JPH1143737A - Cold forging steel excellent in grain coarsening preventing property and cold forgeability, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold forging steel excellent in grain coarsening preventing property and cold forgeability and its production. SOLUTION: This steel has a composition consisting of 0.10-0.60% C, <=0.50% Si, 0.30-2.00% Mn, <=0.025% P, <=0.025% S, <=0.25% Cr, 0.0003-0.0050% B, <=0.0050% N, 0.020-0.100% Ti, and the balance Fe with inevitable impurities and also has a structure where TiC or Ti(CN) of <=0.2 μm diameter exists by >=20 pieces/100 μm<2> in a steel matrix. The steel having the above composition is heated to >=1050 deg.C, hot-rolled into a wire rod or steel rod, and then cooled down to <=600 deg.C. At the time of this cooling, slow cooling is performed at >=2 deg.C/s cooling rate. By this method, the steel, where TiC or Ti(CN) of <=0.2 μm diameter is dispersed by >=20 pieces/μm<2> in the matrix, can be produced.

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 properties and cold forgeability, 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 is performed, for example, according to JIS G 4051, JIS G 4
052, JIS G 4104, JIS G 410
5, using medium-strength carbon steel and alloy steel for mechanical structure of medium carbon stipulated in JIS G 4106 and the like, for example, annealing before cold forging such as hot rolling-annealing-cold forging-quenching-tempering; Alternatively, a step of adding a spheroidizing annealing step is general. This is because medium-carbon carbon steel and alloy steel as described above have high hardness as rolled, and the consumption of tools for cold forging becomes remarkably expensive. This is because there are manufacturing problems such as cracks.

[0003] However, annealing requires energy cost, labor cost,
Since a large cost such as an equipment cost is required, a steel material which can omit this step has 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. The so-called boron steel, which is supplemented by adding a small amount of B, has 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 the steel, BN is generated and the hardenability improving effect of B is lost. It is common to fix in the form of TiN to suppress the generation of BN.

[0004] However, the above-mentioned boron steel is always accompanied by a problem of generation of so-called coarse grains, in which specific austenite crystal grains tend to become abnormally large during quenching heating or carburizing heating as compared with the annealed material. Components with coarsened crystal grains cause deterioration of dimensional accuracy due to quenching strain, impact value, decrease in fatigue life, and decrease in delayed fracture characteristics.
The improvement is a major practical issue. In order to suppress such coarsening of crystal grains, it is effective to disperse a large amount and fine particles for pinning the movement of crystal grain boundaries.

[0005] Boron steel is susceptible to coarsening of crystal grains because most of N in steel is fixed by Ti, and fine AIN which effectively acts as pinning particles in alloy steel and carbon steel is generated. On the other hand, TiN is coarser than AIN, cannot be dispersed in a large amount and finely, and the pinning effect of crystal grain growth decreases.

Techniques for preventing the above-described coarsening of the crystal grain 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

[0007] However, even if the N component is extremely low, a large amount of undissolved Ti (CN) is inevitably generated at low temperature heating, and some solid solution TiC remains undissolved during cooling after rolling or during quenching heating. Since it grows with Ti (CN) as a nucleus, it cannot be finely dispersed and cannot prevent crystal grains from becoming coarse. Even if high-temperature heat rolling is performed, the T
Unless the precipitation state of iC and Ti (CN) or the cooling conditions after rolling is optimized, coarsening of crystal grains cannot be prevented.

For example, Japanese Patent Application Laid-Open No. Sho 52-114515 aims to form a solid solution of TiC at the time of rolling and heating and to precipitate TiC finely at the time of quenching and heating. However, when pinning particles are precipitated during quenching and heating, TiC
The amount of precipitation is affected by the heating rate during quenching heating or carburizing heating, so the expression of the pinning effect is unstable,
Even if the same material is used, there is a high possibility that the coarsening prevention characteristic is deteriorated only by changing the applied parts and the heat treatment furnace, so that there remains a problem in terms of quality stability in actual processes.

[0009]

In the method disclosed above, the annealing before the cold forging or the spheroidizing annealing step is omitted, and the coarsening of the crystal grains cannot be prevented.
The present invention solves such a problem and provides a steel for cold forging having excellent crystal grain coarsening prevention characteristics and excellent cold forgeability, and a method for producing the same.

[0010]

Means for Solving the Problems To achieve the above object, the present inventors have intensively investigated the controlling factors for the coarsening of crystal grains, and (1) pinning to prevent the coarsening of the crystal grains. Fine TiC and Ti (CN) are effective as particles, 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 TiC or Ti (CN); (2) TiC Alternatively, in order to stably exhibit the pinning effect of Ti (CN), it is necessary to previously finely precipitate a certain amount or more of TiC or Ti (CN) before quenching or carburizing heating, (3)
A large amount of TiC or Ti (CN) in the matrix,
In order to disperse finely, it is only necessary to optimize the rolling heating temperature and the cooling conditions after rolling, that is, by increasing the rolling heating temperature to TiC or Ti (C
N) is once dissolved in the matrix, and after hot rolling, T
TiC or Ti (CN) is dispersed in a large amount and finely by gradually cooling the precipitation temperature range of iC or Ti (CN). (4) In order to increase fine TiC or Ti (CN), Ti is required. The inventors have found that it is necessary to add N in an optimum range and to reduce the amount of N as much as possible, leading to the present invention.

The characteristics of the present invention are as follows: C: 0.10 to 0.60
%, Si: 0.50% or less, Mn: 0.30 to 2.00
%, Cr: 0.25% or less ensures the strength of the part after quenching and tempering, P: 0.025% or less (including 0%), S: 0.025% or less (0% Including)
The ductility during cold forging, and quenching,
Ensuring the toughness of the tempered part, B: 0.0003-
0.0050% to ensure hardenability,
Further, N: 0.0050% or less (including 0%), Ti:
By making the content 0.020 to 0.100%, TiC or Ti (CN) is produced, and can be used as pinning particles for preventing crystal grains from becoming coarse.
Alternatively, a steel capable of exhibiting a pinning effect to the maximum by making the size of Ti (CN) 0.2 μm or less in diameter and having a density of 20 pieces / 100 μm ² or more, and preventing crystal grains from becoming coarse. is there.

Another feature of the present invention is that a steel comprising the above-described components is heated to 1050 ° C. or more to form TiC or TiC.
(CN) is once solution-heated, hot-rolled into a wire or a steel bar, and then cooled to a temperature of 600 ° C. or less by 2 ° C. /
s or less, and softens by slow cooling at a cooling rate of not more than 0.2 s.
This is a method for producing steel in which (CN) is dispersed in 20 pieces / 100 μm ² or more.

[0013]

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 0.60%, ductility and toughness are deteriorated, and delayed fracture characteristics are also deteriorated. Therefore, the content needs to be in the range of 0.10 to 0.60%. The preferred range is 0.15 to 0.40%.

[0014] 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. The toughness and ductility are deteriorated, the hardness is increased, and the cold forgeability is deteriorated. A preferred range is 0.30% 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 2.00%, the effect is not only saturated, but also
Since the hardness is increased and the cold forgeability is degraded, 0.30
% To 2.00%. The preferred range is 0.50 to 1.20%.

Since P is an element that increases the deformation resistance during cold forging and deteriorates toughness, the cold forgeability deteriorates. Further, the toughness, torsional strength and delayed fracture characteristics are deteriorated by embrittlement of the crystal grain boundaries of the quenched and tempered parts. Therefore,
It is necessary to limit the content to 0.025% or less.
A preferred range is 0.015% or less.

Since S is an element that easily causes cracking during cold forging, the cold forgeability deteriorates. Further, similarly to P, the toughness, torsional strength and 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 them as much as possible. Therefore, it is necessary to limit the content to 0.025% or less. A preferred range is 0.015% or less.

Cr is an element effective for imparting strength and hardenability to steel and improving temper softening resistance.
%, The hardness increases and the cold forgeability deteriorates. Therefore, the content needs to be within the range of 0.25% or less. The preferred range is 0.10 to 0.20%.

B is an effective element 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 combines with B to form BN, which reduces the effect of B on improving the hardenability.
Harmful with added steel. Also, when combined with Ti in steel, it produces coarse TiN that hardly contributes to pinning,
In order to reduce the amount of fine TiC or Ti (CN) by reducing the amount of Ti that can become TiC or Ti (CN), it is desirable to reduce as much as possible. Also, T
In order to disperse iC or Ti (CN) in a large amount and finely in the matrix, it is necessary to raise the rolling heating temperature so that TiC or Ti (CN) is once dissolved in the matrix. As the amount increases, there is a problem that Ti (CN) becomes stable up to a high temperature and hardly forms a solid solution. Therefore, it is necessary to limit the content to 0.0050% or less. A preferred range is 0.0040% or less.

Ti combines with C and N in steel to form TiC,
It is an element that forms TiN and is effective in miniaturizing crystal grains and suppressing coarsening of crystal grains. Further, when added together with B, it is an element effective for fixing the solute N in the steel in the form of TiN to suppress the generation of BN and obtaining the effect of improving the hardenability by B. %, The effect is insufficient. If it exceeds 0.100%, the effect is not only saturated, but also increases the hardness and deteriorates the cold forgeability, so the range is 0.020 to 0.100%. Need to be inside. A preferred range is from 0.025 to 0.050%.

Although the present invention does not specify the amount of Al added, it is an element effective for deoxidizing steel, and therefore can contain the amount of Al usually used for deoxidation. The usual Al content is about 0.010 to 0.050%. Where A
When using an element (Si, Mn, Ti, etc.) in place of 1 as a deoxidizing agent, it is not always necessary to add Al.

Next, TiC or Ti in the matrix
The dispersion state of (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. 1 shows the relationship between fine TiC or Ti (CN) and the grain coarsening temperature. As is evident from FIG. 1, there is a very close relationship between the coarsening characteristics of the crystal grains and the number of fine precipitate particles, and 20/100 μm ² or more of TiC or Ti (CN) having a diameter of 0.2 μm or less in the matrix. When dispersed, 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 (dia. CN) 20 /
It is necessary that the particles are dispersed at 100 μm ² or more.

Next, the manufacturing conditions will be described. The steel comprising the above-mentioned 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 forging process and, if necessary, a slab rolling process to obtain a rolled material. Next, the rolled material is heated at a temperature of 1050 ° C. or higher. Heating condition is 10
If the temperature is lower than 50 ° C., TiC or Ti (CN) cannot be once dissolved in the matrix, and after hot rolling, TiC or Ti (CN) cannot be dissolved.
Since it is not possible to use steel in which C or Ti (CN) is finely precipitated, it is desirable that the temperature be as high as possible. A preferred range is 1150 ° 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. If the cooling condition is more than 2 ° C./s, it can only pass through the precipitation temperature range of TiC or Ti (CN) for a short time, the amount of precipitation becomes insufficient, and TiC or Ti (CN ) Cannot be a 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 cold rate as much as possible. A preferred range is 1 ° C./s or less. In addition, it is preferable to gradually cool at a cooling rate of 2 ° C./s to a lower temperature range (500 ° C. or less) after hot rolling. When gradually cooled to a low temperature range, the rolled material is further softened, and the cold forgeability is improved.

[0026]

The present invention will be further described below with reference to examples. Converter steel smelting having the structures shown in Tables 1 and 2 was continuously forged and, if necessary, passed through a slab rolling process to obtain a 162 mm square rolled material. Subsequently, the rolled material was heated at a temperature of 1050 ° C. or higher, and was hot-rolled into a steel bar or a wire having a diameter of 5 to 50 mm. For some, the heating temperature was set to 1050 ° C. or lower for comparison. Next, slow cooling was performed using a heat retaining cover provided behind the rolling line. Some did not cool slowly for comparison.

[0027]

[Table 1]

[0028]

[Table 2]

The Vickers hardness of the rolled bar and wire was measured and used as an index of cold forgeability. The required strength and cold forgeability differ between those applied to bolts and gear parts and those applied to induction hardened shafts.
The evaluation was divided according to the range of the amount. That is, Table 1
The steel types of 0.1 to 0.4% C shown in Table 1 (mainly applied to bolts and gear parts, steel numbers 1 to 11) have a hardness of HV after rolling.
Those exceeding 190 were judged to have poor cold forgeability. As for the steel types of 0.4 to 0.6% C shown in Table 2 (applied to induction hardened shafts, steel numbers 12 to 18), those whose hardness after rolling exceeded HV300 were judged to have poor cold forgeability.

TiC or T effective as pinning particles
In order to examine the dispersion state of i (CN), precipitates present in the matrix of the steel bar and the wire were collected by the extraction replica method and observed with a transmission electron microscope. Observation method is 150
Observe about 20 fields at 00x, 0.2μ diameter in one field
m and count the number of TiC or Ti (CN)
It was converted to the number per μm ² .

After the wire or bar manufactured in the above process is subjected to cold drawing with a reduction in area of 70%,
For No. 11, heat-water quenching at 840-1200 ° C. for 30 minutes, and for steel Nos. 12-18, frequency 8 k
Heating-water quenching at 900 to 1250 ° C for 2 seconds at a high frequency of Hz. 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 performed in the temperature range of 900 ° C, steel number 1
As for Nos. 11 to 11, those having a coarse grain generation temperature of less than 900 ° C were judged to be inferior in crystal grain coarsening characteristics. In addition, since induction hardening shafts are often performed in a temperature range of a maximum heating temperature of 900 to 1000 ° C., steel numbers 12 to 18 having a coarse grain generation temperature of less than 1000 ° C. are inferior in crystal grain coarsening characteristics. It was determined. The measurement of the prior austenite grain size was carried out in accordance with JIS G 0551. Observation was performed at about 400 times at about 10 visual fields. If at least one coarse grain having a grain size number of 5 or less was present, it was determined that coarse grains were generated.

[0033]

[Table 3]

[0034]

[Table 4]

Tables 3 and 4 show the results of these various tests. Since H and I in Table 3 and S and T in Table 4 are out of the range of the present invention in Ti or N content, the number of fine TiC or Ti (CN) deposited is insufficient, and the grain coarsening property is deteriorated. doing. L and M in Table 3 and V in Table 4 are such that TiC or Ti (CN) cannot be dissolved in the matrix once because the rolling heating temperature is low.
Since it is not possible to use steel in which C or Ti (CN) is finely precipitated, crystal grain coarsening characteristics are deteriorated.

In addition, N in Table 3 and W and X in Table 4 show that TiC or Ti (C
The precipitation amount of N) is insufficient, and the coarsening characteristics are deteriorated. Further, the hardness after rolling is high and the cold forgeability is inferior. Also,
J, K in Table 3 and U in Table 4 have C, Si, Mn, and Cr amounts out of the range of the present invention, and thus have high hardness after rolling and poor cold forgeability. As is clear from these tables, those satisfying all of the conditions specified in the present invention show excellent properties in both the crystal grain coarsening prevention property and the cold forgeability as compared with the comparative examples.

[0037]

According to the steel for cold forging of the present invention and the method for producing the same, deterioration of dimensional accuracy due to quenching strain due to coarsening of crystal grains, reduction of impact value, fatigue life, and deterioration of delayed fracture characteristics are conventionally achieved. It is possible to provide bolts, gear parts, shafts, and other materials that have extremely low hardness and are low in hardness before cold forging, and that can be cold forged at low cost.

[Brief description of the drawings]

FIG. 1: Fine TiC in matrix of steel after hot rolling
FIG. 3 is a diagram showing an example of analyzing a relationship between the number of precipitated Ti (CN) and the crystal grain coarsening temperature.

Claims (2)

[Claims] 1. C: 0.10 to 0.60%, Si: 0.50% or less, Mn: 0.30 to 2.00%, P: 0.025% or less (including 0%), S : 0.025% or less (including 0%), Cr: 0.25% or less, B: 0.0003 to 0.0050%, N: 0.0050% or less (including 0%), Ti: 0. 0.20 to 0.100%, with the balance being Fe and unavoidable impurities,
TiC having a diameter of 0.2 μm or less
Or a cold forging steel excellent in crystal grain coarsening prevention characteristics and cold forgeability, characterized in that it has at least 20 Ti (CN) / 100 μm ² . 2. C: 0.10 to 0.60%, Si: 0.50% or less, Mn: 0.30 to 2.00%, P: 0.025% or less (including 0%), S : 0.025% or less (including 0%), Cr: 0.25% or less, B: 0.0003 to 0.0050%, N: 0.0050% or less (including 0%), Ti: 0. After the steel containing Fe and unavoidable impurities is heated to 1050 ° C. or more and hot-rolled into a wire or a steel bar, the steel is cooled to a temperature of 600 ° C. or less by 2 ° C.
/ S slowly cooled at a cooling rate of not more than 20/20 TiC or Ti (CN) having a diameter of not more than 0.2 μm in the matrix.
A method for producing a steel for cold forging having excellent crystal grain coarsening prevention characteristics and cold forgeability, characterized in that the steel is dispersed in 100 μm ² or more.