional radius is composed of ferrite having an average particle diameter of 1 to 15 占 퐉 and a ferrite having an average aspect ratio of 2 or less and an average particle diameter of 0.1 to 2 占 퐉 And an inner region from a depth of 25% of a cross-sectional radius from the surface to the center is a steel structure composed of ferrite having an average particle diameter of 15 to 40 탆 and pearlite and / or spheroidizing cementite, Wherein the surface roughness (Ra) in the circumferential direction of the surface after removing the scale is 4 占 퐉 or less and the depth of the grain boundary oxide layer on the surface is 30 占 퐉 or less.

(2)

In terms of% by mass,

Cr: 0.1 to 3.0%

Mo: 0.01 to 1.5%

Cu: 0.1 to 2.0%

Ni: 0.1 to 5.0%

B: 0.0005 to 0.0035%

(1), wherein the steel wire rod or rod steel contains one or more of the following.

(3)

Ca: 0.0002 to 0.005%

Zr: 0.0003 to 0.005%

Mg: 0.0003 to 0.005%

Rem: 0.0001 to 0.015%

(1) or (2), wherein the steel wire rod or rod steel contains one or more of the following.

(4)

Ti: 0.001 to 0.2%

Nb: 0.01 to 0.1%

V: 0.03 to 1.0%

W: 0.01 to 1.0%

(1) to (3), wherein the steel wire rod or rod steel contains one or more of the following.

(5)

Sb: 0.0005 to 0.0150%,

Sn: 0.005 to 2.0%

Zn: 0.0005 to 0.5%

Te: 0.0003 to 0.2%

Bi: 0.005 to 0.5%

Pb: 0.005 to 0.5%

(1) to (4), wherein the steel wire rod or rod steel contains one or more of the above-mentioned steel wire rod or rod steel.

Industrial Applicability The present invention makes it possible to realize cold forging having a high degree of workability, which has not been possible in the prior art, or omission of intermediate annealing in a process which has not been possible for cold forging without conventional intermediate annealing, by preventing the steel material from cracking during cold forging.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram illustrating an outline of a rolling line according to the present invention. Fig.
Fig. 2 is a diagram illustrating an outline of quenching immediately after rolling according to the present invention. Fig.

Hereinafter, embodiments for carrying out the present invention will be described in detail.

First, the reason for limiting the chemical components of the present invention will be described. Hereinafter,% in composition means% by mass.

C: 0.1 to 0.6%

C is an element that greatly affects the basic strength of the steel. However, when the C content is less than 0.1%, sufficient strength can not be obtained, so that it is inevitable to inject a larger amount of other alloying elements. On the other hand, when the C content exceeds 0.6%, the hardness of the material increases, which causes a significant increase in deformation resistance and a drastic decrease in machinability. Therefore, in the present invention, the C content is set to 0.1 to 0.6%. Further, the present invention includes cases where the C content is 0.1% and 0.6%. When high-frequency quenching is performed to secure the strength as a component, the C content is 0.3 to 0.6%, and more preferably the C content is 0.4 to 0.6%.

Si: 0.01 to 1.5%

Si is an element effective for deoxidizing steel and is an effective element for enhancing ferrite strengthening and temper softening resistance. When the content of Si is less than 0.01%, the effect is insufficient, and when the content exceeds 1.5%, the steel becomes brittle and the workability drastically decreases, and the sinkability is impaired. Therefore, the Si content needs to be within a range of 0.01 to 1.5%. The present invention also includes cases where the Si content is 0.01% and 1.5%.

Mn: 0.05 to 2.5%

Mn fixes and disperses S in the steel as MnS. Further, Mn is an element necessary for improving the hardness of the matrix and ensuring the strength after quenching. However, when the Mn content is less than 0.05%, S in the steel is combined with Fe to form FeS, and the steel becomes brittle. On the other hand, when the Mn content is increased, specifically, when the Mn content exceeds 2.5%, the hardness of the substrate is increased to lower the cold workability, and the effect on the strength and hardenability is also saturated. Therefore, the Mn content is set to 0.05% to 2.5%. The present invention also includes cases where the Mn content is 0.05% and 2.5%. The fit range is 0.30 to 1.25%.

Al: 0.015 to 0.3%

In addition to deoxidation of steel, Al generates nitrides to suppress coarsening of crystal grains. Further, Al is useful for securing solid solution B in the case where solid solution N existing in the steel is fixed as AlN and B is contained in the steel. 0.015% or more is necessary to obtain the above effect. However, when it exceeds 0.3%, Al ₂ O ₃ is excessively generated, which causes a decrease in fatigue strength or cracking in cold forging, so that the Al content is set to 0.015 to 0.3%. The present invention also includes cases where the Al content is 0.015% and 0.3%.

N: 0.004 to 0.015%

N combines with Al, Ti, Nb, and V in the steel to produce nitride or carbonitride, thereby suppressing crystal grain coarsening. On the other hand, when the content is less than 0.004%, the effect is insufficient. When the content exceeds 0.015%, the effect is saturated, and the unused carbonitrides remain during hot rolling or hot forging, Which makes it difficult to increase the effective amount of fine carbonitride. Therefore, it is necessary to set the N content within the range of 0.0040 to 0.015%. Further, the present invention includes cases where the N content is 0.004% and 0.015%.

Further, one or more of Cr, Mo, Cu, Ni, and B may be contained as an optional element for improving the hardenability and imparting strength.

Cr: 0 to 3.0%

Cr is an element which imparts a temper softening resistance while improving the quenching property, and the steel which needs high strength may contain Cr. In order to stably obtain these effects, Cr is preferably contained in an amount of 0.1% or more. Further, when Cr is contained in a large amount (specifically, more than 3.0%), Cr carbide is produced and the steel is brittle. Therefore, when Cr is contained, the content thereof is set to 3.0% or less. The present invention also includes a case where the Cr content is 3.0%.

Mo: 0 to 1.5%

Mo is an element which imparts temper softening resistance and improves the quenching property, and the steel which needs high strength may contain Mo. In order to stably obtain these effects, Mo is preferably contained in an amount of 0.01% or more. Even if Mo is contained in excess of 1.5%, the effect is saturated. Therefore, when Mo is contained, its content is set to 1.5% or less. Further, the present invention includes the case where the Mo content is 1.5%.

Cu: 0 to 2.0%

Cu is an element effective for strengthening ferrite, improving quenching and improving corrosion resistance. In order to stably obtain these effects, Cu is preferably contained in an amount of 0.1% or more. Even if Cu is contained in an amount exceeding 2.0%, the effect is saturated in terms of mechanical properties. Therefore, when Cu is contained, its content is set to 2.0% or less. The present invention also includes a case where the Cu content is 2.0%. Cu is particularly preferably added at the same time as Ni since it lowers hot ductility and tends to cause scratches during rolling.

Ni: 0 to 5.0%

Ni is an element effective for strengthening ferrite to improve ductility and also to improve quenching and improve corrosion resistance. In order to stably obtain these effects, Ni is preferably contained in an amount of 0.1% or more. Further, even if Ni is contained in an amount exceeding 5.0%, the effect is saturated in view of the mechanical properties and the machinability is lowered. Therefore, when Ni is contained, its content is set to 5.0% or less. The present invention also includes a case where the Ni content is 5.0%.

B: 0 to 0.0035%

Solid solution B is segregated at the grain boundaries, improving the quenching property and improving the grain boundary strength, thereby improving fatigue strength and impact strength as mechanical parts. In order to stably obtain these effects, B is preferably contained in an amount of 0.0005% or more. In addition, even when B is contained in an amount exceeding 0.0035%, the effect is saturated in view of the mechanical properties, and the hot ductility is remarkably lowered. Therefore, when B is contained, its content is 0.0035% or less. Further, the present invention includes the case where the B content is 0.0035%.

As the optional element, one or more of Ca, Zr, Mg, and Rem may be contained.

Ca: 0 to 0.005%

Ca is an element of deoxidation, which produces oxides. In a steel containing 0.015% or more of total Al (T-Al) as in the steel of the present invention, when calcium is contained, calcium aluminate (CaOAl ₂ O ₃ ) is formed. Since this CaOAl ₂ O ₃ is a low melting point oxide as compared with Al ₂ O ₃ , it becomes a tool protection film at high speed cutting and improves machinability. In order to stably improve machinability, the Ca content is preferably 0.0002% or more. When the Ca content exceeds 0.005%, CaS is generated in the steel, and the machinability is rather lowered. Therefore, when Ca is contained, the content thereof is made 0.005% or less. Further, the present invention includes the case where the Ca content is 0.005%.

Zr: 0 to 0.005%

Zr is a deoxidizing element that produces oxides in the steel. The oxide is thought la ZrO _2. Since this ZrO ₂ serves as precipitation nuclei of MnS, it has an effect of increasing the precipitation sites of MnS and uniformly dispersing MnS. Zr is also dissolved in MnS to form a complex sulfide, and its effect is lowered to suppress the stretching of the MnS shape at the time of rolling and hot forging. As described above, Zr is an element effective for reducing anisotropy. In order to effectively and stably obtain these effects, the Zr content is preferably 0.0003% or more. On the other hand, even if Zr is contained in an amount exceeding 0.005%, the yield becomes extremely poor. In addition, a large amount of hard compounds such as ZrO ₂ and ZrS is produced, and mechanical properties such as machinability, impact value and fatigue characteristics are deteriorated. Therefore, when Zr is contained, the content thereof is made 0.005% or less. The present invention also includes a case where the Zr content is 0.005%.

Mg: 0 to 0.005%

Mg is a deoxidizing element and produces oxides in the steel. When Al deoxidation is a premise, Al ₂ O _3, which is harmful to machinability, is reformed into MgO, Al ₂ O ₃ and MgO which are relatively soft and finely dispersed. Further, the oxide easily becomes a nucleus of MnS and also has an effect of finely dispersing MnS. In order to obtain these effects stably, the Mg content is preferably 0.0003% or more. Further, Mg forms a complex sulfide with MnS to form spherical MnS, but when Mg is excessively contained, concretely, when the Mg content exceeds 0.005%, the production of MgS alone is promoted and the machinability is deteriorated . Therefore, when Mg is contained, the content thereof is made 0.005% or less. Further, the present invention includes the case where the Mg content is 0.005%.

Rem: 0 ~ 0.015%

Rem (rare earth element) is a deoxidizing element, which produces a low melting point oxide and inhibits nozzle clogging during casting. In addition, there is also an action of suppressing the stretching of the MnS shape during rolling and hot forging by reducing the deformability of the MnS by solid solution or bonding. As described above, Rem is an effective element for reduction of anisotropy. In order to stably obtain the effect, the Rem content is preferably 0.0001% or more in total. On the other hand, if the Rem content exceeds 0.015%, a large amount of Rem sulfide is produced and the machinability is deteriorated. Therefore, when Rem is contained, its content is made 0.015% or less. Further, the present invention includes the case where the Rem content is 0.015%.

Further, one or more of Ti, Nb, V, and W may be contained as optional elements in order to increase the strength due to the formation of the carbonitride or to stabilize the austenitic grains by increasing the amount of the carbonitride.

Ti: 0 to 0.2%

Ti is an element contributing to the inhibition or strengthening of austenite lips by forming carbonitride and is used as a stabilizing element for prevention of coarseness in steel which requires high strength and low strength. Further, Ti is a deoxidizing element and has an effect of improving machinability by forming a soft oxide. In order to stably obtain the above effect, the content is preferably 0.001% or more. On the other hand, if the Ti content exceeds 0.2%, coarse carbonitride as a cause of hot cracking is precipitated and the mechanical properties are deteriorated. Therefore, in the case of containing Ti in the present invention, the content thereof is set to 0.2% or less. The present invention also includes a case where the Ti content is 0.2%.

Nb: 0 to 0.1%

Nb is also an element contributing to the strengthening of steel by secondary precipitation hardening and the inhibition and strengthening of austenite lips by forming carbonitride, and steel which requires high strength and low deformation is required for the prevention of coarsening It is used as a sizing element. In order to obtain this effect stably, the Nb content is preferably 0.01% or more. Further, when Nb is added in an amount exceeding 0.1%, coarse carbonitride as a cause of hot cracking precipitates, and the mechanical properties are deteriorated. Therefore, when Nb is contained, its content is set to 0.1% or less. The present invention also includes the case where the Nb content is 0.1%.

V: 0 to 1.0%

V is also an element contributing to the strengthening of steel by secondary precipitation hardening and the inhibition and strengthening of austenite lips by forming carbonitride, and steel which requires high strength and low deformation is required for preventing It is used as a sizing element. In order to obtain this effect stably, the V content is preferably 0.03% or more. In addition, when V is contained in an amount exceeding 1.0%, coarse carbonitride as a cause of hot cracking is precipitated and the mechanical properties are deteriorated. Therefore, when V is contained, its content is set to 1.0% or less. The present invention also includes a case where the V content is 1.0%.

W: 0 to 1.0%

W is also an element capable of forming carbonitride and strengthening the steel by secondary precipitation hardening. In order to obtain this effect stably, the W content is preferably 0.01% or more. In addition, when W is contained in an amount exceeding 1.0%, coarse carbonitride which is a cause of hot cracking is precipitated and the mechanical properties are deteriorated. Therefore, when W is contained, the content thereof is set to 1.0% or less. The present invention also includes a case where the W content is 1.0%.

In order to improve machinability, one or more of Sb, Sn, Zn, Te, Bi, and Pb may be contained as an optional element.

Sb: 0 to 0.0150%

Sb improves machinability by embrittling ferrite appropriately. The effect is remarkable particularly when the amount of solid solution Al is large, and the Sb content is preferably 0.0005% or more. When the Sb content is increased, more specifically, when the Sb content is more than 0.0150%, the macrosegregation of Sb becomes excessive and the impact value greatly decreases. Therefore, the Sb content should be 0.0150% or less. Further, the present invention includes the case where the Sb content is 0.0150%.

Sn: 0 to 2.0%

Sn has the effect of increasing the tool life by brittle of ferrite and enhancing the surface roughness after cutting. In order to obtain the effect stably, the Sn content is preferably 0.005% or more. Even if Sn is contained in an amount exceeding 2.0%, the effect is saturated. Therefore, when Sn is contained, its content is set to 2.0% or less. The present invention also includes a case where the Sn content is 2.0%.

Zn: 0 to 0.5%

Zn has the effect of increasing the tool life by brittle of ferrite and improving the surface roughness after cutting. In order to obtain the effect stably, the Zn content is preferably 0.0005% or more. Even if Zn is contained in an amount exceeding 0.5%, the effect is saturated. Therefore, when Zn is contained, the content thereof is set to 0.5% or less. The present invention also includes a case where the Zn content is 0.5%.

Te: 0 to 0.2%

Te is an element for improving machinability. Further, MnTe production and coexistence with MnS reduce the deformability of MnS and inhibit the stretching of the MnS shape. Thus, Te is an element effective for reducing anisotropy. In order to obtain these effects stably, the Te content is preferably 0.0003% or more. On the other hand, if the Te content exceeds 0.2%, not only the effect is saturated but also the hot ductility is lowered, which is liable to cause scratches. Therefore, when Te is contained, the content thereof is set to 0.2% or less. Further, the present invention includes the case where the Te content is 0.2%.

Bi: 0 to 0.5%

Bi is an element for improving machinability. In order to obtain this effect stably, the Bi content is preferably 0.005% or more. In addition, even if Bi is contained in an amount exceeding 0.5%, not only the machinability improving effect is saturated but also the hot ductility is lowered, which is liable to cause scratches. Therefore, when Bi is contained, its content should be 0.5% or less. The present invention also includes a case where the Bi content is 0.5%.

Pb: 0 to 0.5%

Pb is an element for improving machinability. In order to obtain this effect stably, the Pb content is preferably 0.005% or more. If Pb is contained in an amount exceeding 0.5%, not only the machinability improving effect is saturated but also the hot ductility is lowered, which is liable to cause scratches. Therefore, when Pb is contained, its content is set to 0.5% or less. The present invention also includes a case where the Pb content is 0.5%.

The steel wire rod or bar steel according to the present invention contains the above essential elements and, if necessary, optional elements, and the remaining amount consists of iron and impurities. The impurities include those contained in raw materials such as ores and scrap, and are mixed from the manufacturing environment. However, P and S in impurities are limited to the following ranges.

P: not more than 0.035%

P is contained in the steel as an impurity. However, when the P content exceeds 0.035%, the hardness of the base is increased in the steel, and not only the cold workability but also the hot workability and the casting property are deteriorated. Therefore, the P content should be 0.035% or less. The present invention also includes the case where the P content is 0.035%.

S: not more than 0.025%

S is contained in the steel as impurities. However, when the S content exceeds 0.025%, MnS is coarsened and becomes a starting point of cracking in cold working. For these reasons, it is necessary to set the S content to 0.025% or less. Further, the present invention includes the case where the S content is 0.025%. The fit range is 0.01% or less.

Next, reasons for defining the steel structure and the surface property of the present invention will be described.

The inventors of the present invention have made intensive studies on measures for improving the ductility of the cold forging steel material. As a result, in order to improve the deformability while suppressing an increase in the deformation resistance during cold forging and to prevent forging cracking, it is necessary to make the ferrite of the surface steel layer fine and to coarsen the cementite so that the surface roughness, It is effective to further reduce the depth of the film.

That is, in the present invention, the surface layer region from the surface to the depth of 15% of the cross-sectional radius is composed of ferrite having an average particle diameter of 1 to 15 탆, spherical cementite having an average aspect ratio of 2 or less and an average particle diameter of 0.1 to 2 탆 , And the inner region from the depth to the center of 25% of the cross-sectional radius from the surface is a steel structure consisting of ferrite having an average particle diameter of 15 to 40 占 퐉 and pearlite and / or spheroidizing cementite, Is a steel wire rod or rod steel having a surface roughness (Ra) in the circumferential direction of 4 占 퐉 or less and a depth of the grain boundary oxide layer on the surface of 30 占 퐉 or less.

Experimental investigation of the influence of the ferrite grain size in the case of upsetting the cylindrical column shows that when the ferrite grain size in the surface layer region exceeds 15 탆, the critical compression ratio, which is an index of the deformability in cold forging, is lowered. For this reason, the average ferrite grain size in the surface layer region was limited to 15 탆 or less. When more deformability is required, the average ferrite grain size is preferably 7 占 퐉 or less, more preferably 4 占 퐉 or less. Fine grained steels having an average ferrite grain size of less than 1 탆 remarkably increase the hardness, increase deformation resistance during cold forging, and degrade die life, so that the lower limit is defined as 1 탆 or more. Further, the present invention includes both of the case where the average ferrite grain size in the surface layer region is 15 占 퐉 and the case where the average ferrite grain size is 1 占 퐉.

The larger the particle diameter of the spheroidizing cementite in the surface layer region, the higher the critical compression ratio is. However, when the average particle diameter is less than 0.1 탆, the effect is not exerted. Therefore, it is specified to be 0.1 탆 or more. Preferably 1 mu m or more. When the average particle diameter exceeds 2 탆, the improvement of the critical compression ratio is slowed down, and the spheroidizing annealing time is prolonged and can not be industrially produced. Therefore, the upper limit is set to 2 탆. Further, the present invention encompasses both cases in which the spherical cementite of the surface layer region has an average particle diameter of 2 탆 and 0.1 탆. If the average aspect ratio of the spheroidizing cementite exceeds 2, the critical compression ratio is lowered, so that the average aspect ratio is 2 or less. Further, the present invention includes the case where the average aspect ratio of the spheroidizing cementite is 2.

When the front end face is made of fine ferrite, the deformation resistance during cold forging increases, and the life of the metal mold is lowered. Therefore, in order to improve the critical compression ratio, the micro-ferrite was made to be a surface layer having a depth from the surface of up to 15% of the cross-sectional radius. The average ferrite grain size in the inner region from the depth of 25% of the cross-sectional radius from the surface to the center from the surface was specified to be 15 탆 or more, and the rise of deformation resistance was suppressed. If the average ferrite grain size exceeds 40 占 퐉 and is excessively coarsened, the elongation and draw ratio are lowered. Therefore, the upper limit of the mean ferrite grain size in the inner region from the depth of 25% of the cross-sectional radius to the center of the cross-sectional radius from the surface is defined as 40 탆 or less. Further, the present invention encompasses both cases where the average ferrite grain size in the inner region is 40 占 퐉 and when the average ferrite grain size is 15 占 퐉. The steel structure of the inner region is made of pearlite and / or spheroidized cementite in addition to ferrite. However, since the inner region is in a compressed state at the time of forging, the particle diameter and the aspect ratio of the spheroidizing cementite are not particularly limited from the viewpoint of improvement of the critical compression ratio. The intermediate region from the depth of 15% to the depth of 25% of the cross-sectional radius from the surface is the transition region from the steel structure of the surface layer region to the steel structure of the inner region.

An example of the production conditions for obtaining the above-described structure is shown. The surface of the steel material is once cooled to a temperature not higher than the Ms point temperature by pouring the surface of the steel material immediately after the finish rolling at 750 to 950 캜. Next, the main water is stopped, and the surface temperature of the steel is recuperated to 200 to 700 占 폚 by the retained heat inside. Subsequently, spheroidizing annealing is carried out, which is followed by air cooling once to room temperature, then correction for 20 minutes in the range of Ac1 + 5 deg. C to Ac3-5 deg. C, and gradual cooling at a cooling rate of 5.5 deg. C / h or lower to Ac1-70 deg. The finishing temperature affects the ferrite grain size, and the finishing temperature is set to a low temperature to make the austenite immediately after rolling finer, and the structure after casting and quenching on the surface of the steel becomes finer, and the ferrite grain size after spheroidizing annealing is also miniaturized. The cooling rate of the spheroidizing annealing influences the cementite particle diameter. By slowing the cooling rate, the cementite particle size is coarsened. If the cooling rate is excessively slow, the cementite particle size exceeds 2 탆 in the surface layer region. If the cooling rate is excessively high, pearlite is generated and the spheroidization becomes defective. Therefore, the preferable range of the cooling rate of the spheroidizing annealing is 0.5 to 5.5 占 폚 / h. Here, the Ms point temperature can be calculated from the formula (1), and the Ac1 point temperature can be calculated from the formula (2) (published by Maruzen Maruzen, revised fourth edition metal data book, Feb. 29, 2004 , P162]. The Ac3 point temperature can also be calculated from equation (3) (see "Heat Treatment of Steel 5th Edition" Maruzen, issued on August 20, 1981, P651).

The inner region whose depth from the surface is 25% to the center of the cross-sectional radius in the core water immediately after the rolling is slower in cooling rate than the surface region in which the depth from the surface is 15% or less of the cross-sectional radius. As a result, after the water injection, it is not a quenched structure of martensite or bainite, but a structure in which ferrite and pearlite are mixed. After the spheroidizing annealing, the structure is composed of ferrite and pearlite and / or spheroidizing cementite. In addition, the ferrite grain size becomes larger along the direction from the surface layer to the center.

Next, the reason for defining the surface roughness and the grain boundary oxidation depth will be described.

The characteristic of marginal cracking in the case of upsetting the test piece cut perpendicularly to the longitudinal direction of steel wire rod or bar steel after spheroidizing annealing is affected by the surface roughness of the substrate. As a result of investigating the surface roughness and the marginal cracking characteristics of the bar steel in which the surface roughness was largely changed by shot blasting or pickling under various conditions, the larger the surface roughness, the lower the marginal cracking property. However, if the surface roughness , The marginal cracking property is not lowered, and therefore, the surface roughness (Ra) is defined as 4 m or less. Further, the present invention includes the case where the surface roughness (Ra) is 4 占 퐉.

The marginal cracking characteristics in the upsetting of specimens cut in the direction perpendicular to the longitudinal direction of steel wire rod or bar steel subjected to spheroidizing annealing are affected by the depth of the grain boundary oxide layer on the surface. When the scale produced in the hot rolling is annealed in the state of remaining on the surfaces of the steel wire rods and the steel bar, the scale becomes an oxygen supply source and Si, Mn and Cr having higher affinity with oxygen than Fe are oxidized preferentially during the spheroidizing annealing , A grain boundary oxide layer is generated along the grain boundary from the surface. Depth and marginal cracking characteristics of the grain boundary oxide layer were investigated for steel wire rods and rod steels after spheroidizing annealing, which greatly changed the depth of the grain boundary oxide layer. As a result, it has been found that the critical cracking property decreases as the depth of the intergranular oxide layer becomes deeper, but the critical cracking characteristic is not lowered by setting the depth of the intergranular oxide layer to 30 탆 or less. Therefore, the depth of the intergranular oxide layer was defined to be 30 占 퐉 or less.

In order to reduce the surface roughness Ra to 4 mu m or less and the depth of the grain boundary oxide layer to 30 mu m or less, it is necessary that scale removal before spheroidizing annealing is appropriately performed by a method such as pickling or shot blasting. Excessive pickling or shot blasting deteriorates the surface roughness of the steel. On the contrary, in the case of insufficient pickling or shot blasting, the scale of the surface of the steel remains, and the depth of the grain boundary oxide layer after spheroidizing annealing deteriorates. In order to obtain a surface roughness (Ra) of 4 占 퐉 or more and a depth of the intergranular oxide layer to 30 占 퐉 or less, the pickling is carried out in a hydrochloric acid solution at a concentration of 10% by mass and a temperature of 60 占 폚 for 4 to 14 minutes , More preferably 5 to 10 minutes). Sulfuric acid may be used in addition to hydrochloric acid for pickling. In the case of performing shot blasting, a steel ball having a diameter of 0.5 mm and a hardness of 47.3 HRC is projected at a projection density of 90 kg / m 2 or more and a projection speed of 70 m / s. The projection density X (kg / m 2) is defined by the formula (4) from the mass W (kg / min) of the projection material per unit time, the projection width B (m) of the projection material, do.

Example

Hereinafter, the present invention will be described in detail with reference to Examples. These examples are for the purpose of illustrating the present invention and do not limit the scope of the present invention.

A billet having a chemical composition shown in Table 1 and having a side length of 162 mm was rolled into a rod steel having a diameter of 45 mm under the conditions shown in Table 2, quenched immediately after rolling and repaired, and then air-cooled. The rolling finishing temperature, cooling temperature and double heat temperature were measured with a radiation thermometer. The positional relationship of each radiation thermometer, rolling mill, water cooling apparatus and cooling phase is shown in Fig. 1 and temperature trend is shown in Fig. That is, as shown in Fig. 1 illustrating the outline of the rolling line according to the present invention, the billets heated in the heating furnace 1 are rolled in the hot rolling mill 2 and the finishing temperature is measured by the finishing radial thermometer 3 Respectively. Immediately after the rolling, was quenched in the water-cooling apparatus 4, and the temperature after cooling was measured by a water-cooling radiation thermometer (5). After the heating, the double heating temperature was measured with a double heat radiation thermometer (6), and the cooled room (7) was air-cooled. As shown in Fig. 2 illustrating the quenching immediately after rolling according to the present invention, the surface of the steel material immediately after the finish rolling at a finishing temperature (8) of 750 to 950 deg. 11) was cooled to a cooling temperature (9) below the Ms point temperature and then recuperated to the double reflux temperature (10) from 200 to 700 占 폚 with the retained heat therein, followed by cooling on the cooling phase. On the other hand, as a result of calculation of the temperature change of the steel material center portion by the two-dimensional unsteady heat transfer difference model from the measured values of the finishing temperature and the double heat temperature of the steel material surface layer temperature, the temperature change of the steel material center portion 12 is slower than the surface portion , And was not cooled below the Ms point temperature.

The obtained steel material was scaled off by pickling or shot blast, and then subjected to spheroidizing annealing. After the spheroidizing annealing, test specimens were taken from the rod steel, and microstructure and surface roughness were investigated. Further, up-setting test was performed on a compression test piece cut in a direction perpendicular to the longitudinal direction at a length of 1.5 times the rolled diameter in the longitudinal direction, and the critical compression ratio was examined. The results are summarized in Table 3.

[Microstructure]

The average particle diameter and aspect ratio of spherical cementite were determined by image analysis of a scanning electron microscope photograph. The surface layer region was formed from a surface layer of four directions having a central angle of 90 degrees different from the cut surface (C section) cut perpendicularly to the longitudinal direction of the rod steel at a total of eight sites of a depth of 200 탆 and a depth of 15% , And the internal area was observed at a magnification of 3000 times at nine points in total of nine points of the C section, the depth of 25% of the radius of the four directions whose central angles were 90 degrees different, the depth of 50% of the radius, Were analyzed by an image analysis apparatus. The average particle diameter was a circle equivalent diameter. The aspect ratio was (length of long diameter) / (length of short diameter). The average values of the surface layer regions (8 locations) and the inner regions (9 locations) were obtained.

Electron-Back-Scattering-Diffraction (EBSD) equipment attached to a scanning electron microscope was used to measure the ferrite particle size. In the surface layer region, a 400 × 400 μm area was measured at eight points in total of the depth of 200 μm and the depth of 15% of the radius from the surface layer in four directions of the C-section of the rod wire at 90 ° From the crystal orientation map of one ferrite, the boundary with the azimuth difference of 15 degrees or more is assumed to be a ferrite grain boundary, and the method of Johnson-Saltykov ("Metrology Morphology" Uchida Rokakuho, S47.7.30 publication, DeHoff, FNRhiness. See P189). The inner region is a region measuring 400 占 400 占 퐉 at nine points in total of the depth C of the C-section of the rod wire, the depth of 25% of the radius of the four directions differing from the central angle by 90 degrees, the depth of 50% , And the average particle diameter was determined in the same manner as described above.

[Surface roughness]

The roughness in the circumferential direction was measured to calculate the Ra defined in JIS B0601: '82.

[Depth of grain boundary oxide layer]

The cross section of the C-section was embedded in resin and polished. Then, the layer was etched and etched by an optical microscope to observe the entire circumference at a magnification of 400 times, and the distance between the deepest oxide layer and the surface layer was measured.

[Critical Compression Ratio]

From the upsetting test under the condition that the compression rate is 10 mm / min, the compression ratio at which the failure probability is 50% is examined. Cracking was defined as breaking cracks having a crack length of 0.05 mm or more. Failure probability is the incidence of breakage. In relation to the mold surface pressure, the compression ratio was set to the upper limit of 80%. When the cracking does not occur at 80% (when the fracture probability is less than 50%), the critical compression ratio is set at 80%.

[Deformation resistance]

The deformation resistance was calculated from the equivalent stress at the time of the equivalent deformation 2 by compressing the deformation resistance at a deformation speed equivalent to 10s-1. As is clear from Table 2, it can be seen that the critical compression ratios of the inventive examples (Nos. 1 to 24) are significantly superior to the critical compression ratios of the comparative examples (Nos. 25 to 34).

In Comparative Example No. 25, the rolling finishing temperature was low and the critical compression ratio was sufficient. However, since the ferrite lips are fine to the center portion, the deformation resistance is high and the life of the mold is lowered.

In Comparative Example No. 26, the cooling rate at the time of spheroidizing annealing was slow and the average cementite particle size was coarse, which exceeded the specification of the present invention, and the critical compression ratio was lowered. In Comparative Example No. 27, the critical compression ratio was lowered because the rolling finishing temperature was high and the average ferrite grain size was coarse and exceeded the specification of the present invention. In Comparative Example No. 28, the cooling rate at the time of spheroidizing annealing was fast, pearlite was generated during cooling, and the average cementite aspect ratio increased, exceeding the specification of the present invention, and the critical compression ratio was lowered.

In Comparative Examples Nos. 29 and 30, the chemical components of P or S which lowered the cold workability exceeded the specification of the present application, and as a result, the processing limit was lowered.

In Comparative Example No. 31, since the shot blast was excessive, in Comparative Example No. 34, the pickling was excessive and the surface roughness was increased, exceeding the specification of the present invention, and the critical compression ratio was lowered.

In Comparative Examples Nos. 32 and 33, spheroidization annealing was performed in a state in which scale removal was insufficient, so that a grain boundary oxide layer having a depth of 30 占 퐉 or more was generated, exceeding the requirements of the present application, and the critical compression ratio was lowered.

1: heating furnace
2: Hot rolling mill
3: Finishing radiation thermometer
4: Water cooling device
5: Water cooling thermometer
6: Double Radiation Thermometer
7: Cooling phase
8: Finishing temperature
9: cooling temperature below Ms point temperature
10: Double temperature
11: steel surface layer
12: steel core

Claims (9)

The chemical composition, in% by mass,
C: 0.1 to 0.6%
Si: 0.01 to 1.5%
Mn: 0.05 to 2.5%
Al: 0.015 to 0.3%
N: 0.004 to 0.015%
As an optional component
Cr: 0 to 3.0%
Mo: 0 to 1.5%
Cu: 0 to 2.0%
Ni: 0 to 5.0%
B: 0 to 0.0035%,
Ca: 0 to 0.005%,
Zr: 0 to 0.005%,
Mg: 0 to 0.005%,
Rem: 0 to 0.015%,
Ti: 0 to 0.2%
Nb: 0 to 0.1%
V: 0 to 1.0%
W: 0 to 1.0%
Sb: 0 to 0.0150%,
Sn: 0 to 2.0%
Zn: 0 to 0.5%
Te: 0 to 0.2%
Bi: 0 to 0.5%
Pb: 0 to 0.5%
, The balance being composed of iron and impurities,
Wherein P and S in the impurity are,
P: not more than 0.035%
S: 0.025% or less,
In steel wire rod or rod steel,
The surface layer region from the surface to the depth of 15% of the cross-sectional radius is a steel structure composed of ferrite having an average particle diameter of 1 to 15 占 퐉 and spherical cementite having an average aspect ratio of 2 or less and an average particle diameter of 0.1 to 2 占 퐉 ,
The inner region from the depth to the center of 25% of the cross-sectional radius from the surface is a steel structure composed of at least one of ferrite having an average particle diameter of 15 to 40 탆 and pearlite and spheroidizing cementite,
The surface roughness (Ra) in the circumferential direction of the surface after removing the surface scale is 4 占 퐉 or less,
A steel wire rod or rod steel having a depth of the intergranular oxide layer on the surface of 30 μm or less. The method according to claim 1,
In terms of% by mass,
Cr: 0.1 to 3.0%
Mo: 0.01 to 1.5%
Cu: 0.1 to 2.0%
Ni: 0.1 to 5.0%
B: 0.0005 to 0.0035%
Steel wire rod or rod steel containing one or more of the above. 3. The method according to claim 1 or 2,
Ca: 0.0002 to 0.005%
Zr: 0.0003 to 0.005%
Mg: 0.0003 to 0.005%
Rem: 0.0001 to 0.015%
Steel wire rod or rod steel containing one or more of the above. 3. The method according to claim 1 or 2,
Ti: 0.001 to 0.2%
Nb: 0.01 to 0.1%
V: 0.03 to 1.0%
W: 0.01 to 1.0%
Steel wire rod or rod steel containing one or more of the above. 3. The method according to claim 1 or 2,
Sb: 0.0005 to 0.0150%,
Sn: 0.005 to 2.0%
Zn: 0.0005 to 0.5%
Te: 0.0003 to 0.2%
Bi: 0.005 to 0.5%
Pb: 0.005 to 0.5%
Steel wire rod or rod steel containing one or more of the above. The method of claim 3,
Ti: 0.001 to 0.2%
Nb: 0.01 to 0.1%
V: 0.03 to 1.0%
W: 0.01 to 1.0%
Steel wire rod or rod steel containing one or more of the above. The method of claim 3,
Sb: 0.0005 to 0.0150%,
Sn: 0.005 to 2.0%
Zn: 0.0005 to 0.5%
Te: 0.0003 to 0.2%
Bi: 0.005 to 0.5%
Pb: 0.005 to 0.5%
Steel wire rod or rod steel containing one or more of the above. 5. The method of claim 4,
Sb: 0.0005 to 0.0150%,
Sn: 0.005 to 2.0%
Zn: 0.0005 to 0.5%
Te: 0.0003 to 0.2%
Bi: 0.005 to 0.5%
Pb: 0.005 to 0.5%
Steel wire rod or rod steel containing one or more of the above. The method according to claim 6,
Sb: 0.0005 to 0.0150%,
Sn: 0.005 to 2.0%
Zn: 0.0005 to 0.5%
Te: 0.0003 to 0.2%
Bi: 0.005 to 0.5%
Pb: 0.005 to 0.5%
Steel wire rod or rod steel containing one or more of the above.

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