JP2003073742A - Manufacturing method of high-quenching high-carbon hot-rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-carbon hot rolled steel sheet with high hardenability, which can be produced without employing high temperature and multistage annealing, hardly generates cracks due to press forming and cold forging, is extremely soft, and has superior workability. SOLUTION: The method for manufacturing a high-carbon hot rolled steel sheet with hardenability comprises hot rolling a steel containing C of 0.2-0.7 mass % to control the structure so as to have a bainite phase with a volume factor of greater than 20%, and annealing it to make a spheroidizing structure. The method further comprises hot rolling the steel at a finishing temperature (Ar3 transformation point -20 deg.C) or higher, cooling it at a higher cooling rate than 120 deg.C/second and at a cooling finishing temperature of 620 deg.C or lower, subsequently winding it up at a winding temperature of 600 deg.C or lower, pickling it, then annealing it at an annealing temperature of 640 deg.C or higher but Ac1 transformation point or lower. Alternatively, the method can include controlling the structure so as to have the bainite phase with a volume factor of greater than 70%, or further cooling it at the finishing temperature of 550 deg.C or lower, and winding it at a winding temperature of 500 deg.C or lower.

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 high hardenability high carbon hot rolled steel sheet which contains 0.2 to 0.7 mass% of C and is excellent in workability.

[0002]

2. Description of the Related Art High carbon steel sheets used for tools or automobile parts (gears, missions) are
Heat treatment such as quenching and tempering is performed. In recent years, manufacturers of tools and parts, that is, users of high-carbon steel plates, have been able to reduce costs by performing cutting work on cast materials and processing parts by hot forging, and then press forming steel plates (including cold forging). Simplification of the processing process is being studied. Along with this, it is strongly demanded that the high-carbon steel sheet as a raw material has high hardenability and excellent workability capable of processing a complicated shape even in a small number of steps, particularly softness.

Several techniques have been studied for softening such a high carbon steel sheet. For example,
Japanese Patent Laid-Open No. 9-157758 proposes a method for producing a high carbon steel strip in which after hot rolling, the ferrite-austenite two-phase region is heated at a predetermined heating rate and annealed at a predetermined cooling rate. In this technique, a high-carbon steel strip is annealed in the ferrite-austenite two-phase region where Ac is ₁ point or more, and coarse spheroidized cementite is uniformly distributed in the ferrite matrix.

Details of this prior art are as follows: C: 0.2-0.8%, Si:
0.03 to 0.30%, Mn: 0.20 to 1.50%, Sol.Al: 0.01 to 0.10%,
High-carbon steel with N: 0.0020 to 0.0100% and Sol.Al/N: 5 to 10 is hot-rolled, pickled, descaled, and then 95 volume.
In an atmosphere furnace consisting of hydrogen and balance nitrogen, the heating rate Tv (℃ / Hr): 500 × (0.01-N (%) asAl in the temperature range of 680 ℃ and above.
N) to 2000 × (0.1-N (%) asAlN), soaking temperature TA (° C): Ac ₁ point to 2
22 × C (%) ² -411 × C (%) + 912, soaking time: 1 to 20 hours annealing, cooling rate: 100 ℃ / Hr to cool to room temperature at a cooling rate below .

Further, JP-A-64-25946 and JP-A-8-24
Japanese Patent No. 6051 also proposes a method of graphitizing carbon in steel to achieve softness and high ductility.

[0006]

However, these techniques have the following problems.

In the technique described in Japanese Patent Laid-Open No. 9-157758, a high carbon steel strip is annealed in a ferrite-austenite two-phase region with Ac ₁ point or more to obtain coarse spheroidized cementite.
It is clear that such coarse cementite deteriorates the hardenability because of its slow dissolution rate. Also, regarding the hardness after annealing, Hv 132-141 (HRB 72-7
5), which is not necessarily soft.

Further, JP-A-64-25946 and JP-A-8-24
The technique described in Japanese Patent No. 6051 has a problem that carbon in steel is graphitized and the rate of dissolution of graphite is slow, so that the hardenability is poor.

Recently, the demands on the processing level from the viewpoint of improving productivity have become more severe than ever before.
Therefore, even in press working of a high carbon steel sheet, cracks are likely to occur due to an increase in workability and the like. Therefore, high carbon steel sheets are required to be extremely soft and have high workability.

The present invention has been made in view of the above circumstances.
-No need for high temperature annealing like austenite region,
Another object of the present invention is to provide a high-hardenability high-carbon hot-rolled steel sheet that can be manufactured without using multi-step annealing, is hardly cracked by press forming or cold forging, and is extremely soft and excellent in workability.

[0011]

The above-mentioned problems can be solved by the following inventions. The invention is characterized in that steel containing C in an amount of 0.2 to 0.7 mass% is hot-rolled to control a structure having a bainite phase having a volume ratio of more than 20% and then annealed to form a spheroidized structure. A method for producing a high-hardenability high-carbon hot-rolled steel sheet.

Further, in the present invention, after hot rolling is performed at a finishing temperature (Ar ₃ transformation point -20 ° C) or higher, a cooling rate is 120 ° C.
/ Sec and cooling at a cooling end temperature of 620 ° C or less, then winding at a winding temperature of 600 ° C or less, pickling, and annealing temperature of 640
It is also possible to use a method of manufacturing a high-hardenability high-carbon hot-rolled steel sheet characterized by annealing at a temperature of ℃ or higher and Ac ₁ transformation point or lower.

In these inventions, the volume ratio is 70%.
It is also possible to provide a method for producing a high-hardenability high-carbon hot-rolled steel sheet, which is controlled to have a structure having a bainite phase that exceeds, or further, a cooling end temperature of 550 ° C or less and a winding temperature of 500 ° C or less. .

These inventions have been made while earnestly studying the influence of the microstructure and manufacturing conditions on the hardness of a high carbon steel sheet. In the process, it was found that not only the annealing conditions but also the microstructure of the hot-rolled steel sheet before annealing had a great influence as a factor affecting the hardness after spheroidizing annealing. As a result, in order to achieve both high hardenability and workability, the spheroidized cementite after spheroidizing annealing, which was difficult with the conventional technique, was uniformly fine, and succeeded in obtaining coarse ferrite grains. For that purpose, it is effective to include bainite in the structure of the hot rolled steel sheet. Hereinafter, details of the present invention will be described.

C content: 0.2 to 0.7% by mass C is an important element that forms a carbide and imparts hardness after quenching. If the C content is less than 0.2%, sufficient strength cannot be obtained as a machine structural part after quenching. C content is
If it exceeds 0.7%, sufficient workability cannot be obtained even after annealing. Further, in that case, the hardness of the steel sheet after hot rolling is high and brittle, which is inconvenient to handle and the strength after quenching is saturated. Therefore, the C content is set within the range of 0.2 to 0.7%.

Microstructure of hot rolled steel sheet: volume ratio of bainite phase
Regarding the structure of the hot-rolled steel sheet before the 20% super-spheroidizing annealing, the structure having a bainite phase is more preferable after the spheroidizing annealing than the structure having a pearlite single phase. When the volume ratio of the bainite phase exceeds 20%, carbides are finely spheroidized during spheroidizing annealing, and ferrite grains are uniformly coarsened. Therefore, the volume ratio of the bainite phase is controlled to a value exceeding 20%.

Further, when the volume ratio of the bainite phase is 70% or more, the dispersed state of the carbide becomes more uniform and fine, uniform coarse ferrite grains can be obtained, and extremely excellent workability and hardenability can be imparted. .

The finishing temperature: is lower than a finishing temperature of (Ar ₃ transformation point -20 ° C.) or higher hot rolling (Ar ₃ transformation point -20 ° C.), since a part in the ferrite transformation proceeds, ferrite + pearlite + bainite And a bainite phase with a volume ratio of more than 20% cannot be obtained. Therefore, even after the spheroidizing annealing, the carbides are not uniformly dispersed, the hardenability is deteriorated, the ferrite grains are not easily coarsened, and the softening is not sufficiently performed. Therefore, the finishing temperature is set to (Ar ₃ transformation point −20 ° C.) or higher.

Cooling condition after rolling: Cooling rate> 120 ° C./sec In the present invention, rapid cooling (cooling) is required after rolling in order to reduce the volume ratio of the ferrite phase after transformation. When the cooling method is slow cooling, the degree of supercooling of austenite is small and proeutectoid ferrite is generated. If the cooling rate is 120 ° C / sec or less,
Since the formation of proeutectoid ferrite becomes remarkable, it becomes impossible to obtain a bainite phase with a volume ratio exceeding 20%. Therefore, the cooling rate after rolling is set to a rate exceeding 120 ° C / sec.

Cooling end temperature: 620 ° C. or less Ending the rapid cooling after rolling When the cooling end temperature is higher than 620 ° C., not only ferrite is formed during cooling until winding (slow cooling) or after winding, but also pearlite The lamella spacing becomes coarse and the volume ratio of bainite phase decreases to 20% or less. Therefore, finely dispersed fine carbides cannot be obtained after spheroidizing annealing, and hardenability deteriorates. Therefore, the cooling end temperature of rapid cooling (cooling) after hot rolling is set to 620 ° C or lower.

Further, by setting the cooling end temperature to 550 ° C. or less, the volume ratio of the bainite phase becomes 70% or more, and during the spheroidizing annealing, ferrite grains are uniformly coarsened and extremely softened, which is excellent processing. Sex is obtained.

Winding temperature: 600 ° C. or less In the winding after cooling, when the winding temperature exceeds 600 ° C., pro-eutectoid ferrite is formed and the lamellar spacing of pearlite becomes large, and the bainite phase having a volume ratio of more than 20% is obtained. Will not be obtained. Therefore, not only is the carbide after annealing coarsened and the hardenability deteriorates, but also sufficient softening cannot be obtained and workability deteriorates. Therefore, the coiling temperature should be 600 ° C or lower.

Further, by setting the coiling temperature to 500 ° C. or less, the bainite phase volume ratio becomes 70% or more, and the dispersed state of the carbide becomes more uniform and finer, resulting in extremely excellent workability and hardenability. can get. The lower limit of the coiling temperature is not particularly specified, but the lower the temperature, the more the shape of the steel sheet deteriorates.

Annealing temperature: 640 ° C. or more and Ac ₁ transformation point or less After hot-rolling the hot-rolled steel sheet, it is annealed in order to sufficiently coarsen ferrite grains and to spheroidize carbides. If the annealing temperature is lower than 640 ° C., the coarsening of ferrite grains and the spheroidization of carbides are insufficient, and the workability deteriorates because the softening is not sufficient. On the other hand, if the annealing temperature is Ac ₁
If it exceeds the transformation point, a part of it becomes austenite and pearlite is generated again during cooling, so that the workability also deteriorates. Therefore, the annealing temperature is set to 640 ° C. or higher and the Ac ₁ transformation point or lower.

In order to obtain extremely soft and excellent workability and hardenability, the annealing temperature is preferably 690 ° C. or higher. Further, box annealing is preferable in order to stably obtain extremely soft and excellent workability.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION In the steel used in the present invention, the C content is 0.2 to 0.7 mass%, and the metal structure may be controlled to the above-mentioned state. The C content is preferably 0.3% or more in order to sufficiently secure the strength of the gear parts after quenching. Other chemical components are not particularly specified, and there is no problem even if elements such as Mn, Si, P, S, Al and N are contained in the normal range. However, the following is preferable.

First, regarding Si, the carbide is graphitized,
Hardenability tends to be impaired, so 2% or less is desirable. Regarding Mn, excessive addition tends to cause a decrease in ductility, so 2% or less is desirable.

Regarding P and S, if they are contained excessively, the ductility is deteriorated and cracks are apt to be generated, so that both are less than 0.
It is desirable that it is 03% or less.

Regarding Al, if added excessively, Al
Since a large amount of N precipitates and deteriorates hardenability, it is desirable to set it to 0.08% or less. Also with respect to N, if it is contained excessively, ductility is lowered, so 0.01% or less is desirable.

Further, depending on the purpose, various elements such as B, Cr, Cu, Ni, Mo, Ti, Nb, W, V, and Zr may be added within the range usually added. These elements do not particularly affect the effects of the present invention. Further, various elements such as Sn and Pb may be mixed as impurities in the manufacturing process, but such impurities do not particularly affect the effect of the present invention.

A high carbon steel having the components thus prepared is ingot-cast.
-The slab is made by slab rolling or continuous casting. Hot rolling is performed on this slab, but at that time, the slab heating temperature is set to avoid deterioration of the surface state due to scale generation.
It is desirable that the temperature is 1280 ° C or lower.

In order to secure the finishing temperature, the rolled material may be heated by a heating means such as a bar heater during hot rolling. After finish rolling, quenching is performed under the above-mentioned conditions. On this occasion,
If the cooling is started within the time of more than 0.1 second and less than 1.0 second after the finish rolling, the workability can be further improved. Further, in order to promote spheroidization or reduce hardness, the coil may be kept warm by means such as a slow cooling cover after winding.

The annealing after hot rolling may be box annealing or continuous annealing. Then, temper rolling is performed if necessary. This temper rolling does not affect the hardenability, so there are no particular restrictions on the conditions.

The composition of the high carbon steel of the present invention is prepared by
Either a converter or an electric furnace can be used. Further, the continuous casting slab may be subjected to the direct feed rolling in which the slab is rolled as it is or while keeping the heat for the purpose of suppressing the temperature decrease. Alternatively, finish rolling may be performed by applying a thin slab casting technique or the like without performing rough rolling.

Since the steel sheet of the present invention produced in this manner is excellent in workability and hardenability, it is also suitable as a material for a press temper or die quench for simultaneously performing press forming and quenching.

[0036]

Example A continuously cast slab of steel having the chemical composition shown in Table 1 was heated to 1250 ° C., hot-rolled and annealed under the conditions shown in Table 2 to produce a steel plate having a thickness of 5.0 mm.

[0037]

[Table 1]

[0038]

[Table 2]

In Table 2, steel sheets Nos. 1 to 10 are inventive examples whose manufacturing conditions are within the scope of the present invention. Among the invention examples, especially steel plate No.
2,4,6,8,10 have a cooling end temperature of 550 ° C or less and a winding temperature of 5
It is a steel plate that is set to a low temperature of 00 ° C or lower and a high annealing temperature of 690 ° C or higher. Steel sheets Nos. 11 to 20 are comparative examples in which the manufacturing conditions were outside the scope of the present invention. Manufacturing conditions that are outside the scope of the present invention in the comparative examples are rolling end temperature (finishing temperature) in steel sheets No. 11 and 17, cooling conditions after rolling in steel sheets No. 12, 13, 15, 18, and 19, steel sheet No. .16 and 19, winding temperature, steel plate No.14,
At 20 is the annealing temperature.

Samples were taken from these steel sheets, and the volume ratio of the bainite phase of the hot rolled steel sheet, the hardness of the annealed sheet, and the hardenability were measured. The test and measurement methods and conditions are shown below, and the measurement results are shown in Table 3.

Measurement of Bainite Phase Volume Ratio After polishing and corrosion of the plate thickness cross section of the sample, the bainite phase volume ratio was measured with a scanning electron microscope.

Hardness measurement The plate surface of the sample of the annealed plate was measured at 5 points by Rockwell B scale hardness (HRB), and the average value was calculated and used as the hardness of the sample.

Hardenability measurement The hardenability test is carried out by holding a test piece cut out from an annealed plate in a size of 50 × 100 mm at 900 ° C. for 30 seconds and quenching it in oil at 20 ° C. Hardness (HR
C) was measured to evaluate the hardenability.

[0044]

[Table 3]

As shown in Table 3, steel sheets Nos. 1 to 10 whose manufacturing conditions are within the scope of the present invention have a bainite phase volume ratio of 20%.
It is an example of an invention that exceeds the above. Among them, especially steel sheet No. 2, 4, 6, 8, 10 whose cooling end temperature and coiling temperature were set low
Has a bainite phase volume ratio of 70% or more. The steel sheets Nos. 11 to 13 and 15 to 19 of the comparative example have a bainite phase volume ratio of
20% or less. The steel sheets Nos. 14 and 20 of the comparative example have a volume ratio of bainite phase exceeding 20%.

With respect to the hardness after annealing, invention examples 1 to 10 are lower than the comparative examples 11 to 20 of the same steel type by 8 points or more, respectively, and it can be confirmed that the hardness is softened. In particular, steel sheets Nos. 2, 4, 6, 8 and 10 having a bainite phase volume ratio of 70% or more and a high annealing temperature (720 ° C.) have low hardness after annealing and are extremely soft. In Comparative Steel Sheet Nos. 14 and 20, the volume ratio of the bainite phase exceeds 20%, but since the annealing temperature is too low, the hardness after annealing is high.

Regarding the hardness after quenching, Invention Examples 1 to 10
Has a high hardness and corresponds to a completely hardened structure. Comparative Examples 11 to 20 have lower hardness after quenching as compared to Invention Examples 1 to 10 of the same steel type, indicating that quenching was insufficient.

[0048]

Industrial Applicability The present invention controls not only the annealing conditions after hot rolling but also the structure of the hot rolled steel sheet before annealing to a structure having a predetermined amount of bainite, so that uniform fine dispersion of carbides and ferrite can be achieved after annealing. Achieve grain coarsening. As a result, it is possible to provide a high-carbon hot-rolled steel sheet that is extremely soft and has excellent workability while having high hardenability.

By using such a high-carbon hot-rolled steel sheet, it is possible to easily process parts having complicated shapes such as gear parts represented by gears with a low load, and as a result, the manufacturing process is improved. By omitting it, it becomes possible to manufacture components and the like at low cost.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Toshiaki Urabe             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Dr. Nakata             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Yasuhide Ishiguro             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. F-term (reference) 4K037 EA06 EA15 EA18 EA23 EA25                       EA27 FC07 FD04 FE06 FF02                       FF03

Claims (4)

[Claims] 1. A steel containing 0.2 to 0.7% by mass of C is hot-rolled to control a structure having a bainite phase exceeding 20% by volume and then annealed to form a spheroidized structure. And a method for producing a high-hardenability high-carbon hot-rolled steel sheet. 2. The finishing temperature for hot rolling (Ar ₃ transformation point -20 ° C.)
After the above, cooling is performed at a cooling rate of more than 120 ° C / sec and a cooling end temperature of 620 ° C or less, then winding at a winding temperature of 600 ° C or less, pickling, and then an annealing temperature of 640 ° C or more Ac ₁ transformation point Annealing is performed below, The manufacturing method of the high hardenability high carbon hot rolled steel sheet of Claim 1 characterized by the above-mentioned. 3. A steel containing 0.2 to 0.7 mass% of C is hot-rolled to control a structure having bainite having a volume ratio of more than 70%, and then annealed to obtain a spheroidized structure. A method for manufacturing a high-hardenability high-carbon hot-rolled steel sheet. 4. The finishing temperature for hot rolling (Ar ₃ transformation point -20 ° C.)
After the above, cooling is performed at a cooling rate of more than 120 ° C / sec and a cooling end temperature of 550 ° C or less, then winding at a winding temperature of 500 ° C or less, pickling, and an annealing temperature of 640 ° C or more Ac ₁ transformation point Annealing is performed below, The manufacturing method of the high hardenability high carbon hot-rolled steel sheet of Claim 3 characterized by the above-mentioned.