JPWO2013183648A1 - Steel wire rod or bar - Google Patents

Abstract

Chemical component is mass%, C: 0.1-0.6%, Si: 0.01-1.5%, Mn: 0.05-2.5%, Al: 0.015-0.3% , N: 0.004 to 0.015%, may further contain a predetermined optional element, the balance is iron and impurities, and P and S in the impurities are P: 0.035% or less, S: Steel wire rod or steel bar of 0.025% or less, and the surface layer region from the surface to a depth of 15% of the cross-sectional radius is ferrite having an average particle diameter of 1 to 15 μm and an average aspect ratio of 2 or less. , A steel structure that is spheroidized cementite having an average particle diameter of 0.1 to 2 μm, and an internal region from the surface to a depth of 25% of the cross-sectional radius to the center includes ferrite having an average particle diameter of 15 to 40 μm, It is a steel structure composed of pearlite and / or spheroidized cementite and has a surface scale. Circumferential surface roughness Ra of the surface after removal of the is at 4μm or less, the steel wire rod or steel bar depth of the grain boundary oxidized layer on the surface is 30μm or less.

Description

  The present invention relates to a steel wire rod or steel bar (including a burn-in coil, the same applies hereinafter) suitable for cold forging and the like. This application claims priority based on Japanese Patent Application No. 2012-131316 for which it applied to Japan on June 8, 2012, and uses the content here.

  In recent years, there has been a growing need for cold forging application of medium carbon steel because of the reduction and omission of machining represented by cutting due to the improvement of productivity. However, medium carbon steel has higher deformation resistance than low carbon steel and low alloy steel, and has poor deformability (ductility), so there is a problem that mold cracks and steel material cracks are likely to occur. Therefore, it is common to subject the steel material used for cold forging to spheroidizing annealing for spheroidizing cementite with the aim of reducing deformation resistance and improving deformability. In Patent Document 1, the region from the surface to the depth of the rod wire radius × 0.15 has a ferrite structure area ratio of 10% or less, the balance being tempered martensite or tempered martensite, and bainite and pearlite. It discloses a bar wire rod for cold forging after spheroidizing annealing, which is composed of one or two types and the center portion is ferrite pearlite.

  Patent Document 2 has a ferrite decarburized layer having a depth of 0.01 to 0.5 mm in the steel surface layer portion, and the total decarburized region layer including the ferrite decarburized layer is 0.039 to 0 as a ratio to the steel material radius. The steel for cold forging having excellent workability by having a spheroidized cementite structure in the range of .37 is disclosed.

  Patent Document 3 discloses that in a region of 10% or more from the surface, cementite having a ferrite particle diameter of 2 to 5.5 μm, a major axis of 3 μm or less, and an aspect ratio of 3 or less represented by the major axis / minor axis is total cementite. The steel wire material which is 70% with respect to this and excellent in cold work is disclosed.

  Patent Document 4 discloses a high-carbon steel strip having excellent cold workability and hardenability by defining ferrite grain size, cementite grain size, cementite aspect ratio, and cementite density ratio.

  Patent Document 5 discloses a high-carbon steel sheet excellent in formability and hardenability that forms cementite having an average particle diameter of 1.1 μm or less and an average aspect ratio of 1.5 or less and ferrite grains having an average particle diameter of 2 μm or more. The manufacturing method is disclosed.

  In Patent Document 6, when the surface layer has a depth of 5 to 30% of the wire diameter as a surface layer, the average particle diameter of the surface layer is 5 μm or less, and 0% from the resurface of the surface layer. Cold working, wherein the average particle diameter of the outermost surface layer is 2 μm or more and the average particle diameter inside the surface layer is 10 μm or less when the depth position of 3 to 0.4 mm is the outermost surface layer. A steel wire having excellent properties is disclosed.

  In Patent Document 7, in the region of 10% or more of the wire diameter from the surface, the average particle diameter of ferrite is 2 to 5.5 μm, the major axis is 3 μm or less, and the aspect ratio indicated by the major axis / minor axis is 3 or less. Discloses a steel wire material excellent in cold workability in which no cementite is 70% or more of the total cementite. However, only the steel structure of ferrite and cementite is specified, and the surface roughness and the intergranular oxidation depth of the surface that have a large effect on cracking by cold forging are not specified.

  The methods disclosed in Patent Documents 1 to 7 relate to a technique for preventing cracking of a steel material, which is essentially a problem in cold working with a high degree of work, but recently, further improvements in cold forgeability are required. Has been.

Japanese Patent No. 4435954 Japanese Patent No. 3167550 JP 2000-192148 A Japanese Patent No. 3468172 Japanese Patent No. 3577957 JP 2000-119806 A Japanese Patent No. 3527641

  The present invention has been made in view of the above-described circumstances. This invention makes it a subject to provide the steel wire which can prevent the crack of the steel materials which are the obstructive factor of cold forging in the process with a high workability, or a bar steel.

  The present inventors diligently studied to solve the above problems. As a result, the present inventors have improved the deformability to prevent cracking of the steel during cold forging, in addition to the steel composition and the steel structure after spheroidizing annealing, as well as the surface roughness of the steel and the depth of the grain boundary oxide layer. It was found that it is useful to appropriately control the thickness.

  The present invention has been made on the basis of the above novel findings, and the gist of the present invention is as follows.

(1)
Chemical composition is mass%,
C: 0.1 to 0.6%
Si: 0.01 to 1.5%,
Mn: 0.05 to 2.5%
Al: 0.015-0.3%,
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 consists of iron and impurities,
P and S in the impurity are
P: 0.035% or less,
S: 0.025% or less,
In the steel wire rod or steel bar, the surface layer region from the surface to a depth of 15% of the cross-sectional radius is ferrite having an average particle diameter of 1 to 15 μm, an average aspect ratio of 2 or less, and an average particle diameter of 0. .1 to 2 μm spheroidized cementite steel structure, the inner region from the surface to the depth of 25% of the cross-sectional radius to the center is ferrite with an average particle size of 15 to 40 μm, pearlite and / or spherical A steel structure comprising a cementitious cementite, the surface roughness Ra in the circumferential direction after removing the surface scale is 4 μm or less, and the depth of the grain boundary oxide layer on the surface is 30 μm or less. Or steel bar.

(2)
% 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%,
The steel wire rod or steel bar according to (1), containing one or more of them.

(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%,
The steel wire or steel bar according to (1) or (2), containing one or more of the above.

(4)
Ti: 0.001 to 0.2%,
Nb: 0.01 to 0.1%,
V: 0.03-1.0%,
W: 0.01 to 1.0%
The steel wire rod or steel bar according to any one of (1) to (3), containing one or more of them.

(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%,
The steel wire rod or steel bar according to any one of (1) to (4), containing one or more of them.

  In the present invention, by preventing cracking of the steel material that occurs during cold forging, it is possible to achieve cold forging with a high degree of workability, which was impossible in the past, or cold forging is impossible without conventional intermediate annealing. This makes it possible to omit intermediate annealing in the process.

It is a figure which illustrates the outline | summary of the rolling line in connection with this invention. It is a figure which illustrates the outline | summary of the rapid cooling immediately after rolling concerning this invention.

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

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

C: 0.1 to 0.6%
C is an element that greatly affects the basic strength of steel. However, if the C content is less than 0.1%, sufficient strength cannot be obtained, and a larger amount of other alloy elements must be added. On the other hand, if the C content exceeds 0.6%, the material hardness increases, leading to a significant increase in deformation resistance and a significant decrease in machinability. Therefore, in the present invention, the C content is set to 0.1 to 0.6%. In addition, this invention includes the case where C content is 0.1% and 0.6%. In the case of induction hardening in order to ensure the strength as a part, the C content is 0.3 to 0.6%, more preferably the C content is 0.4 to 0.6%.

Si: 0.01 to 1.5%
Si is an element effective for deoxidation of steel, and is also an element effective for improving the strengthening and temper softening resistance of ferrite. If Si is less than 0.01%, the effect is insufficient, and if it exceeds 1.5%, it becomes brittle, a significant reduction in machinability, and further, carburization is hindered. It must be in the range of ~ 1.5%. In addition, this invention includes the case where Si content is 0.01% and 1.5%.

Mn: 0.05 to 2.5%
Mn fixes and disperses S in steel as MnS. Mn is an element necessary for solid solution in the matrix to improve the hardenability and ensure the strength after quenching. However, if the Mn content is less than 0.05%, S in the steel combines with Fe to become 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 and the cold workability is lowered, and the influence on the strength and hardenability is also affected. Saturates. Therefore, the Mn content is 0.05% to 2.5%. In addition, this invention includes the case where Mn content is 0.05% and 2.5%. The preferred range is 0.30 to 1.25%.

Al: 0.015-0.3%
In addition to deoxidizing steel, Al generates nitrides and suppresses coarsening of crystal grains. Further, Al is useful for securing the solid solution B when the solid solution N existing in the steel is fixed as AlN and B is contained in the steel. In order to acquire said effect, 0.015% or more is required. However, if it exceeds 0.3%, Al2O3 is excessively generated, causing a decrease in fatigue strength and cold forging cracks, so the Al content was made 0.015 to 0.3%. In addition, this invention includes the case where Al content is 0.015% and 0.3%.

N: 0.004 to 0.015%
N combines with Al, Ti, Nb, and V in steel to form nitrides or carbonitrides, and suppresses the coarsening of crystal grains. In addition, if it is less than 0.004%, the effect is insufficient, and if it exceeds 0.015%, the effect is saturated, and insoluble carbonitride during hot rolling or hot forging heating As a result, it becomes difficult to increase the amount of fine carbonitride effective for suppressing the coarsening of crystal grains. Therefore, the N content needs to be in the range of 0.0040 to 0.015%. In addition, this invention includes the case where N content is 0.004% and 0.015%.

  Furthermore, you may contain 1 type (s) or 2 or more types of Cr, Mo, Cu, Ni, and B as an arbitrary containing element for the improvement of hardenability or provision of intensity | strength.

Cr: 0 to 3.0%
Cr is an element that improves hardenability and imparts temper softening resistance, and steel that requires 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 generated and the steel becomes brittle. Therefore, when it contains Cr, the content shall be 3.0% or less. In addition, this invention includes the case where Cr content is 3.0%.

Mo: 0 to 1.5%
Mo is an element that imparts resistance to temper softening and improves hardenability, and steel that requires high strength may contain Mo. In order to stably obtain these effects, Mo is desirably contained in an amount of 0.01% or more. Moreover, even if it contains Mo exceeding 1.5%, the effect will be saturated. Therefore, when it contains Mo, the content shall be 1.5% or less. In addition, this invention includes the case where Mo content is 1.5%.

Cu: 0 to 2.0%
Cu is an element effective for strengthening ferrite and improving hardenability and corrosion resistance. In order to stably obtain these effects, it is desirable to contain Cu by 0.1% or more. Moreover, even if it contains Cu exceeding 2.0%, an effect will be saturated in terms of mechanical properties. Therefore, when it contains Cu, the content shall be 2.0% or less. In addition, this invention includes the case where Cu content is 2.0%. Cu is particularly preferable to be added at the same time as Ni because it lowers the hot ductility and tends to cause defects during rolling.

Ni: 0 to 5.0%
Ni strengthens ferrite, improves ductility, and is an element effective for improving hardenability and corrosion resistance. In order to stably obtain these effects, Ni is desirably contained in an amount of 0.1% or more. Moreover, even if it contains Ni exceeding 5.0%, an effect will be saturated in terms of mechanical properties, and machinability will fall. Therefore, when it contains Ni, the content shall be 5.0% or less. In addition, this invention includes the case where Ni content is 5.0%.

B: 0 to 0.0035%
The solid solution B segregates at the grain boundary, improves the hardenability and improves the grain boundary strength, and improves the fatigue strength and impact strength as a machine part. In order to stably obtain these effects, B is preferably contained in an amount of 0.0005% or more. Moreover, even if it contains B exceeding 0.0035%, the effect is saturated in terms of mechanical properties, and further, hot ductility is remarkably reduced. Therefore, when it contains B, the content shall be 0.0035% or less. In addition, this invention includes the case where B content is 0.0035%.

  Furthermore, you may contain 1 type (s) or 2 or more types of Ca, Zr, Mg, and Rem as an arbitrary containing element.

Ca: 0 to 0.005%
Ca is a deoxidizing element and generates an oxide. In steel containing 0.015% or more as total Al (T-Al) like the steel of the present invention, when Ca is contained, calcium aluminate (CaOAl2O3) is formed. Since CaOAl2O3 is a low melting point oxide compared to Al2O3, it becomes a tool protective film during high-speed cutting and improves machinability. In order to stably improve the machinability, the Ca content is preferably 0.0002% or more. Moreover, when Ca content exceeds 0.005%, CaS will produce | generate in steel and on the contrary, machinability will fall. Therefore, when it contains Ca, the content is made into 0.005% or less. In addition, this invention includes the case where Ca content is 0.005%.

Zr: 0 to 0.005%
Zr is a deoxidizing element and generates an oxide in steel. The oxide is believed to be ZrO2. Since this ZrO2 becomes a precipitation nucleus of MnS, there is an effect of increasing MnS precipitation sites and uniformly dispersing MnS. Zr also has a function of forming a composite sulfide in MnS, reducing its deformability, and suppressing the elongation of the MnS shape during rolling and hot forging. Thus, Zr is an element effective for reducing anisotropy, and in order to obtain these effects effectively and stably, the Zr content is preferably 0.0003% or more. On the other hand, even if the Zr content exceeds 0.005%, the yield is extremely deteriorated. In addition, a large amount of hard compounds such as ZrO 2 and ZrS are generated, and mechanical properties such as machinability, impact value, and fatigue characteristics are lowered. Therefore, when it contains Zr, the content is made 0.005% or less. In addition, this invention includes the case where Zr content is 0.005%.

Mg: 0 to 0.005%
Mg is a deoxidizing element and generates an oxide in steel. If Al deoxidation is premised, Al2O3 harmful to machinability is modified into MgO or Al2O3 and MgO that are relatively soft and finely dispersed. In addition, the oxide tends to be a nucleus of MnS and has an effect of finely dispersing MnS. In order to stably obtain these effects, the Mg content is desirably 0.0003% or more. In addition, Mg forms a composite sulfide with MnS and spheroidizes MnS. When Mg is excessively contained, specifically, when Mg content exceeds 0.005%, a single MgS is formed. Generation is promoted and machinability deteriorates. Therefore, when it contains Mg, the content shall be 0.005% or less. In addition, this invention includes the case where Mg content is 0.005%.

Rem: 0 to 0.015%
Rem (rare earth element) is a deoxidizing element, generates a low melting point oxide, and suppresses nozzle clogging during casting. In addition, it has a function of suppressing the elongation of the MnS shape at the time of rolling and hot forging by dissolving or bonding to MnS to reduce its deformability. Thus, Rem is an element effective for reducing anisotropy, and the Rem content is desirably 0.0001% or more in total in order to stably obtain the effect. On the other hand, if the Rem content exceeds 0.015%, a large amount of Rem sulfide is generated, and the machinability deteriorates. Therefore, when it contains Rem, the content shall be 0.015% or less. In addition, this invention includes the case where Rem content is 0.015%.

  Furthermore, to increase the strength by forming carbonitride and to refine austenite grains by increasing the amount of carbonitride, it contains one or more of Ti, Nb, V, W as optional elements You may do it.

Ti: 0 to 0.2%
Ti is an element that forms carbonitrides and contributes to the suppression and strengthening of austenite grain growth. For steels that require high strength and steels that require low strain, adjustment is required to prevent coarse grains. Used as a granulating element. Ti is also a deoxidizing element and has the effect of improving machinability by forming a soft oxide. In order to stably obtain the above effects, the content is preferably 0.001% or more. On the other hand, if the Ti content exceeds 0.2%, undissolved coarse carbonitrides that cause hot cracking precipitate, and the mechanical properties are impaired. Therefore, when Ti is contained in the present invention, the content is made 0.2% or less. In addition, this invention includes the case where Ti content is 0.2%.

Nb: 0 to 0.1%
Nb is also an element that forms carbonitrides and contributes to steel strengthening by secondary precipitation hardening, suppression of austenite grain growth and strengthening, and steel that requires high strength and steel that requires low strain It is used as a sizing element for preventing coarse grains. In order to obtain this effect stably, the Nb content is desirably 0.01% or more. On the other hand, when Nb is added in excess of 0.1%, undissolved coarse carbonitrides that cause hot cracking are precipitated, and mechanical properties are impaired. Therefore, when it contains Nb, the content is made 0.1% or less. In addition, this invention includes the case where Nb content is 0.1%.

V: 0 to 1.0%
V is also an element that forms carbonitrides and contributes to steel strengthening by secondary precipitation hardening, suppression of austenite grain growth and strengthening, and steel that requires high strength and steel that requires low strain. It is used as a sizing element for preventing coarse grains. In order to stably obtain this effect, the V content is preferably 0.03% or more. On the other hand, if V is contained in excess of 1.0%, undissolved coarse carbonitride that causes hot cracking is precipitated, and mechanical properties are impaired. Therefore, when it contains V, the content shall be 1.0% or less. In addition, this invention includes the case where V content is 1.0%.

W: 0 to 1.0%
W is also an element that forms carbonitride and can strengthen steel by secondary precipitation hardening. In order to stably obtain this effect, the W content is desirably 0.01% or more. Moreover, when it contains W exceeding 1.0%, the undissolved coarse carbonitride which causes a hot crack will precipitate and a mechanical property will be impaired on the contrary. Therefore, when it contains W, the content shall be 1.0% or less. In addition, this invention includes the case where W content is 1.0%.

  Furthermore, in order to improve machinability, you may contain 1 type (s) or 2 or more types of Sb, Sn, Zn, Te, Bi, and Pb as an arbitrary containing element.

Sb: 0 to 0.0150%
Sb moderately embrittles ferrite and improves machinability. The effect is particularly remarkable when the amount of dissolved Al is large, and the Sb content is preferably 0.0005% or more. Further, when the Sb content increases, specifically, when it exceeds 0.0150%, the macrosegregation of Sb becomes excessive and the impact value is greatly reduced. Therefore, the Sb content is 0.0150% or less. In addition, this invention includes the case where Sb content is 0.0150%.

Sn: 0 to 2.0%
Sn has the effect of making the ferrite brittle and extending the tool life and improving the surface roughness after cutting. In order to obtain the effect stably, the Sn content is desirably 0.005% or more. Moreover, even if it contains Sn exceeding 2.0%, the effect will be saturated. Therefore, when it contains Sn, the content shall be 2.0% or less. In addition, this invention includes the case where Sn content is 2.0%.

Zn: 0 to 0.5%
Zn has the effect of embrittlement of ferrite to extend the tool life and improve the surface roughness after cutting. In order to obtain the effect stably, the Zn content is preferably 0.0005% or more. Moreover, the effect will be saturated even if it contains Zn exceeding 0.5%. Therefore, when it contains Zn, the content shall be 0.5% or less. In addition, this invention includes the case where Zn content is 0.5%.

Te: 0 to 0.2%
Te is a machinability improving element. In addition, the generation of MnTe and coexistence with MnS have the effect of reducing the deformability of MnS and suppressing the extension of the MnS shape. Thus, Te is an element effective for reducing anisotropy. In order to stably obtain these effects, the Te content is desirably 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 tends to cause wrinkles. Therefore, when it contains Te, the content is made 0.2% or less. In addition, this invention includes the case where Te content is 0.2%.

Bi: 0 to 0.5%
Bi is an element that improves machinability. In order to stably obtain this effect, the Bi content is desirably 0.005% or more. Moreover, even if Bi is contained exceeding 0.5%, not only the machinability improving effect is saturated, but also the hot ductility is lowered and it is liable to cause flaws. Therefore, when it contains Bi, the content is made 0.5% or less. In addition, this invention includes the case where Bi content is 0.5%.

Pb: 0 to 0.5%
Pb is a machinability improving element. In order to stably obtain this effect, the Pb content is desirably 0.005% or more. Further, if Pb is contained in excess of 0.5%, not only the machinability improving effect is saturated, but also the hot ductility is lowered, which tends to cause wrinkles. Therefore, when it contains Pb, the content is made 0.5% or less. In addition, this invention includes the case where Pb content is 0.5%.

  The steel wire rod or steel bar of the present invention contains the above essential elements and optionally contained elements as necessary, and the balance consists of iron and impurities. Impurities are those contained in raw materials such as ores and scrap, and are introduced from the manufacturing environment. However, P and S in the impurity are limited to the following ranges.

P: 0.035% or less P is contained as an impurity in the steel. However, when the P content exceeds 0.035%, the hardness of the substrate increases in the steel, and not only cold workability but also hot workability and casting characteristics are deteriorated. Therefore, the P content is 0.035% or less. In addition, this invention includes the case where P content is 0.035%.

S: 0.025% or less S is contained as an impurity in the steel. However, if the S content exceeds 0.025%, MnS becomes coarse and becomes a starting point of cracking during cold working. For these reasons, the S content needs to be 0.025% or less. In addition, this invention includes the case where S content is 0.025%. The preferred range is 0.01% or less.

Next, the steel structure of the present invention and the reason for defining the surface properties will be described.
The present inventor has intensively studied on measures for improving the ductility of steel for cold forging. As a result, in order to improve the deformability and prevent forging cracks while suppressing an increase in deformation resistance during cold forging, the ferrite of the surface steel structure is refined, cementite is coarsened, and the surface is further increased. It has been found that it is effective to reduce the roughness and the depth of the grain boundary oxide layer on the surface.

  That is, in the present invention, the surface layer region from the surface to a depth of 15% of the cross-sectional radius is a ferrite having an average particle diameter of 1 to 15 μm, an average aspect ratio of 2 or less, and an average particle diameter of 0.1 to 2 μm. The inner region from the surface to a depth of 25% of the cross-sectional radius to the center is composed of ferrite with an average particle diameter of 15 to 40 μm and pearlite and / or spheroidized cementite. It is a steel wire rod or steel bar that has a steel structure and has a surface roughness Ra in the circumferential direction of the surface after removal of the surface scale of 4 μm or less and a depth of the grain boundary oxide layer on the surface of 30 μm or less.

  As a result of experimentally investigating the effect of ferrite grain size when a cylindrical steel material is installed, if the ferrite grain size in the surface layer exceeds 15 μm, the critical compressibility, which is an index of deformability during cold forging, is reduced. Declined. For this reason, the average ferrite grain size in the surface layer region is limited to 15 μm or less. When more deformability is required, the average ferrite particle size is preferably 7 μm or less, more preferably 4 μm or less. When an ultrafine-grained steel having an average ferrite grain size of less than 1 μm is obtained, the hardness is remarkably increased, the deformation resistance during cold forging is increased, and the die life is reduced. Therefore, the lower limit is defined as 1 μm or more. The present invention includes both cases where the average ferrite grain size in the surface layer region is 15 μm and 1 μm.

  As the particle size of the spheroidized cementite in the surface layer region is larger, the limit compression ratio is improved. However, when the average particle size is less than 0.1 μm, the effect is not exhibited. Therefore, it was defined as 0.1 μm or more. Preferably it is 1 micrometer or more. When the average particle diameter exceeds 2 μm, the improvement in the critical compression rate becomes slow, and the spheroidizing annealing time becomes long and cannot be industrially produced. Therefore, the upper limit is set to 2 μm. In addition, this invention includes both the case where the average particle diameter of the spheroidized cementite in the surface layer region is 2 μm and 0.1 μm. When the average aspect ratio of the spheroidized cementite exceeds 2, the critical compression ratio decreases, so it was set to 2 or less. The present invention includes the case where the average aspect ratio of spheroidized cementite is 2.

  If the entire cross section is made of fine ferrite, the deformation resistance during cold forging increases and the die life is reduced. Therefore, in order to improve the critical compressibility, the fine ferrite is a surface region whose depth from the surface is up to 15% of the cross-sectional radius. And the average ferrite particle diameter of the internal region from the depth of 25% of the cross-sectional radius to the center to the center was defined as 15 μm or more, and the increase in deformation resistance was suppressed. When the average ferrite particle size exceeds 40 μm and is excessively coarse, elongation and drawing are reduced. For this reason, the upper limit of the average ferrite grain size in the inner region from the depth of 25% of the cross-sectional radius to the center to the center is defined as 40 μm or less. In addition, this invention includes both the case where the average ferrite particle diameter of an internal area | region is 40 micrometers, and the case of 15 micrometers. The steel structure in the inner region is pearlite and / or spheroidized cementite in addition to ferrite. However, since the inner region is in a compressed state during forging, the particle size and aspect ratio of the spheroidized cementite are not particularly limited from the viewpoint of improving the critical compression ratio. The intermediate region from the surface to the depth of 15% to 25% of the cross-sectional radius is a transition region from the steel structure in the surface layer region to the steel structure in the inner region.

An example of manufacturing conditions for obtaining the above structure is shown. By pouring water on the surface of the steel material immediately after finish rolling at 750 to 950 ° C., the steel material surface temperature is once cooled below the Ms point temperature. Next, water injection is stopped, and the steel material surface temperature is reheated to 200 to 700 ° C. with internal heat retention. Continuously or after air cooling to room temperature, spheroidizing annealing is carried out for 20 minutes in the range of Ac1 + 5 ° C. to Ac3-5 ° C., and gradually cooled to Ac1-70 ° C. at a cooling rate of 5.5 ° C./h or less. Do. The finishing temperature affects the ferrite grain size, and by lowering the finishing temperature, the austenite immediately after rolling is refined, the structure after water quenching is performed on the steel surface, and the ferrite grains after spheroidizing annealing are refined. The diameter is also refined. The cooling rate of spheroidizing annealing affects the cementite particle size, and slowing the cooling rate coarsens the cementite particle size. If the cooling rate is excessively slow, the cementite particle size exceeds 2 μm in the surface layer region. If the cooling rate is excessively high, pearlite is generated and the spheroidization becomes poor. Therefore, the suitable range of the cooling rate of the spheroidizing annealing is 0.5 to 5.5 ° C./h. Here, the Ms point temperature can be calculated from the equation (1), and the Ac1 point temperature can be calculated from the equation (2) (see “Revised 4th edition Metal Data Book” Maruzen, published on February 29, 2004, P162). . Further, the Ac3 point temperature can be calculated from the equation (3) (“Steel Heat Treatment Revised 5th Edition” Maruzen, issued on August 20, 1981, P651).
Ms (° C.) = 550−361 × (% C) −39 × (% Mn) −35 × (% V) −20 × (% Cr) −17 × (% Ni) −10 × (% Cu) −5 × (% Mn +% W) + 15 × (% Co) + 30 × (% Al) (1)
Ac1 (° C.) = 723-10.7 × (% Mn) −16.9 × (% Ni) + 29.1 × (% Si) + 16.9 × (% Cr) + 6.38 × (% W (2)
Ac3 (° C.) = 908−223.7 × (% C) + 438.5 × (% P) + 30.49 × (% Si) −34.43 (% Mn) −23 × (% Ni) +2 × {100 × (% C) −54 + 6 × (% Ni)} (3)

  When water is poured on the surface of the steel material immediately after rolling, the cooling rate is lower in the inner region where the depth from the surface is 25% of the cross-sectional radius to the center than the surface layer region where the depth from the surface is 15% or less of the cross-sectional radius. . Therefore, after water injection, it does not become a quenched structure of martensite or bainite, but a structure in which ferrite and pearlite are mixed. After spheroidizing annealing, a structure composed of ferrite and pearlite and / or spheroidized cementite is formed. In addition, the ferrite grain size becomes coarser from the surface layer toward the center.

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

  The critical crack characteristics when the steel wire rod or steel bar after spheroidizing annealing is installed with a test piece cut in a direction perpendicular to the longitudinal direction is affected by the surface roughness of the substrate. As a result of investigating the surface roughness and critical cracking properties of steel bars that have been shot blasted or pickled under various conditions and the surface roughness has been greatly changed, the critical cracking properties decrease as the surface roughness increases, but Ra is 4 μm or less. If the surface roughness is made smaller, the critical cracking characteristics will not be lowered, so the surface roughness Ra is specified to be 4 μm or less. In addition, this invention includes the case where surface roughness Ra is 4 micrometers.

  The critical cracking property when a steel wire or steel bar subjected to spheroidizing annealing is installed with a test piece cut in a direction perpendicular to the longitudinal direction is affected by the depth of the surface grain boundary oxide layer. When the scale generated by hot rolling remains spheroidized while remaining on the surface of the steel wire rod and steel bar, the scale becomes an oxygen supply source, and Si, Mn, which has a stronger affinity for oxygen than Fe during spheroidizing annealing, Cr is preferentially oxidized, and a grain boundary oxide layer is generated along the grain boundary from the surface. The depth of the grain boundary oxide layer and the critical crack characteristics were investigated for the steel wire rod and steel bar after spheroidizing annealing in which the depth of the grain boundary oxide layer was greatly changed. As a result, it has been found that the limit cracking property decreases as the depth of the grain boundary oxide layer increases, but the limit cracking property does not decrease by setting the depth of the grain boundary oxide layer to 30 μm or less. Therefore, the depth of the granulated oxide layer was specified to be 30 μm or less.

In order for the surface roughness Ra to be 4 μm or less and the depth of the grain boundary oxide layer to be 30 μm or less, it is necessary to appropriately remove the scale before spheroidizing annealing by a method such as pickling or shot blasting. is there. Excess pickling or shot blasting deteriorates the surface roughness of the steel material. Conversely, inadequate pickling or shot blasting leaves a scale on the surface of the steel material, which worsens the depth of the grain boundary oxide layer after spheroidizing annealing. When the surface roughness Ra is 4 μm or more and the depth of the grain boundary oxide layer is 30 μm or less, when pickling, it is 4 to 14 minutes (preferably 4 to 12 minutes) in a hydrochloric acid solution having a concentration of 10 mass% and a temperature of 60 ° C. , More preferably 5 to 10 minutes). For pickling, sulfuric acid may be used in addition to hydrochloric acid. When shot blasting is performed, 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 calculated from the mass W (kg / min) of the projection material projected per unit time, the projection width B (m) of the projection material, and the steel material conveyance speed V (m / min) (4 ) Expression.
X = W / (B × V) (4) formula

  Hereinafter, the present invention will be specifically described by way of examples. These examples are for explaining the present invention, and do not limit the scope of the present invention.

  A 162 mm square billet having the chemical components shown in Table 1 was rolled into a φ45 mm steel bar under the conditions shown in Table 2, quenched immediately after rolling, reheated, and then air cooled. The rolling finishing temperature, cooling temperature, and recuperation temperature were measured with a radiation thermometer. The positional relationship among each radiation thermometer, rolling mill, water cooling device, and cooling bed is illustrated in FIG. 1, and the temperature transition is illustrated in FIG. That is, as shown in FIG. 1 illustrating the outline of the rolling line according to the present invention, the billet heated in the heating furnace 1 was rolled with a hot rolling mill 2 and the finishing temperature was measured with a finishing radiation thermometer 3. Immediately after rolling, it was quenched with a water cooling device 4, and the temperature after cooling was measured with a water-cooled radiation thermometer 5. After the recuperation, the recuperation temperature was measured with a recuperation radiation thermometer 6 and air cooled with a cooling bed 7. And as shown in FIG. 2 which illustrates the outline of the rapid cooling immediately after rolling according to the present invention, the steel material is poured by pouring water onto the surface of the steel material immediately after finish rolling at a finishing temperature 8 of 750 to 950 ° C. above the A1 point. The temperature transition of the surface layer portion 11 was cooled to a cooling temperature 9 below the Ms point temperature, then reheated to a recuperation temperature 10 of 200 to 700 ° C. with internal retained heat, and then air-cooled on the cooling bed. On the other hand, the temperature transition of the steel material center part is calculated from the surface temperature of the steel material by the two-dimensional unsteady heat conduction difference model from the measured values of the finishing temperature and the recuperation temperature. As a result, it was not cooled below the Ms point temperature.

The obtained steel material was subjected to spheroidizing annealing after scale removal by pickling or shot blasting. After spheroidizing annealing, specimens were collected from the steel bars and examined for microstructure and surface roughness. In addition, an upsetting test was performed on a compression test piece cut in a direction perpendicular to the longitudinal direction at a length that is 1.5 times the rolling diameter in the longitudinal direction, and the limit compression rate was investigated. The results are summarized in Table 3.
[Microstructure]
The average particle diameter and aspect ratio of spherical cementite were determined by image analysis of scanning electron micrographs. The surface layer region is a total of 8 parts with a depth of 200 μm and a depth of 15% of the radius from the surface layer in the 4 directions different from the central angle of the cut surface (C cross section) obtained by cutting the steel bar in the direction perpendicular to the longitudinal direction. In each part, the inner region is observed at a magnification of 3000 times in a total of nine places in the center section, 25% depth of the radius in four directions, 50% of the radius, and 90% of the center angle. The photograph was analyzed with an image analyzer. The average particle diameter was the equivalent circle diameter. The aspect ratio was (length of major axis) / (length of minor axis). The average values of the surface layer regions (8 locations) and the internal regions (9 locations) were determined.

An Electron-Back-Scattering-Diffraction (EBSB) apparatus attached to the scanning electron microscope was used for the measurement of the ferrite particle size. As for the surface layer area, 400 x 400 μm area was measured in each of the 8 areas of 200 μm depth and 15% depth of the radius from the surface layer of the C direction of the bar wire in 4 directions with different central angles of 90 degrees. From the ferrite crystal orientation map, the boundary with an orientation difference of 15 degrees or more is defined as the ferrite grain boundary, and the Johnson-Saltykov method (“Metric morphology” by Uchida Otsutsuru, S47.30, original work: RT) DeHoff, F. N. Rhiness. P189), the average particle size was determined. The inner region is a region of 400 × 400 μm at a total of nine locations of 25% depth, 50% depth, and central portion of the radius in four directions with a 90 ° difference in the central angle of the cross section of the rod and wire. And the average particle size was determined by the same method as described above.
〔Surface roughness〕
The roughness in the circumferential direction was measured, and Ra defined in JIS B0601: '82 was calculated.
[Depth of grain boundary oxide layer]
The C cross-section filled with resin and polished was subjected to nital etching, and the entire circumference was observed with an optical microscope at a magnification of 400 times, and the distance between the deepest grain boundary oxide layer and the surface layer was measured.
[Limit compression ratio]
The compression rate at which the fracture probability was 50% was investigated from an upsetting test under the condition that the compression rate was 10 mm / min. A crack having a crack length of 0.05 mm or more was regarded as a crack. The failure probability is the occurrence rate of cracks. Due to the mold surface pressure, the upper limit of the compression rate is 80%. When cracking did not occur at 80% (when the failure probability was less than 50%), the critical compression ratio was set to 80%.
(Deformation resistance)
The deformation resistance was obtained by compressing at a strain rate equivalent to 10 s-1 and calculating from the equivalent stress at the time of equivalent strain 2. As is clear from Table 2, it can be seen that the critical compression ratio of the inventive examples (No. 1 to 24) is significantly superior to that of the comparative example (No. 25 to 34).

  Comparative Example No. No. 25 has a low rolling finishing temperature and a sufficient critical compression ratio, but is not preferable because ferrite grains are fine up to the center and deformation resistance is high and the mold life is reduced.

  Comparative Example No. In No. 26, the cooling rate during spheroidizing annealing was slow, the average cementite particle size was coarsened and exceeded the provisions of the present application, so the critical compression ratio was lowered. Comparative Example No. No. 27 had a high rolling finish temperature, and the average ferrite grain size was coarsened and exceeded the provisions of the present application, so the critical compression ratio decreased. Comparative Example No. No. 28 had a high cooling rate during spheroidizing annealing, and pearlite was generated during cooling, so that the average cementite aspect ratio increased and exceeded the provisions of the present application.

  Comparative Example No. In Nos. 29 and 30, the chemical component of P or S that decreases the cold workability exceeds the provisions of the present application, and as a result, the processing limit decreases.

  Comparative Example No. Since No. 31 has excessive shot blasting, Comparative Example No. Since the pickling 34 was excessively pickled, the surface roughness increased and exceeded the provisions of the present application, and the critical compression ratio decreased.

  Comparative Example No. Nos. 32 and 33 were subjected to spheroidizing annealing with insufficient scale removal, and a grain boundary oxidation layer having a depth of 30 μm or more was generated, exceeding the provisions of the present application, and the critical compression ratio was lowered.

DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Hot rolling mill 3 Finishing radiation thermometer 4 Water cooling device 5 Water cooling radiation thermometer 6 Recuperated radiation thermometer 7 Cooling floor 8 Finishing temperature 9 Cooling temperature below Ms point temperature 10 Recuperated temperature 11 Steel material surface layer part 12 Steel center

Claims (5)

Chemical composition is mass%,
C: 0.1 to 0.6%
Si: 0.01 to 1.5%,
Mn: 0.05 to 2.5%
Al: 0.015-0.3%,
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 consists of iron and impurities,
P and S in the impurity are
P: 0.035% or less,
S: 0.025% or less,
A steel wire or a steel bar,
The surface layer region from the surface to a depth of 15% of the cross-sectional radius is composed of ferrite having an average particle diameter of 1 to 15 μm and spheroidized cementite having an average aspect ratio of 2 or less and an average particle diameter of 0.1 to 2 μm. Steel structure
The inner region from the surface to a depth of 25% of the cross-sectional radius to the center is a steel structure composed of ferrite having an average particle size of 15 to 40 μm and pearlite and / or spheroidized cementite,
The surface roughness Ra in the circumferential direction of the surface after removing the surface scale is 4 μm or less,
A steel wire rod or steel bar having a surface grain boundary oxide layer depth of 30 μm or less. % 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%,
The steel wire rod or steel bar of Claim 1 containing 1 type, or 2 or more types. Ca: 0.0002 to 0.005%,
Zr: 0.0003 to 0.005%,
Mg: 0.0003 to 0.005%,
Rem: 0.0001 to 0.015%,
The steel wire rod or steel bar according to claim 1 or 2, containing one or more of them. Ti: 0.001 to 0.2%,
Nb: 0.01 to 0.1%,
V: 0.03-1.0%,
W: 0.01 to 1.0%
The steel wire rod or steel bar in any one of Claims 1-3 containing 1 type, or 2 or more types. 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%,
The steel wire rod or steel bar in any one of Claims 1-4 containing 1 type, or 2 or more types of these.

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