JP2000192147A - Directly spheroidized annealing method of low alloy wire rod - Google Patents

Abstract

PROBLEM TO BE SOLVED: To directly obtain a spheroidized structure in a short time with inexpensive equipment without needing to troublesome rolling control. SOLUTION: Hot rolling is applied to a low alloy steel material containing 0.1-1.2 mass % C, 0.25-1.60 mass % Cr under condition of 900-1,200 deg.C the last finish temp. to make a wire rod and successively, after cooling to the temp. of <= (Ar1 transformation temp. -30 deg.C), when this wire rod is heated and cooled to execute the spheroidized annealing, the max. heating temp. is regulated to the temp. range of (Ac1 transformation temp. +30 deg.C) to (Ar1 transformation temp. +70 deg.C) at the heating time of the wire rod, and the cooling speed from (Ar1 transformation temp. -30 deg.C) to the max. heating temp. is regulated to <=1.0 deg.C/sec, and thereafter, the wire rod is continuously cooled at the cooling speed of 0.2-5 deg.C/sec from the max. heating temp. to the temp. of <= (Ar1 transformation temp.).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for direct spheroidizing annealing of an alloy steel wire rod, and more particularly, to a method for reducing the annealing time by a simple method when performing direct spheroidizing annealing after hot rolling. It is intended to achieve advantageous shortening.

[0002]

2. Description of the Related Art In general, a machine part formed by cold forging or cutting an alloy steel wire is subjected to primary pickling for the purpose of removing the scale of the steel wire and then to spheroidizing annealing. Next, a secondary pickling is performed to remove the decarburized layer and scale generated by the spheroidizing annealing, and a wire drawing process of about 10% is performed to further improve dimensional accuracy. It is processed and molded. The spheroidizing annealing of the alloy steel wire as described above is an indispensable process for sufficiently reducing the hardness of the material at the time of forming to secure good workability. This is carried out by charging into a pot furnace in a state of being wound around and giving a predetermined heat history.

[0003] However, the above-mentioned spheroidizing annealing method has the following problems. (a) It takes a long time (usually about 20 to 30 hours) to heat and cool to provide a heat history of heating or cooling in the state of being wound on the coil, and low productivity due to batch processing. , Heat treatment costs increase. (b) Since the heat histories are significantly different at each part in the coil, there is a large variation in the quality of the wire after annealing. (c) Even if the coil weight is increased to improve productivity, a large pot furnace is required to process a heavy coil, so only excessive capital investment is required. Instead, the cost of maintaining it increases significantly.

To solve the above-mentioned problem, Japanese Patent Application Laid-Open No. 63-230821 discloses C: 0.10 to 1.00 mass%.
Is hot rolled, the temperature of the material to be rolled on the entrance side of the group of finishing mills is set to 650 to 850 ° C., and the final finishing temperature of the material to be rolled on the exit side of the group of finishing mills is 750. ~
The steel wire is adjusted to 900 ° C., and then the steel wire is cooled at a cooling rate of 2 ° C./sec or more to a temperature of 650 ° C. or less, and then the cooled steel wire is cooled to 2 ° C./sec or more. at a heating rate to a temperature range of Ac ₁ to Ac ₁ +160 ° C. by heating, and, to the time maintained within 5 minutes in the temperature range, then heated to the temperature range, retained the steel wire rod, 1) any Cooling at a cooling rate to a temperature of Ar _{1 to} Ar ₁ −160 ° C. and holding in said temperature range for 5 to 60 minutes, or 2) cooling to the temperature of Ar ₁ at any cooling rate, then ,
A method for directly spheroidizing a hot-rolled steel wire, characterized in that the steel wire cooled to the temperature is cooled to a temperature of Ar ₁ -80 ° C. at a cooling rate of 2 ° C./sec or less. I have.

[0005]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No.
In the method disclosed in Japanese Patent No. 230821, for the purpose of shortening the spheroidizing time, a method of obtaining a fine austenite structure and increasing the number of pearlite precipitation sites is employed to promote the transformation from austenite to ferrite. For this reason, the temperature before and after finish rolling needs to be considerably reduced, and new installation or modification of equipment such as installing a water cooling zone before finishing rolling or increasing the capacity of the rolling mill is indispensable. Had a problem where it was extremely bulky.

A main object of the present invention is to propose a direct spheroidizing annealing method of an alloy steel wire rod capable of directly obtaining a spheroidized structure without having to perform complicated rolling control under inexpensive equipment. It is in.

[0007]

Means for Solving the Problems In view of the above-mentioned objects, the present inventors have assumed that there is no increase in equipment costs and that there is no need to perform complicated rolling control, and the direct production of alloy steel wires is A spheroidizing annealing method (a method of continuously performing spheroidizing annealing online after wire rod rolling) was studied.

[0008] Generally, spheroidizing annealing is a method in which a material is Ac
_After heating above the transformation point to partially dissolve part of the layered pearlite, the Ar ₁ transformation point is slowly cooled (slow cooling method) or isothermically maintained below the Ar ₁ transformation point (two-step method), This is a heat treatment for precipitating C in the supersaturated austenite as a solid solution based on the core of the residual carbide. As described above, as the spheroidizing annealing method, the slow cooling method and the two-step method are known, and the thermal history is shown in FIGS. 1 (a) and (b), respectively. The spheroidizing annealing processing time by the slow cooling method and the two-step method requires the processing time indicated by Y in FIG. What is important here is that the number of residual carbides before slow cooling is equal to the number of finally obtained spheroidized carbides because no new nuclei are formed during cooling.

The time required to obtain a complete spheroidized structure becomes shorter as the amount of residual carbide after heating indicated by X in FIG. 1 increases. For this reason, in order to shorten the annealing time, it is generally considered necessary to increase the amount of residual carbide at the point indicated by X in FIG. However, if this residual carbide is too large, there arises a problem that the hardness of the wire after annealing does not decrease to a predetermined value. This is because, as the amount of residual carbides increases, the number of pearlite precipitation sites increases, so that the transformation is promoted and the annealing time is shortened.
The number of carbides finally obtained tends to increase.
It is considered that the hardness is not sufficiently reduced due to the dispersion strengthening by the carbide. Therefore, there is a need for a direct spheroidizing annealing method that simultaneously achieves a reduction in processing time and sufficient softening.

Here, in order to sufficiently reduce the hardness by the spheroidizing annealing and effectively shorten the spheroidizing annealing time, the inventors have determined the temperature of the spheroidizing annealing. We found that strict control of the pattern was important. That is, even if the temperature before and after the finish rolling is not particularly low, the heating temperature range during the spheroidizing annealing and the heating speed corresponding to each temperature, the cooling speed during cooling and the cooling stop temperature within a predetermined range. By controlling, even if the number of residual carbides after heating is the same, it was found that the processing time was shortened. Thus, it was possible to set spheroidizing annealing conditions that shorten the processing time and satisfy the hardness. The present invention is based on the above findings.

That is, according to the present invention, a low-alloy steel material containing 0.1 to 1.2 mass% of C and 0.25 to 1.60 mass% of Cr is hot-rolled to a final finishing temperature of 90%.
The temperature is reduced to a temperature of (Ar ₁ transformation point −30 ° C.) or lower, and then the wire is heated and cooled to perform spheroidizing annealing. When heating, set the maximum heating temperature to (Ac ₁ transformation point + 30 ° C)
(Ac ₁ transformation point + 70 ° C) and the cooling rate from the temperature of (Ar ₁ transformation point -30 ° C) to the maximum heating temperature is 1.0 ° C / sec or less. ₁
(Transformation point) A direct spheroidizing annealing method for a low alloy wire, characterized by continuously cooling at a cooling rate of 0.2 to 5 ° C./sec to the following temperature.

In the present invention, the cooling rate is set to 0.
It is more preferable to determine within the range of 2 to 5 ° C./sec according to the carbon content of the wire.

Here, when the spheroidizing annealing after rolling of the steel wire is performed in a single thread or in units of several loops, not only heating and cooling are completed in a short time, but also temperature control is easy. However, it is needless to say that the spheroidization time can be shortened by applying the present invention to a coil state or a steel bar.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. First, a steel material targeted in the present invention will be described. The present invention can be applied to all steel materials for mechanical parts which require spheroidizing annealing in a manufacturing process. For reference, the representative steel types are high carbon chromium bearing steel (JIS G 4805; for example, SUJ
2) Carbon steel for machine structure (JIS G 4051; for example, S45
C) and chrome steel (JIS G 4104; for example, SCr420)
And the like.

Next, the reasons for limiting the above-mentioned components, particularly C and Cr, of the steel materials for machine parts as described above will be described. C: 0.1 to 1.2 mass% C is a useful component that forms a solid solution to strengthen the matrix and improves sufficient strength and wear resistance as mechanical parts. If the C content is less than 0.1 mass%, it is not necessary to perform spheroidizing annealing before cold working, so the lower limit of C was set to 0.1 mass%. On the other hand, when C exceeds 1.2 mass%, proeutectoid cementite precipitates in a net shape, and the cold workability deteriorates.
The upper limit of C was set to 1.2 mass%.

Cr: 0.25 to 1.60 mass% Cr effectively contributes to improving the hardenability of the steel material and spheroidizing the carbide. If the Cr content is less than 0.25 mass%, the effect is small, and no matter how the spheroidizing annealing conditions are changed, a good spheroidized structure cannot be obtained because lamellar pearlite is precipitated. On the other hand, if the Cr content exceeds 1.60 mass%, the machinability decreases and the cost increases in view of the chemical composition.
It was added in the range of 0.25 to 1.60 mass%. that's all,
Although the essential components have been described, in the present invention, these two components are used.
As long as the component satisfies the above range, the other components are not particularly limited, and can be appropriately contained according to the characteristic value required for each steel material.

Next, the reason why the rolling conditions and the spheroidizing annealing conditions in the present invention are limited to the above ranges will be described. Finish rolling temperature: 900 to 1200 ° C. In the present invention, even if the finish rolling temperature is lower than 900 ° C., it is possible to obtain a direct spheroidized structure which is the object of the present invention. The temperature was increased to 900 ° C or more because the rolling mill needed to be strengthened. On the other hand, if the finish rolling temperature exceeds 1200 ° C, the amount of decarburization increases, and
1200 ° C. was set as the upper limit because surface defects increased rapidly.

Cooling stop temperature: (Ar ₁ transformation point −30 ° C.) or less For direct spheroidization, the structure after rolling must once be a structure mainly composed of fine pearlite or bainite or martensite. For this purpose, the cooling stop temperature after rolling needs to be equal to or lower than the Ar ₁ transformation point, and is set to at least the Ar ₁ transformation point −30 ° C. or lower from the viewpoint of temperature control.

The maximum heating temperature is (Ac ₁ transformation point + 30 ° C.)
The temperature range is (Ac ₁ transformation point + 70 ° C), and the cooling rate from the temperature (Ar ₁ transformation point -30 ° C) to the maximum heating temperature is 1.0 ° C / sec or less. This is one of the major points of the invention. The hardness after spheroidizing annealing depends on the number of residual carbides, and tends to become softer as the number decreases, that is, as the diameter of the carbides increases. Therefore, in order to reduce the hardness of the material, it is necessary to reduce the number of carbides from the stage of heating. For this purpose, it is efficient and important that the heating rate is slowed down, the transformation is carried out gradually to partially dissolve the carbide, and C is sufficiently dissolved in the parent phase.

The degree of spheroidization, hardness and processing time were examined by changing the heating temperature range and heating rate variously, and the maximum heating temperature was found to be (Ac ₁ transformation point + 30 ° C.) to (Ac ₁ transformation point +70).
° C), and the cooling rate from the temperature of (Ar ₁ transformation point-30 ° C) to the maximum heating temperature must be 1.0 ° C / sec or less. If the maximum heating temperature is too high, the number of residual carbides will be too small and the hardness will be low, but the transformation time will increase significantly, or the residual carbides will disappear and spheroidization itself will not be achieved, Because lamellar pearlite is precipitated during the cooling process, the upper limit of the heating temperature is set to (Ac ₁ transformation point + 30 ° C.) to (Ac ₁
(Transformation point + 70 ° C).

The heating rate is closely related to the heating temperature range. When the cooling rate from the temperature of (Ar ₁ transformation point −30 ° C.) to the maximum heating temperature exceeds 1.0 ° C./sec, sufficient transformation occurs. It does not occur, and the dissolution of some carbides is insufficient. Therefore, the cooling rate was set to 1.0 ° C./sec or less. Although the lower limit of the cooling rate is not particularly specified, it is preferably 0.1 ° C./sec from the viewpoint of shortening the processing time. The lower limit of the temperature range of the cooling rate is set to (Ar ₁ transformation point −30 ° C.) because at lower temperatures, the processing time only increases, and
The reason why (Ar ₁ transformation point is −30 ° C.) is that the temperature is low enough to gradually perform transformation while dissolving the carbide in the matrix.

Further, from the above maximum heating temperature (Ar ₁ transformation point)
In order to promote spheroidization by continuously cooling to the following temperature at a cooling rate of 0.2 to 5 ° C./sec, instead of precipitating pearlite during cooling, spherical carbides are precipitated by using residual carbides as nuclei, Need to grow. For that purpose, it is important to select a cooling rate and a cooling stop time. In order to obtain a stable spheroidized carbide, it is necessary to cool to a temperature not higher than the Ar ₁ transformation point at a rate of 5 ° C./sec or less. When the cooling rate is too high, or when the cooling stop temperature is too high, the lamellar pearlite precipitates and a poor spheroidized structure is obtained, so that the above range is set. The lower limit of the cooling rate is 0.2 ° C./sec or more from the viewpoint of shortening the processing time.

According to the present invention, the cooling rate is set to 0.2 to
It is more preferable to set the temperature within the range of 5 ° C./sec according to the carbon content. In order to obtain a good spheroidized structure, it is advantageous to set a cooling rate according to the amount of supersaturated C to be precipitated. That is, the above-mentioned supersaturated C
When the amount is large, it is preferable to reduce the cooling rate in order to precipitate spherical carbides by using the residual carbides as nuclei and to sufficiently grow the carbides. When the amount of supersaturated carbon is small, the cooling rate can be increased, and the processing time can be shortened. In order to obtain a stable spheroidized carbide, the temperature is at least 5 ° C / sec or less, and the lower limit is 0.2 ° C / sec or more from the viewpoint of shortening the processing time.

[0024]

EXAMPLES Ac ₁ having the component composition shown in Table 1 and having the composition shown in Table 1 was obtained.
The steel at the transformation point and the Ar ₁ transformation point was melted in a converter, and then continuously cast into a bloom of 400 mm × 560 mm. Then
Under the manufacturing conditions (a) to (g) shown in FIG. 2, a wire having a diameter of 6.50 mm was obtained. A part of the bloom was 6.85 mmφ by hot rolling.
After being wound around a coil and allowed to cool to room temperature, it is subjected to spheroidizing annealing under normal conditions (Fig. 1 (a) long-time slow cooling method), and is further drawn to 6.50 mmφ after pickling and lubrication. (Conventional method 1). Further, according to the method disclosed in JP-A-63-230821, a wire having a diameter of 6.50 mm was manufactured (conventional method 2).

[0025]

[Table 1]

A microscopic sample was taken from the 6.50 mm diameter wire rod, microstructure was observed after corrosion with picral, and the spheroidization ratio was measured. The spheroidization rate was observed and photographed at 8000 times with a scanning electron microscope.
The major axis and minor axis were individually measured for 500 or more carbides, and the ratio of the number of carbides having a major axis / minor axis ratio of 2.0 or less to the total number of carbides was expressed as a percentage of spheroidization in%. Also,
A sample of 6 mmφ × 8 mm was cut out, and the cold forgeability was evaluated by a visual inspection of the occurrence of surface cracks in the sample when the sample was compressed in the cold. Table 2 shows the obtained results.

[0027]

[Table 2]

The production condition (a) is a comparative example. Since the rolling finish temperature is 1250 ° C., which is higher than the range of the present invention, the decarburization amount is large and surface flaws are generated. Also, manufacturing conditions
(b) is also a comparative example, in which the maximum heating temperature is Ac ₁ transformation point + 90 ° C.
And higher than the range of the present invention, the amount of residual carbide is extremely reduced, layered pearlite is generated, and the spheroidization ratio is reduced and the hardness is increased as compared with the conventional example. Further, the manufacturing conditions (c-1), (c-2) and (d) are also comparative examples, and the heating rate is as high as 2.0 ° C./sec, the heating start temperature is as high as the Ar ₁ transformation point, or the cooling rate is Since the rate is as high as 10 ° C./sec, the transformation is insufficient and the dissolution of the residual carbide does not progress relatively, and the number of carbide finally obtained is large and the hardness is high.

On the other hand, the production condition (e) is such that the rolling temperature and the subsequent heat history satisfy the appropriate range of the present invention.
All of the A-1 steel, B-1 steel and C-1 steel have a C content which does not satisfy the lower limit of the present invention. Therefore, a good spheroidized structure cannot be obtained, pearlite is precipitated, and the spheroidization ratio is lower and the hardness is higher than in the conventional example.

On the other hand, A-2 steel, B-2 steel and C-2 steel obtained under the manufacturing conditions (e) to (g) are applicable examples of the present invention, all of which are transformed during cooling. Was completed, and a spheroidization ratio equal to or higher than that of the conventional example and a hardness equal to or lower than that of the conventional example could be obtained. Comparing (e) and (f), No. 9, 10, 2
In the case of 0, 21, 31, and 32, it can be seen that the hardness is further reduced by decreasing the cooling rate as the C amount increases.

The production condition (h) is a composition mainly composed of bainite which is quenched by using a salt bath after finish rolling, but compared with the production condition (e) which is the same spheroidizing annealing condition. The spheroidization rate is improved by 5-10% on average, and the hardness is average Hv
5-10>l; j was lower. Further, the cold forgeability obtained by the present invention is equal to or higher than that of a material obtained by performing cold drawing on conventional spheroidizing annealing, and the energy saving effect by short-time treatment is extremely large.

[0032]

Thus, according to the present invention, there is no need to lower the rolling temperature, and a spheroidized structure can be stably obtained in a short time, and as a result, without increasing the equipment, etc. Productivity can be significantly improved.

[Brief description of the drawings]

FIG. 1 shows a thermal history in a general spheroidizing annealing, in which (a) is a slow cooling method and (b) is a two-step method.

FIG. 2 is a view showing heat treatment conditions in spheroidizing annealing.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Toshiyuki Hoshino 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. Inside the Mizushima Works of Kawasaki Steel Corporation (72) Inventor Ken-ichi Amano, Kawasaki-dori Mizushima, Kurashiki-shi, Okayama 1-chome (without address) F-term in Kawasaki Steel Corporation Mizushima Works (reference) 4K032 AA05 AA06 AA11 AA12 AA16 AA27 AA29 AA31 BA02 CC04 CF02 CF03 4K043 AA02 AB04 AB05 AB06 AB10 AB11 AB15 AB25 AB26 AB27 BA11 BA06 DA05 FA

Claims (2)

[Claims] 1. A low alloy steel containing C: 0.1 to 1.2 mass% and Cr: 0.25 to 1.60 mass% is hot rolled to a final finishing temperature of 90%.
The temperature is reduced to a temperature of (Ar ₁ transformation point −30 ° C.) or lower, and then the wire is heated and cooled to perform spheroidizing annealing. When heating, set the maximum heating temperature to (Ac ₁ transformation point +
30 ° C) to (Ac ₁ transformation point + 70 ° C), and
The cooling rate from the temperature of (Ar ₁ transformation point −30 ° C.) to the maximum heating temperature is 1.0 ° C./sec or less, and the cooling rate from the maximum heating temperature to the temperature of (Ar ₁ transformation point) or less
A method for direct spheroidizing annealing of a low alloy wire, characterized by continuously cooling at a cooling rate of 0.2 to 5 ° C / sec. 2. A method for direct spheroidizing annealing of a low alloy steel wire, wherein the cooling rate is determined within the range of 0.2 to 5 ° C./sec according to the carbon content of the wire.