JPH1192868A - Steel for cold forging excellent in crystal grain-coarsening preventability and delayed fracture resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the crystal coarsening preventability and delayed fracture resistance of a steel by incorporating the matrix of a steel having a specified componental compsn. with one or more kinds of the particles of TiC and Ti (CN) with specified diameters by specified total pieces. SOLUTION: A steel having a compsn. contg., by weight, 0.10 to 0.40% C, >=0.15% Si, 0.30 to 1.00% Mn, 0.50 to 1.20% Cr, 0.0003 to 0.0050% B and 0.020 to 0.100% Ti, in which the content of P is limited to <=0.015% (including 0%), that of S to <=0.015% (including 0%) and that of N to <=0.0100% (including 0%), and the balance Fe with inevitable impurities is prepd. At this time, the matrix of the steel is incorporated with one or more kinds of the particles of TiC and Ti (CN) with <=0.2 μm diameters by >=20 pieces/100 μm<2> in total. If required, the steel may furthermore be incorporated with 0.003 to 0.100% Nb and one or more kinds of 0.005 to 0.30% V and 0.003 to 0.100% Zr as well.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for cold forging which is excellent in crystal grain coarsening prevention characteristics and delayed fracture resistance characteristics and a method for producing the same.

[0002]

2. Description of the Related Art Cold forging (including rolling) has good surface texture and dimensional accuracy of products, has lower manufacturing costs than hot forging, and has a good yield. And has been applied to many fields. Cold forging of these parts is performed, for example, according to JIS G 4051,
JIS G 4052, JIS G 4104, JIS
G 4105, JIS G 4106, etc., using carbon steel and alloy steel for machine structural use of medium carbon, for example, before cold forging such as hot rolling-annealing-cold forging-quenching-tempering. Or a step of adding a spheroidizing annealing step. This is because medium-carbon carbon steel and alloy steel as described above have high hardness as rolled, and the consumption of cold forging tools when forming parts such as bolts becomes extremely expensive and the ductility of the material is insufficient. This is because there are manufacturing problems such as cracks occurring during the molding of parts.

[0003] However, annealing requires energy cost, labor cost,
Since a large cost such as equipment cost is required, a material and a process that can omit this step have been demanded. Therefore, by reducing the amount of C and alloying elements in the steel material, the hardness during hot rolling is reduced, the ductility is improved, the annealing step is omitted, and the hardenability is reduced by reducing the amount of alloying elements such as Cr and Mo. Are supplemented by adding a small amount of B, so-called low-carbon boron steels have been proposed, for example, in Japanese Patent Application Laid-Open Nos. 5-339676, 5-63524 and 61-253347. B can improve the hardenability by adding a small amount, but if solid solution N is present in steel, BN is generated and the hardenability improving effect of B is lost.
It is common practice to add Ti to fix N in steel in the form of TiN and to suppress the formation of BN.

[0004] As the need for increasing the strength of parts has increased, attempts have been made to apply the above low-carbon boron steel to parts having higher strength. However, since low carbon boron steel has a reduced C content and alloying element content, it has a tensile strength of 10%.
When the heat treatment is performed so as to be at least 00 MPa, there is a problem that the delayed fracture characteristic is deteriorated. It is known that when low-temperature tempering is performed to obtain high strength, the delayed fracture strength decreases.However, in order to obtain high strength even at high-temperature tempering and to reduce the delayed fracture strength to a level at which there is no practical problem. If the amount of C added is increased or alloy steels such as SCR and SCM are used, the strength of the material increases and annealing cannot be omitted. Low-carbon boron steel, which can eliminate annealing, is economical,
In order to obtain high strength, the tempering temperature must be lowered, and as a result, the delayed fracture strength is reduced, which is a practical problem. Therefore, application to high strength parts has been difficult.

[0005] In order to meet the demand for the application of boron steel to high-strength parts, a steel in which the amount of impurities is reduced and the delayed fracture characteristics are made comparable to that of an alloy steel has been proposed, for example, as disclosed in Japanese Patent Application Laid-Open No. H8-60245. ing. However, boron steel as described above shows a delayed fracture characteristic superior to that of alloy steel in the evaluation of a test piece of a cutting surface, but a part is created on an actual production line, and the delayed fracture characteristic Upon evaluation, it has been found that boron steel components have worse delayed fracture properties than alloy steel. Therefore, there is a limit in responding to the enhancement of the strength of the component by the above-described technology.

In addition to the above-mentioned problems, boron steel has a problem that specific austenite crystal grains tend to become abnormally large during quenching and heating as compared with annealed material. For parts with coarsened crystal grains, deterioration of dimensional accuracy due to quenching strain, reduction of impact value and fatigue life, especially of high-strength parts, cause a decrease in delayed fracture characteristics, so boron steel is applied to high-strength parts. Therefore, it is necessary to suppress the coarsening of the crystal grains and to make the crystal grains fine. In order to suppress the coarsening of the crystal grains, it is effective to disperse a large amount and fine particles for pinning the movement of the crystal grain boundaries.

[0007] Techniques for preventing the above-described coarsening of crystal grains of boron steel have been proposed. For example, JP
1-217553 indicates that the content of Ti and N is 0.02 <Ti−
The purpose is to generate TiC by setting it to 3.42 N and to pin the crystal grain boundaries. However, only by defining the components, TiC cannot be finely dispersed, and coarsening of crystal grains cannot be prevented. For example, Japanese Patent Publication No. 63-64495 has an object to prevent the crystal grain from becoming coarse by performing low-temperature heat rolling of a component having an extremely low N of 0.0035% or less and an excessive amount of Ti relative to the amount of N. And However, TiC, Ti before quenching heating
Unless the precipitation state of (CN) is optimized, coarsening of crystal grains cannot be prevented.

For example, Japanese Patent Application Laid-Open No. Sho 52-114545 discloses a method in which TiC is dissolved in a raw material phase and T
The purpose is to finely precipitate iC. However, when pinning particles are precipitated during quenching heating, the amount of TiC deposited is affected by the heating speed during quenching heating or carburizing heating, so the expression of the pinning effect is unstable. Since there is a high possibility that the coarsening prevention characteristics are deteriorated only by changing the size and the heat treatment furnace, there remains a problem in terms of the stability of the quality in the actual process.

[0009]

In the disclosed method as described above, the annealing before the cold forging or the spheroidizing annealing step is omitted, and the delay in actual parts when the heat treatment is performed with high strength. Fracture characteristics cannot be equal to or higher than alloy steel. The present invention solves such a problem and provides a steel for cold forging which is excellent in the crystal grain coarsening prevention property and the delayed fracture resistance property, and a method for producing the same.

[0010]

Means for Solving the Problems In order to achieve the above object, the present inventors have conducted intensive studies on the effects of various factors on the delayed fracture characteristics of heat-treated skin of real parts. The properties of the surface greatly affect the lag characteristics, that is, the actual bolts with heat-treated scale (heat-treated skin) are the same as the test pieces (cut skin) with the surface layer removed by cutting or grinding. Even if a delayed fracture test is performed under the same conditions, the characteristics are significantly different, and the actual component to which the heat treatment scale is attached is inferior in delayed fracture characteristics. (A) In order to improve delayed fracture characteristics on heat-treated skin, Cr is added in a certain optimum range, and the scale generated during heat treatment of the component is a dense scale in which Cr is concentrated, and the scale and the corrosion resistance are increased. The ability to reduce the amount of hydrogen generated in the course of corrosion of the steel surface layer inside the scale and improve delayed fracture characteristics. (C) When boron steel is applied to high-strength parts such as bolts having a tensile strength of 1000 MPa or more, it is necessary to limit the amounts of P and S to a certain amount or less in order to improve delayed fracture characteristics, and It is necessary to prevent coarsening of crystal grains. (D) To prevent coarsening of crystal grains, fine TiC, Ti (CN), NbC, Nb (C
N) and (Nb, Ti) (CN) particles are effective, and there is a very close relationship between the crystal grain coarsening characteristics and the size and dispersion state (the number of precipitated particles) of these precipitates. To stably exhibit the pinning effect, a certain amount or more of TiC, Ti (CN), NbC, N
The present inventors have found that it is necessary to previously precipitate one or more particles of b (CN) and (Nb, Ti) (CN) finely, and have reached the present invention.

The characteristics of the present invention are as follows: (1) C: 0.10
0.40%, Si: 0.15% or less, Mn: 0.30%
By setting it to 1.00%, the strength of the part after quenching and tempering is secured, and P: 0.015% or less (including 0%);
S: Improves delayed fracture characteristics by limiting the content to 0.015% or less (including 0%), and B: 0.0003-0.
By limiting to 0050%, hardenability is secured,
By setting the Cr content to 0.50 to 1.20%, the delayed fracture characteristic on the heat-treated skin can be improved, and the delayed fracture characteristic when a real part is obtained can be significantly improved. Further, N:
0.0100% or less (including 0%), Ti:
By making the content 0.020 to 0.100%, TiC,
It produces Ti (CN) and can be used as pinning particles for preventing crystal grains from becoming coarse. The total number of particles of one or two of TiC and Ti (CN) having a diameter of 0.2 μm or less in the matrix is 20 particles / 100 μm.
is maximize the pinning effect by having m ² or more, a cold forging steel for the austenite grain can be miniaturized while preventing grain coarsening during heating for quenching.

(2) Another feature of the present invention is that, in addition to the above components, Nb: 0.003 to 0.100% is contained,
TiC, Ti having a diameter of 0.2 μm or less in the matrix
(CN), NbC, Nb (CN), (Nb, Ti) (C
N) The total number of one or more particles is 20 particles / 100 μ
a cold forging steel capable of preventing the coarsening of crystal grains by having m ² or more.

(3) Another feature of the present invention is that, in addition to the component (1) or (2), V: 0.05 to
0.30%, Zr: 1 out of 0.003 to 0.100%
By containing one or two species, the prior austenite crystal grains can be further refined, and TiC, Ti (CN), Nb having a diameter of 0.2 μm or less are contained in the matrix.
C, Nb (CN), (Nb, Ti) (CN) For cold forging, wherein the total number of at least one type of particles is at least 20/100 μm ^2, whereby coarsening of crystal grains can be prevented. It is steel.

(4) Another feature of the present invention is that a steel comprising the above components (1), (2) and (3) is heated at 1050 ° C.
By heating as above, TiC, Ti (CN), NbC, Nb
(CN), (Nb, Ti) (CN) are once dissolved in a matrix and hot-rolled into a wire or a steel bar.
When cooled to a temperature of 0 ° C. or less, the material is gradually cooled at a cooling rate of 2 ° C./s or less to be softened, and fine TiC, Ti (CN) having a diameter of 0.2 μm or less in a matrix.
This is a method for producing a steel for cold forging, in which the total number of one or more particles of NbC, Nb (CN), and (Nb, Ti) (CN) is dispersed into 20/100 μm ² or more.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. First, the reasons for limiting the components will be described. C is an element effective in giving the necessary strength to steel, but 0.1
If it is less than 0%, the required tensile strength cannot be secured,
If it exceeds 0.40%, the cold forgeability decreases, and the annealing before the cold forging or the spheroidizing annealing step cannot be omitted. In addition, the ductility and toughness of the parts tend to deteriorate, and the delayed fracture characteristics also tend to deteriorate.
It must be within the range of 0%. The preferred range is 0.20
0.30%.

Si is an element effective for deoxidizing steel, and is an element effective for imparting necessary strength and hardenability to steel and improving temper softening resistance. Since toughness and ductility are deteriorated, the hardness is increased, and the cold forgeability is deteriorated, it is necessary to be within a range of 0.15% or less. A preferred range is 0.10% or less.

Mn is an element effective for deoxidizing steel and is an element effective for imparting necessary strength and hardenability to steel. However, if it is less than 0.30%, the effect is insufficient.
If it exceeds 1.00%, the hardness will increase and the cold forgeability will deteriorate, so it is necessary to be in the range of 0.30% to 1.00%. The preferred range is 0.40 to 0.70%.

P is an element that increases the deformation resistance during cold forging and deteriorates toughness, so that cold forgeability is deteriorated. Further, since the grain boundary of the quenched and tempered component is embrittled to degrade the delayed fracture characteristics, it is desirable to reduce as much as possible. Therefore, its content is reduced to 0.1.
It is necessary to limit to 015% or less. The preferred range is 0.0
10% or less.

Since S is an element that easily causes cracking during cold forging, the cold forgeability deteriorates. Further, similarly to P, the delayed fracture characteristics are deteriorated by embrittlement of the crystal grain boundaries of the parts after quenching and tempering, so that it is desirable to reduce as much as possible. Therefore, the content is 0.0
It must be limited to 15% or less. The preferred range is 0.01
0% or less.

[0020] Cr is an element that imparts strength and hardenability to steel and is effective in improving temper softening resistance, and is also an element that significantly improves delayed fracture characteristics, particularly in heat-treated skin. Cr has the effect of improving the delayed fracture characteristics by reducing the amount of hydrogen generated in the course of corrosion of the scale by increasing the corrosion resistance by increasing the scale formed during the heat treatment to a dense scale in which Cr is concentrated. 1350MP tensile strength
FIG. 1 shows the effect of the amount of Cr on the delayed fracture characteristics when tempering near a.

FIG. 1 shows the test results in 0.1N HCl, but shows almost the same tendency in 1% H ₂ SO ₄ .
As is clear from FIG. 1, the amount of Cr has a great influence on the delayed fracture characteristics on the heat-treated skin, and if it is less than 0.50%, a sufficient effect of improving the delayed fracture characteristics cannot be obtained.
Addition of more than 0% not only increases the hardness and deteriorates the cold forgeability, but also promotes the generation of grain boundary oxidation of the surface layer generated during the heat treatment, and rather deteriorates the delayed fracture characteristics. This tendency appears more remarkably as the component strength increases.
Therefore, the addition amount of Cr needs to be in the range of 0.50 to 1.20%. The preferred range is 0.60 to 0.9
0%.

B is an element effective for imparting hardenability to steel with a small amount of addition, but if its content is less than 0.0003%, its effect is insufficient, and if it exceeds 0.0050%, its effect is saturated. , 0.0003 to 0.0050%. The preferred range is 0.0010 to 0.003
0%.

N is harmful to B-added steel as in the present invention because N combines with B to form BN and reduces the effect of B on improving the hardenability. In addition, when combined with Ti in steel, coarse TiN that hardly contributes to pinning is generated, the amount of Ti that can be carbonitride containing Ti is reduced, and the amount of fine precipitates is reduced. It is desirable to do. Therefore, keeping the content as low as possible is the point of suppressing the coarsening of the crystal grains, and as will be described later, the smaller the amount of N, the smaller the amount of Ti added. However, since it is difficult to completely remove N in an actual manufacturing process, the range is set to 0.0100% or less. A preferred range is 0.0050% or less.

Ti combines with C and N in steel to form TiC,
It is an element that forms Ti (CN) and is effective in refining crystal grains and suppressing coarsening of crystal grains. Further, when added together with B, it is an effective element for suppressing the generation of BN by fixing solid solution N in the steel in the form of TiN and Ti (CN), and obtaining the effect of improving the hardenability by B. Is 0.
If it is less than 020%, the effect is insufficient. If it exceeds 0.100%, the effect is not only saturated, but also the hardness is increased and the cold forgeability is deteriorated.
% Must be within the range. The preferred range is 0.025 to
0.50%. Of course, all the solute N in steel
In order to fix in the form of N, it is necessary to increase the amount of Ti according to the amount of N. In order to secure a sufficient amount of fine TiC and Ti (CN) effective for pinning the crystal grain boundaries. Also,
It is necessary to increase the Ti amount according to the N amount. At least the addition of Ti exceeding 3.4 N% is necessary.

Nb is combined with C and N in steel to form NbC,
Nb (CN) and (Nb, Ti) (CN) are formed and are effective elements for refining crystal grains and suppressing coarsening of crystal grains. When Nb is added together with Ti, most of them form stable (Nb, Ti) (CN) and can obtain a stable pinning effect, but if less than 0.003%, the effect is insufficient. If it exceeds 0.100%, the effect is not only saturated, but also the hardness is increased, and the cold forgeability is deteriorated. Therefore, it is necessary to be within the range of 0.003 to 0.100%. The preferred range is 0.005 to 0.030%
It is.

V is combined with C and N in the steel to form VC, VN
And is an element effective for refining crystal grains.
If it is less than 05%, the effect is insufficient. If it exceeds 0.30%, the effect not only saturates, but also increases the hardness and deteriorates the cold forgeability. Must be within range. The preferred range is 0.10 to 0.20%
It is.

Zr combines with C and N in steel to form ZrC,
It is an element that forms ZrN and is effective in refining crystal grains. However, if the content is less than 0.003%, the effect is insufficient.
If it exceeds 00%, the effect is not only saturated, but also the hardness is increased and the cold forgeability is deteriorated.
It must be within the range of 0.100%. The preferred range is 0.005 to 0.030%.

Although V and Zr are not essential elements in the present invention, they can be added as needed for the purpose of making crystal grains fine.

Although the present invention does not specify the amount of Al added,
Since it is an element effective for deoxidizing steel, it may contain an Al amount usually used for deoxidizing. Normal Al content is 0.0
It is about 10 to 0.050%. However, when an element (such as Si, Mn, Ti, or Zr) instead of Al is used as the deoxidizing agent, it is not always necessary to add Al.

Next, TiC, Ti (C
N), NbC, Nb (CN), (Nb, Ti) (CN)
Will be described. In order to suppress the coarsening of the crystal grains, it is effective to disperse a large amount and finely of the particles that pin the crystal grain boundaries, and the number of the pinned particles increases as the diameter of the particles decreases or the amount increases. Is preferred. FIG. 2 shows the relationship between the fine TiC and Ti (CN) and the crystal grain coarsening temperature. Note that NbC, Nb (CN), (N
b, Ti) (CN) has the same effect.
Follow the relationship.

As is evident from FIG. 2, there is a very close relationship between the coarsening characteristics of the crystal grains and the number of fine precipitate particles, and TiC and Ti (C
N), NbC, Nb (CN), (Nb, Ti) (CN)
The total number of one or more particles is 20 particles / 100 μm ²
When dispersed, the crystal grains are not coarsened in a practical quenching heating or carburizing heating temperature range, and excellent crystal grain coarsening prevention properties are obtained. Therefore, TiC or Ti having a diameter of 0.2 μm or less is contained in the matrix. (CN), NbC, N
It is necessary that the total number of at least one kind of particles of b (CN), (Nb, Ti) (CN) is dispersed at least 20 particles / 100 μm ² .

Next, the manufacturing conditions will be described. The steel comprising the above-described component of the present invention is melted by a usual method such as a converter or an electric furnace, the components are adjusted, and the material is subjected to a casting process and, if necessary, a slab rolling process to obtain a rolled material. In addition, if the slab is subjected to soaking diffusion treatment in which the slab is kept at a temperature of about 1200 to 1350 ° C. for several hours before the slab rolling step, segregation of impurity elements such as P is reduced, and delayed fracture characteristics in actual parts are reduced. In addition to the further improvement, coarse precipitates precipitated in the casting step can be once solutionized, and the precipitates can be easily dissolved in the matrix in the next step, so that further characteristics can be obtained by performing this treatment. .

Next, the rolled material is heated at a temperature of 1050 ° C. or higher. The heating conditions are as follows: TiC, T
i (CN), NbC, Nb (CN), (Nb, Ti)
(CN) cannot be dissolved in the matrix once, and after hot rolling, TiC, Ti (CN), NbC, Nb
It is not possible to use a steel in which one or more particles of (CN) and (Nb, Ti) (CN) are finely precipitated. Coarse TiC, Ti (CN) that could not be dissolved,
NbC, Nb (CN), and (Nb, Ti) (CN), when present in large amounts, degrade the ductility of the component and adversely affect delayed fracture characteristics. Furthermore, if many coarse precipitates remain, they act as precipitation nuclei during cooling after rolling and grow more coarsely, making it difficult to finely disperse the pinned particles in the matrix. Therefore, it is desirable that the heating temperature be as high as possible. The preferred range is 115
0 ° C. or higher.

Next, after the rolled material heated to 1050 ° C. or more is hot-rolled into a wire or a steel bar, the material is gradually cooled at a cooling rate of 2 ° C./s or less when cooled to a temperature of 600 ° C. or less. When the cooling condition exceeds 2 ° C./s, TiC, T
i (CN), NbC, Nb (CN), (Nb, Ti)
(C) can only pass through the precipitation temperature range for a short time, the amount of precipitation becomes insufficient, and TiC, Ti (CN), NbC, Nb (CN), (N
b, Ti) (CN) cannot be made into steel with a large amount and fine precipitation. Further, if the cooling rate is high, the hardness of the rolled material increases, and the cold forgeability deteriorates. Therefore, it is desirable to reduce the cooling rate as much as possible. A preferred range is 1 ° C./s or less. In addition, after hot rolling, a lower temperature range (500
(Lower than or equal to ° C) at a cooling rate of 2 ° C / s. When gradually cooled to a low temperature range, the rolled material is further softened, and the cold forgeability is improved.

[0035]

The present invention will be further described below with reference to examples. Continuously cast converter steel having the composition shown in Table 1,
If necessary, pass through the soaking diffusion process
A 62 mm square rolled material was used. Next, rolled material was added to 105
It was heated at a temperature of 0 ° C. or more, and hot-rolled into a steel bar or a wire having a diameter of 5 to 50 mm. Some have a heating temperature of 10 for comparison.
The temperature was set to 50 ° C. or less. Next, slow cooling was performed using a heat retaining cover provided behind the rolling line. Some did not cool slowly for comparison.

[0036]

[Table 1]

TiC, Ti effective as pinning particles
(CN), NbC, Nb (CN), (Nb, Ti) (C
The dispersion state of N) was measured by extracting precipitates present in the matrix of the steel bar and the wire rod by the extraction replica method and observing them with a transmission electron microscope. The observation method is to observe about 20 visual fields at 15000 magnifications, and TiC, Ti (CN), NbC, Nb having a diameter of 0.2 μm or less in one visual field.
The total number of (CN), (Nb, Ti) (CN) was counted, and 10
It was converted to a number per 0 μm ² .

[0038] The crystal grain coarsening temperature of the wire rod or the steel bar manufactured in the above process was measured. After subjecting the rolled material to cold drawing with a surface reduction rate of 70%, the temperature is reduced to 840 to 1200 ° C. by 30%.
Heat-water quenched for minutes. Thereafter, the cut surface was polished and corroded, and the old austenite grain size was observed to determine the coarse grain generation temperature (crystal grain coarsening temperature).

In the quenching process of actual parts such as bolts, A _c3 ~
Since it is often carried out in a temperature range of 900 ° C., those having a coarse grain generation temperature of less than 900 ° C. were judged to be inferior in crystal grain coarsening characteristics. The measurement of the prior austenite grain size is based on JIS
The measurement was performed according to G 0551, and the observation was performed at a magnification of about 400 for about 10 visual fields.

Next, in order to investigate the delayed fracture characteristics of these materials, a material subjected to cold drawing of 70% was processed into a delayed fracture test piece having an annular V notch. Then 90
0 ° C. × 30 minutes-quenching, followed by tempering, tempering to a tensile strength of 1350 MPa class, and production of a delayed fracture test piece of heat-treated skin close to the surface skin of the actual part. The delayed fracture test piece was immersed in 0.1N HCl, and the time until fracture was measured while changing the applied stress. The test time was a maximum of 200 hours, and the maximum applied stress that did not break for 200 hours was measured. The value obtained by dividing the maximum applied stress that does not break for 200 hours by the breaking stress in the atmosphere was defined as “delayed fracture strength ratio” and used as an index of delayed fracture characteristics.

At present, the tensile strength is 1000-1400MP
Since the delayed fracture strength ratio of SCM435, which is often used for a-class parts, is about 0.5, it was determined that the delayed fracture strength ratio of less than 0.5 was inferior to the delayed fracture characteristic. On the other hand, the grain size of the test piece subjected to the delayed fracture test was examined. In the case of sizing, the average particle size of the matrix was measured, and in the case of mixed or coarse particles, the particle size number of the largest particle in the observation visual field was also measured. The measurement of the prior austenite grain size was performed in the same manner as in the above-described investigation of the crystal grain coarsening temperature.

[0042]

[Table 2]

Tables 2, 3 and 4 show the results of these various tests.
Shown in Symbols N and O in Table 2 indicate that the amount of Ti or N is out of the range of the present invention.
N), NbC, Nb (CN), (Nb, Ti) (CN)
Is insufficient, and the crystal grain coarsening characteristics are deteriorated.
The symbols V, X, and Y are Ti
C, Ti (CN), NbC, Nb (CN), (Nb, T
i) Since (CN) cannot be once dissolved in the matrix and cannot be made into steel in which precipitates are finely precipitated upon cooling after hot rolling, the crystal grain coarsening characteristics are deteriorated.

Further, the symbols W and Z indicate that the cooling rate after rolling is too high, so that the amount of fine precipitates becomes insufficient and the crystal grain coarsening characteristics are deteriorated.

[0045]

[Table 3]

[0046]

[Table 4]

Table 3 shows the delayed fracture characteristics when the rolled material in Table 2 was tempered to about 1350 MPa and Table 4 was tempered to about 1200 MPa. Symbols P, Q, and T in Table 3 indicate that the amount of Cr added is out of the range of the present invention.
The delayed fracture characteristics have deteriorated. In the symbols R and S, since the P amount or the S amount is out of the range of the present invention, the delayed fracture characteristics are deteriorated. Incidentally, those having poor crystal grain coarsening properties (symbols N, O, V, W, X, Y, and Z) have deteriorated delayed fracture properties because coarse grains are generated in the delayed fracture test specimen. In Table 4, since the tensile strength is about 1200 MPa, the delayed fracture characteristics are improved as compared with Table 3. The component number 21 in Table 1 and the symbol U in Tables 2 and 3 are examples of alloy steels that are currently frequently used and for which annealing cannot be omitted. As is clear from these tables, those satisfying all of the conditions specified in the present invention show superior properties in both the grain coarsening prevention property and the delayed fracture resistance property as compared with the comparative example.

[0048]

According to the steel for cold forging and the method for producing the same of the present invention, annealing before cold forging can be omitted, and dimensional accuracy deterioration and impact value due to quenching distortion due to coarsening of crystal grains during heat treatment. Further, it is possible to provide a material such as a bolt, a gear component, and a shaft, which has less decrease in fatigue strength than the conventional one and has particularly excellent delayed fracture characteristics in an actual component used on a heat-treated skin.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of analyzing the effect of the amount of Cr on delayed fracture characteristics on heat-treated skin

FIG. 2 Fine Ti in matrix of steel before quenching and heating
The figure which shows an example which analyzed the relationship between the total number of C and Ti (CN) and the crystal coarsening temperature.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hideo Kanisawa 12 Nakamachi, Muroran-shi, Hokkaido Inside Nippon Steel Corporation Muroran Works (72) Inventor Atsushi Murakami 1-4-1 Chuo, Wako-shi, Saitama Stock Company Inside the Honda R & D Center (72) Inventor Masao Ishida 1-4-1 Chuo, Wako-shi, Saitama Pref.

Claims (4)

[Claims] C .: 0.10 to 0.40%, Si: 0.15% or less, Mn: 0.30 to 1.00%, Cr: 0.50 to 1.20% by weight%, B: 0.0003 to 0.0050%, Ti: 0.020 to 0.100%, P: 0.015% or less (including 0%), S: 0.015% or less (0% , N: 0.0100% or less (including 0%), and the balance is Fe and unavoidable impurities, and TiC, Ti (CN) having a diameter of 0.2 μm or less in a steel matrix. ), Wherein the total number of one or two kinds of particles is at least 20 particles / 100 μm ² or more. 2. In% by weight, C: 0.10 to 0.40%, Si: 0.15% or less, Mn: 0.30 to 1.00%, Cr: 0.50% to 1.20% , B: 0.0003 to 0.0050%, Ti: 0.020 to 0.100%, Nb: 0.003 to 0.100%, and P: 0.015% or less ( S: 0.015% or less (including 0%), N: 0.0100% or less (including 0%), and the balance consists of Fe and unavoidable impurities, and TiC, Ti (CN), NbC, Nb (CN), (N
b, Ti) (CN), the total number of one or more particles is 2
A cold forging steel excellent in the crystal grain coarsening prevention characteristic and the delayed fracture resistance characteristic characterized by having 0/100 μm ² or more. 3. In addition to the steel component according to claim 1 or 2, further contains one or two of V: 0.05 to 0.30%, Zr: 0.003 to 0.100%, and the balance Is composed of Fe, unavoidable impurities, and TiC, Ti (CN), NbC, Nb having a diameter of 0.2 μm or less in a steel matrix.
(CN), (Nb, Ti), (CN), wherein the total number of one or more particles is at least 20 particles / 100 μm ² or more. Steel for forging. 4. The steel component according to claim 1 or 2 or 3,
After being heated to 050 ° C or higher and hot-rolled to a wire or steel bar, when cooled to a temperature of 600 ° C or lower, 2 ° C /
Slow cooling at a cooling rate of not more than s
TiC, Ti (CN), NbC, Nb (C
N), a steel in which the total number of one or more particles of (Nb, Ti) and (CN) is dispersed at least 20 particles / 100 μm ² , characterized in that the crystal grain coarsening prevention property and the delayed fracture resistance are improved. Excellent cold forging steel manufacturing method.