JP3150523B2 - Method for producing hypoeutectoid graphite deposited steel wire - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hypoeutectoid graphite-precipitated steel wire which facilitates graphitization of carbide.

[0002]

2. Description of the Related Art Graphitization of carbides in steel improves machinability by the lubricating action of graphite, and the two-phase structure of ferrite and graphite makes the base material softer, thereby significantly improving cold workability. well known. For example, according to the Journal of the Japan Institute of Metals, No. 12, vol. 52 (1988), p. 1285, as the graphitization rate increases, the cutting resistance decreases, and the curl radius of the chips decreases, improving the processability. It has been reported that Journal of the Japan Institute of Metals, No. 2, vol. 53
According to (1989), p. 206, even if the graphite ratio is increased, the cold workability is superior to that of sulfur free-cutting steel, and it is possible to obtain a performance that combines machinability and cold workability. It has been reported.

[0003] Graphitization is a reaction in which cementite, which is a pure stable phase, is decomposed into graphite and iron, which are stable phases, in principle. Graphitization is not easy due to the accompanying volume expansion despite being a stable phase. Therefore, the knowledge for promoting the graphitization by introducing a graphite nucleation site is described in Journal of the Japan Institute of Metals, No.
3, vol. 30 (1966), p. 279 and No. 7, vol. 43 (197
9), p. 640. In other words, it states that the existence of precipitates serving as graphite nucleation sites, working strain, and martensitic transformation strain are effective as graphite sites. The prior art applying the above findings is introduced below.

[0004] As a method of dispersing a precipitate serving as a graphite nucleus generation site, there is JP-A-2-111842.
It is characterized in that BN precipitates are used as nucleation sites, and after hot rolling, they are annealed to be graphitized. That is,
C: 0.1-1.5%, Mn: 0.05-2.0%,
O: 30 ppm or less, B: 5 to 80 ppm, N: 5 to 80 pp
m, Si: 0.5 to 2.0%, and Ni:
0.1 to 3.0%, one or two kinds of Cu: 0.1 to 1.0%, Ca: 0.0008 to 0.008%, REM:
Contains 0.001 to 0.005% of one or two kinds, and
A hot-rolled steel material excellent in machinability and hardenability, having a ferrite-graphite or ferrite-graphite-cementite structure. However, in this manufacturing method, the annealing time is extremely long in order to obtain a graphitization rate of 100%, and there is a problem in manufacturing cost.

As a method utilizing processing distortion, Japanese Patent Publication No. Sho 63
No. -9580. According to this, C: 0.01
5 to 0.140%, Mn: 0.3% or less, Sol. A
l: 0.02 to 0.30%, N: 0.006% or less,
P: not more than 0.01%, S: not more than 0.010%, and satisfy the formula P (%) × S (%) ≦ 10 × 10 ⁻⁶ ,
Further, Si: 0.03 to 2.50%, Ni: 0.1 to
4.0%, Cu: After the hot rolling of steel containing at least one of 0.03 to 1.00% and the balance being Fe and impurities, cold rolling is performed at a rolling reduction of 30% or more. This is a manufacturing method in which strain is introduced and then annealed. However, this method cannot be said to be a realistic manufacturing method because a new step of cold rolling the steel wire after hot rolling at a rolling reduction of 30% is required. Similarly, a method utilizing the processing strain is introduced in Japanese Patent Publication No. Hei 5-79743.

Japanese Patent Application Laid-Open No. 49-67817 discloses a method utilizing the martensitic transformation strain. According to this, C (Total): 0.45 to 1.5%, graphite:
0.45 to 1.50%, Si: 0.5 to 2.5%, M
n: 0.1 to 2.0%, P: 0.02 to 0.15%,
S: 0.001 to 0.015%, N: 0.008 to 0.
02%, Ni: 0.1 to 2.0%, one or two of Al and Ti, 0.015 to 0.5%, Ca: 0.0005 to
After hot rolling, a steel containing 0.030% is 750-950.
This is a production method of reheating to ℃ and quenching to transform to martensite, heating this again and annealing at 600 to 750 ° C. This manufacturing method has a drawback that the annealing time for graphitization becomes extremely long because no processing strain is added, and also has a drawback that the manufacturing cost becomes high because two heating steps are required after hot rolling. .

[0007] Japanese Patent Publication No. Hei 5-79743 discloses a manufacturing method combining a method of dispersing a precipitate serving as a graphite nucleus generation site and a method of utilizing a processing strain. Even with this method, the above-mentioned disadvantages have not been solved. As described above, there is a problem in the production method for graphitization, and it has not yet been used on an industrial scale. The development of an industrially effective manufacturing method capable of achieving a graphitization ratio of 100% by a short-time heat treatment is strongly desired by the industry.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to solve these problems, and more specifically, to provide a method for producing a hypoeutectoid graphite-precipitated steel wire which can be graphitized to 100% by short-time annealing. The purpose is to provide.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and its gist is that the content of C: 0.
3 to 0.80%, Si: 0.4 to 1.0%, Mn: 0.
3 to 1.0%, P: 0.02% or less, S: 0.015 to
0.035%, B: 0.001 to 0.004%, Ti:
0.001 to 0.02%, Al: 0.01 to 0.03%
, A wire rod immediately after hot rolling consisting of the balance Fe and unavoidable impurities, the cooling start temperature is _{Ar 1} point or more, the cooling end temperature is Ms point or less by a cooling device installed on the rear surface of the hot rolling line. After forced cooling with an average cooling rate of 5 ° C. to 30 ° C./s, further natural cooling was performed,
A method for producing a hypoeutectoid graphite-precipitated steel wire, wherein the graphitization treatment is performed at 0 ° C to 710 ° C.

As described above, it is said that the martensitic transformation strain becomes a graphite nucleation site. The total amount of strain included in martensite was considered to be the sum of the martensite transformation strain and the rolling strain remaining in martensite due to rapid cooling after hot rolling. That is, the present inventors have conducted various studies on the cooling conditions after hot rolling on the graphitization, and as a result, have succeeded in developing a new method for producing hypoeutectoid graphite-precipitated steel and made the present invention.

[0011]

The reasons for defining the scope of the steel of the present invention as described above will be described below. C is an indispensable element for producing graphite. From the viewpoint of securing a certain amount of graphite, the lower limit is set to 0.3 in order to secure the amount of martensite transformation.
%. The upper limit is set to 0.8% or less in order to prevent burning cracks during heat treatment.

Si is an essential element because it has a small bonding force with carbon atoms in steel and is one of the powerful elements that promote graphitization. However, there is a disadvantage in that the solid solution of the ferrite increases the hardness and shortens the life of the cutting tool and the cold working jig. Graphite rate of 1 when quenching + annealing
It is necessary to add Si in order to make it 00%, and its lower limit must be limited to 0.4%.
The reason for limiting the upper limit to 1.0% is to prevent an increase in hardness due to solid solution hardening of ferrite.

Mn needs to be added to the amount necessary for fixing and dispersing sulfur in the steel as MnS and the amount necessary for securing the strength by dissolving it in the matrix and securing the strength. 0.3%. If the amount of Mn increases, graphitization is inhibited, so the upper limit is set to 1.0%. Since P is present in the steel as a phosphorus compound precipitated at the grain boundaries and as P dissolved in ferrite, the machinability is improved and the hot workability is significantly impaired.
2%.

S bonds with Mn and exists as MnS inclusions. When the amount of MnS inclusions in steel increases, tool and MnS
The chance of contact with inclusions increases, and the MnS inclusions plastically deform on the tool rake face to form a coating. as a result,
Since the chance of contact between the ferrite and the tool is reduced, adhesion is suppressed and the properties of the cut surface are improved. In order to suppress the adhesion, the lower limit of S needs to be 0.015%. Since S inhibits graphitization, the upper limit is set to 0.035%.

B segregates at the ferrite crystal grain boundary to become a graphite precipitation site, and has the effect of shortening the graphitizing annealing time. In order to obtain a sufficient shortening effect, 0.001% or more of B must be added. If B exceeds 0.004%, the shortening effect is saturated, so the upper limit is 0.004%.
And Ti is inevitably mixed N: 0.002
It is sufficient to fix 0.008% and to add an amount necessary to make the graphitization promotion by B effective. The lower limit is 0.001% and the upper limit is 0.02%.

Al is a deoxidizing element, and it is necessary to add 0.01% or more when the carbon content of steel is high, and 0.03% when the carbon content is low. The reason why the steel wire immediately after hot finish rolling is forcibly cooled by a cooling device installed on an extension of the hot rolling line is to leave rolling strain due to hot rolling in the quenched martensitic structure. According to this method, the heat energy of the red-hot steel wire after hot rolling can be used for quenching and reheating is not required, and as a result, the heat treatment cost can be reduced.

The cooling start temperature measured on the surface of the steel wire must be equal to or higher than the _Ar1 point in order to increase the degree of graphitization by simultaneously generating martensitic transformation strain and rolling strain. The cooling end temperature must be below the _MS point in order to obtain a sufficient martensitic transformation structure and facilitate graphitization.
The lower limit of the average cooling rate was set at 5 ° C./s in order to obtain a martensitic transformation structure and to maintain the processing strain to facilitate graphitization, and set the upper limit to 30 ° C./s. The reason for this is that the amount of martensite transformation does not increase even if cooling is performed more rapidly. The lower limit of the annealing temperature is limited to 610 ° C. and the upper limit is set to 710 ° C. because the graphitization time in this temperature range is the shortest.

Here, means for producing the steel of the present invention will be described. A steel wire having a diameter of 5.5 to 19 mm immediately after hot finish rolling was placed on the rear surface of the hot rolling line at a water temperature of 20 ° C to 10 ° C.
The sample was kept in a water bath at 0 ° C. for 20 to 200 seconds, then cooled naturally, and further graphitized in an offline annealing furnace.
The cooling start temperature was adjusted by controlling the time from the end of rolling to the water tank, and the cooling end temperature was adjusted by controlling the residence time in the water tank.

Next, the effects of the present invention will be described more specifically by way of examples.

[0020]

EXAMPLES Tables 1 and 2 show the chemical composition and production conditions of the test steel.

[0021]

[Table 1]

[0022]

[Table 2]

The steel wire used in the test was 5.5 to 19 in diameter.
In mm, it was cooled in a water tank installed on an extension of the hot rolling line. The distance from the finishing mill stand to the water tank is about 45
m, the rolling speed is 10 to 100 m / s, and the required time from the end of rolling to the water tank is about 1 to 3 seconds. The wire was wound into a coil and inserted into a water bath having a water temperature of 20 to 100 ° C. The residence time in the water tank is between 20 and 200 seconds. The cooling start temperature and the cooling end temperature are values obtained by measuring the surface temperature of the steel material with a radiation thermometer, and the average cooling rate was determined by dividing the difference between the cooling start temperature and the cooling end temperature by the cooling time.

Table 3 shows the annealing time and the graphitization rate.

[0025]

[Table 3]

The graphitization ratio was calculated by the following equation. (Graphite content in steel / Carbon content of steel) × 100 (%) The carbon content of steel was quantified by chemical analysis. The graphite content was calculated from the average graphite particle diameter, the density and the number of graphite particles.
The graphitization rate of the steel wire obtained by the method of the present invention is 100%, which is a remarkably excellent result, although the annealing time is as short as about 10 hours. In the case of the comparative method, the graphitization rate is as low as about 70%.

Here, the comparative method F in Table 1 shows the average cooling rate and the graphitizing annealing temperature, the comparative method G shows the B, Ti, Al component and the graphitizing annealing temperature, and the comparative method H shows Si, Mn, Ti.
The components and cooling rates were outside the scope of the present invention, and Comparative Methods I and J were off-line quenching without direct quenching after hot rolling. The cooling device on the rear side of the hot rolling line in the present invention is not limited to the water tank of the present embodiment. Average cooling rate of 5-30 ° C /
s, preferably may be a cooling device according to possible Naru liquid and / or gaseous refrigerant cooled cooling end temperature below M _S point 6 to 26 ° C. / s. At that time, the cooling start temperature is A _r1
The temperature may be equal to or higher than the temperature, preferably the temperature of _Ar1 point + 50.
In the present invention is to then naturally cooled, the natural cooling end temperature may be a M _S point -200 ° C.. Further, the heating temperature of the graphitizing annealing in the next treatment is in the range of 610 to 710 ° C, preferably 630 to 700 ° C.

[0028]

As is clear from the above embodiments, according to the present invention, it is possible to provide an excellent production method capable of realizing a graphitization ratio of 100% with an extremely short annealing time.
The industrial effects are very significant.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/06, 9/52 C22C 38/00, 38/14

Claims (1)

(57) [Claims] (1) C: 0.3 to 0.80% by mass %, S:
i: 0.4 to 1.0%, Mn: 0.3 to 1.0%, P:
0.02% or less, S: 0.015 to 0.035%, B:
0.001 to 0.004%, Ti: 0.001 to 0.0
2%, Al: 0.01 to 0.03%, with the balance Fe
And the wire rod immediately after hot rolling consisting of unavoidable impurities, by a cooling device installed on the rear surface of the hot rolling line, the cooling start temperature is _{Ar 1} point or more, the cooling end temperature is Ms point or less,
A method for producing a hypoeutectoid graphite-precipitated steel wire, comprising forcibly cooling at an average cooling rate of 5 ° C. to 30 ° C./s, further natural cooling, and then graphitizing at a heating temperature of 610 ° C. to 710 ° C.