JP2524156B2 - High carbon steel tough parts manufacturing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a high carbon steel tough component with high accuracy and economically.

[Prior Art] As a method for efficiently and economically forming a steel part from a simple shape to a relatively complicated shape, a hot or sub-hot plastic working method generally performed in a temperature range of 900 ° C or higher, 600 to 850 ° C The warm plastic working method performed in the temperature range of is known for a long time.

In the hot or sub-hot temperature range, the deformation resistance of the steel material is generally small and the deformability is large. However, since the amount of oxide film generated on the surface of the material is large and the heat load applied to the mold is large, the mold is easily damaged (wear, set, crack, etc.) and the molding accuracy is low.

In contrast, the degree of material oxidation is low in the warm temperature range,
Since the heat load on the die is also reduced, high precision machining is possible. However, the deformability decreases and the deformation resistance increases.

In particular, high-carbon steel containing 0.6% by weight or more of carbon has a large deformation resistance in the warm region and insufficient deformability, so it is generally formed in the hot or sub-hot region. For the reasons described above, it has been difficult to economically obtain highly accurate parts by the plastic working method.

By the way, a desired hardness and strength are given to a high carbon steel part by quenching and tempering after forming. However, mechanical properties, particularly plasticity and fatigue strength, are not always sufficient, and several thermomechanical treatment methods have been proposed in order to improve the mechanical properties.

For example, in the ausforming shown in FIG. 9, it is said that a tough martensitic structure can be obtained by rapid warming after applying strong warm plastic working (WF) in the supercooled austenite region. However, if this method is applied to high-chromium bearing steel, for example, it is necessary to perform plastic working at a low temperature of about 300 to 400 ° C. Therefore, the deformation resistance during working is extremely large and the deformability is poor. Processing is extremely difficult. Furthermore, the obtained component strength has a strong anisotropy, and there are many difficulties in the temperature control method during processing.

Forging and quenching shown by the line (a) in FIG. 10 are also well known. In this method, hot plastic working (HF) is performed in a stable austenite state, so the workability of the material is good, but the obtained part precision is low. In addition, the heat energy consumption can be saved compared to normal quenching, but the improvement in strength is not sufficient, and the size of the former austenite crystal grains in the quenched structure is affected by the distribution of the hot workability, which causes the It has also been pointed out that it tends to be uniform.

As an improved method of this forging and quenching, Fig. 10 (b)
As shown by the line, after the material of hypoeutectoid steel is hot forged at about 1250 ° C, the forged product is once cooled to a temperature range of 720 to 620 ° C or 390 to 250 ° C to substantially eliminate the austenite structure. After that, a method for improving the mechanical properties by obtaining a relatively fine and uniform quenched structure of austenite crystal grains by reheating to a predetermined quenching temperature and quenching has been proposed (JP-A-58-58). 141331, JP-A-58-141333).

However, the plastic working strain added to austenite by hot working is almost completely released by recrystallization. Therefore, when reheating to the quenching temperature, almost no strain remains effective for refining the austenite crystal grains, and it is difficult to significantly refine the obtained austenite crystal grains. Further, since hot forging is used in this case as well, the above-mentioned problems concerning the forming accuracy have not been solved.

For hypo-eutectoid and hyper-eutectoid steels, quenching by austenitizing, cooling to room temperature, and then warm working (WF) or cold working (CF) as shown in Figs. A method for refining the former austenite crystal grains of the structure has been disclosed (JP-A-61-210120 and JP-A-61-21).
0121, JP-A-61-210122, JP-A-61-279620).
According to this method, austenite is preferentially nucleated at grain boundaries of ferrite deformed by warm working or cold working, and finally fine austenite of several μm is obtained.

However, if this method is applied to high carbon steel parts, the deformation resistance is large and the deformability is small in the warm region, which makes the processing difficult, and is not a practical method.

[Problems to be Solved by the Invention] As described above, high-carbon steel, particularly high-carbon steel having high deformation resistance such as containing undissolved carbide and relatively low deformability, is economically and highly accurately manufactured. The method is not yet known. Also, the mechanical properties were not sufficient.

The present invention has been made in view of the above circumstances, the material of the high carbon steel is efficiently and precisely formed by plastic working, has a quenching structure of extremely fine old austenite crystal grains, excellent toughness The purpose of the invention is to provide an economical and practical method for obtaining steel parts.

[Means for Solving Problems] As a result of intensive studies to achieve the above-mentioned object, the inventors of the present invention have determined that a high-carbon steel material has the following steps (a), (b) and (c): It has been found that a high carbon steel tough component can be obtained by processing through. FIG. 1 is a diagram schematically showing the process. In addition, [] shows the case where the matrix structure is hypereutectic.

(A) The base material is heated to austenite.

(B) Subsequently, plastic working is performed on the material in each of the hot temperature range in which the matrix structure is austenite and the warm temperature range in which the matrix structure is substantially ferrite and pearlite or pearlite.

(C) Subsequently, the material is reheated to just above the austenitizing temperature of the base structure to austenite the base structure, followed by quenching and then tempering.

The high carbon steel targeted by the present invention is a steel containing at least 0.6% carbon by weight. For example, high carbon chrome bearing steel, carbon tool steel and the like can be cited, and those that are usually hardened and tempered and used with high hardness (generally H _R C60 or more) are targeted.

In particular, hyper-eutectoid steel has a high deformation resistance, and when the structure thereof contains undissolved carbides, the deformability is extremely low, so the effect of the present invention is high.

Steel with less than 0.6% carbon has a hardness of about Hv 300 or less after quenching and tempering when it is generally used, and often has practically sufficient toughness. Since it has a deformation resistance and a deformability capable of performing sufficient interworking, it has little significance for the present invention.

In the step (a), the heating temperature may be higher than the temperature at which the matrix structure of the material is austenitized, but in the next step (b), the heating temperature is substantially austenite even if the temperature of the material is lowered. It is preferable to select a temperature slightly higher than the austenitizing temperature of the matrix structure so that plastic working can be performed.

When using hyper-eutectoid steel containing undissolved carbides, in connection with the heating time, ensure that the amount of undissolved carbides does not fall below the amount specified by the application of the part (excessive solid solution in the matrix). The upper limit of the heating temperature is set. Further, in order to avoid the formation of coarse austenite grains during heating, the upper limit is preferably set to 1200 ° C or lower.

In the step (b), the plastic working is performed by rolling, forging, etc. to form the final shape of the component.

In this plastic working, the material is plastically worked through at least the following two steps. That is, the first stage processing is performed in the state where the matrix structure of the material is austenite (indicated by the time range of Fh in FIG. 1), and the second stage processing is that the matrix structure of the material is substantially pearlite. It is performed in the state of being ferrite or pearlite (indicated by the time range of Fw in FIG. 1).

Here, as shown in FIG. 1, when the material temperature is continuously decreased in the time range of Fh or Fw, it does not matter if the matrix structure contains a large amount of austenite in the first half of Fw, or A small amount of austenite may remain at the end of the second stage processing in Fw. In this case, the first-step processing and the second-step processing may be performed at intervals (FIG. 1) or may be continuously performed (FIG. 2). Also, the first stage and the second
The steps of processing may be carried out while maintaining a substantially constant temperature (FIG. 3).

In short, the first stage processing can be performed with austenite or a mixed structure of austenite and undissolved carbide, and the second stage processing can be subsequently performed with ferrite and pearlite or pearlite or a mixed structure of these and undissolved carbide. Good.

In addition, in order to minimize the increase in deformation resistance and the decrease in deformability, the lower limit of the processing temperature in the second step is 550.
It is desirable that the temperature is ℃ or higher.

In the step (c), the material is reheated following the step (b). The reheating temperature is directly above the austenitizing temperature determined by the carbon concentration of the matrix structure,
The upper limit is the austenitizing temperature + 150 ° C, preferably the austenitizing temperature + 100 ° C. The holding time at the reheating temperature (indicated by Ht in FIGS. 1 to 3) is preferably within several minutes after completion of the austenite transformation. By setting the reheating temperature and the holding time within the above ranges, growth (coarsening) of austenite equiaxed crystals is prevented, and an extremely fine and uniform structure of austenite equiaxed crystals is obtained.

Subsequent to the reheating treatment, quenching is performed using a generally known refrigerant, and then tempering is performed under normal conditions.

In addition, among the steps (a) to (c), the process up to the quenching of (c) needs to be continuously performed in terms of time,
Do not insert any steps or operations other than those mentioned above between each step or process. The tempering treatment after quenching does not necessarily have to be performed subsequent to the quenching, and an arbitrary rest time may be provided before the tempering treatment.

[Operation] In the present invention, first, the matrix structure of the material is austenitized in the step (a), and then in this state (b)
The first stage plastic deformation is performed. In this state, the material exhibits low deformation resistance and high deformability, so that it can be arbitrarily rough-processed with a low processing pressure and without the risk of cracking.

Next, (b) plastic deformation is performed in the second stage, and finally, precision molding is performed into a desired part shape. This second stage is warm zone working, but since a considerable part of the workability required to reach the final molded shape has been deformed in the first stage, the workability at this stage is low,
Therefore, the risk of mold damage, material cracking, etc. is small.

In this second stage, plastic strain is accumulated in the matrix structure containing at least ferrite and pearlite or pearlite. The matrix structure in which the plastic strain is accumulated is transformed into a fine and uniform austenite equiaxed crystal in the subsequent step (C), and is hardened before growth to become a hardened structure of fine prior austenite grains. Further, tempering can improve the toughness of the quenched martensite.

[Effects of the Invention] As described above, according to the present invention, a high carbon steel material having high deformation resistance and poor ductility can be stably and reasonably plastically processed into a precise component, and at the same time, extremely fine old austenite crystal grains can be formed. It is possible to obtain a high toughness component having a quenched structure of

Further, since the process of (a) to the quenching process of (c) are carried out continuously in terms of time, the heat energy can be restricted as compared with the conventional method in which these processes are carried out separately. In addition, since precision processing is possible, there is little waste of material in post-processing such as cutting and grinding, and the processing time can be shortened, which is an extremely economical and practical method.

[Examples] Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to the following Examples as long as the gist thereof is not exceeded.

(1-A) JIS high carbon chrome bearing steel SUJ2 (0.9
5 to 1.10% C, 0.15 to 0.35% Si, <0.5% Mn, <0.025% P, <
0.025% S, 1.30 to 1.60% Cr) was used to roll the ring product.

 The material is a ring with an outer diameter of 44 mm, an inner diameter of 24 mm, and a width of 20 mm.

This ring material was heated in an electric furnace at 850 ° C for 20 minutes to austenite the matrix structure. At this time, the undissolved carbide was present in the austenite, and the heating time was selected so that the carbon concentration in the austenite was about 0.5 to 0.6%. The austenitizing temperature of the matrix structure in this case is estimated to be about 750 to 760 ° C from the phase diagram. This material is rolled between a mandrel and a cylindrical roller, the outer diameter is 72.5 mm,
It was molded into a ring product with an inner diameter of 62.5 mm and a width of 20.2 mm. The processing start temperature was 830 to 840 ° C, and the processing completion temperature was 690 to 710 ° C, during which rolling was continuously performed.

The roller pushing speed was 0.13 mm per revolution of the material, and the processing time was about 7 seconds.

Immediately after the rolling process was completed, the molded product was inserted into an electric furnace at 850 ° C and the matrix structure was transformed into austenite (transformation was
(Occurred at 0 ° C.), held in oven for 1-3 minutes and then oil cooled. Then, after holding at 160 ° C. for 2 hours, it was tempered by cooling with water.

The dimensional accuracy of the obtained ring product is ± 0.2 mm in diameter error.
(Including roundness error), width error was ± 0.1 mm. The surface finish was smooth and there were no defects such as cracks, and it was possible to finish the grinding as it was. The hardness of the molded product was Hv760-790, and the quenched structure consisted of undissolved carbide and martensite matrix.

The holding time after the austenite transformation and the size of the former austenite crystal grains in the reheating step are shown in FIG. 4A.
Fine particles of 4 to 7 μm were obtained within a holding time of 4 minutes.
The former austenite grains were equiaxed and extremely uniform regardless of location.

(1-B) On the other hand, a ring of the same size as the above ring product is cut from a round bar of the same material as the above material, heated in a furnace at 850 ° C for 20 minutes to austenite, and then oil quenched. The size of the former austenite crystal grains was measured after tempering at 160 ° C. for 2 hours, and the result is shown in FIG. 4B. The amount of undissolved carbides in this normal heat-treated product (hardness Hv784) was the same as in the case of A above. As you can see in the figure,
It can be seen that the product of the present invention has extremely fine prior austenite grains as compared with the normally heat-treated product.

Next, the crush test of the ring products obtained in the above A and B was performed. As shown in Fig. 5, a ring product (1) with a part cut away is used to test the upper and lower flat plates (2, 3) by a material testing machine.
The amount of ring deflection Δl and the load at that time (crushing load), which is compressed at a speed of 30 mm / min between
Was measured. Results are shown in FIG. It can be seen that the product A of the present invention has a significantly larger compression value and deflection than the heat-treated product B and is toughened. It is also excellent in strength.

Further, in FIG. 7, the crushing load of the product A of the present invention decreases as the holding time after austenitizing in the reheating step increases, and when the holding time is several minutes or more, the crushing load greatly differs from that of the normally heat-treated product B. Disappear. This is mainly due to the coarsening of austenite crystal grains as the holding time after austenitization increases, as described above.

The side surfaces of the ring products A and B were ground and finished, and the surfaces were subjected to a rolling fatigue test using a Mori type (thrust type) bearing steel durability tester. Stress repetition rate is 1800cp
m, lubricating oil is # 60 spindle oil (immersion), diameter 5/16
Using an inch steel ball, the working surface pressure (Hertz pressure) ρmax is 5
It was performed at 34 kgf / mm ² . The results are shown in Fig. 8. The rolling life of the ring of the product A of the present invention is about twice as long as that of the product B of the normal heat treatment.

(2) A material similar to that of (1-A) was used, heated at 920 ° C. for 20 minutes to form austenite, and then continuously rolled in the same manner. The processing start temperature was 820 to 880 ° C, and the processing end temperature was 610 to 720 ° C.

Here, after the rolling process was completed, oil quenching was performed to examine the hardness at that time. H if the rolling end temperature is 690 ° C or lower
There was obviously a pearlite transformation in v360-425,
When the rolling end temperature was 700 ° C or higher, it was shown that the rolling process was substantially completed in the austenite state at about Hv800.

After completion of the rolling process, the molded product was immediately put in a furnace at 800 ° C. and 920 ° C., austenitized, and then oil-quenched in 1 to 3 minutes. When the size of old austenite grains was examined for the obtained quenched structure, when the reheating temperature was 800 ° C, the grain size No. 12 to
No. 14 was fine, but those with a rolling end temperature of 700 ° C. or higher (those not satisfying the process of the present invention (b)) had grain size No. 11 or less and were insufficient in fineness. When the reheating temperature was 920 ° C, the particle size was No. 11 or less in all cases.

Here, in this experiment, since the austenitizing temperature before rolling was higher than (1-A), the amount of undissolved carbide was (1
-Less than A). Therefore, the carbon content in the base organization is 0.5-
More than 0.6%, it is considered that the austenitizing temperature of the base structure upon reheating is lower than 750 to 760 ° C, and from the above results, when the reheating temperature is considerably higher than the austenitizing temperature of the base structure, It can be seen that a sufficiently fine austenite grain structure cannot be obtained.

(3) Using a carbon steel ring material having a carbon content of 0.7 to 0.8%, rolling was performed in the same manner as (1-A). The austenite formation before rolling is 1125 to 12 by high frequency induction heating.
This is done by heating to 00 ° C, and the processing start temperature is
The temperature was 910 to 920 ° C, and the finishing temperature was 560 to 800 ° C. The processing end temperature was adjusted by changing the time from the processing start to the processing end by intermittently repeating the rolling process several times.

Immediately after the rolling process was completed, the molded product was reheated to 800 to 860 ° C for about 30 seconds by high frequency induction heating and immediately cooled with oil.

There was no crack in the obtained molded product, and the hardness of the quenched structure was Hv 800 to 850 in all, but the former austenite grains have the grain size No. 11.5 to 605 at 560, 605 and 635 ° C. 1
The particle size was 3.5, and those of 780 and 800 ° C had a particle size of No. 11 or less. The austenitizing temperature of this steel was about 730 ° C.

(4) Using the same material as in (1-A), compression processing was performed in the material width direction (axial direction) by a knuckle joint press. First, the material was heated in a furnace at 850 ° C for 20 minutes to form a mixed structure of undissolved carbide and austenite. Subsequently, compression processing with a processing rate of 50% was performed at 800 ° C, and then compression processing with a processing rate of 30% was further performed at 650-700 ° C. Immediately thereafter, the matrix structure was austenitized in a furnace at 850 ° C., held for 2 to 4 minutes and oil-quenched.

The obtained molded product had no cracks and had a former austenite quenched structure with grain size Nos. 11.5 to 12.5.

[Brief description of drawings]

1 to 3 are process explanatory diagrams of the present invention, FIG. 4 is a diagram showing the relationship between the holding time after austenitization and the particle size in this example, and FIG. 5 is the crushing test method in this example. Fig. 6 to Fig. 7 show the results of the crushing test in this example, Fig. 8 shows the result of the rolling life test in this example, and Figs. 9 to 12 show the conventional machining. It is a process explanatory view of a heat treatment method.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiaki Tanaka Nagakute-cho, Aichi-gun, Aichi Prefecture, Nagachoji 1 No. 41 Yokomichi, Toyota Central Research Institute Co., Ltd. No. 41 Yokomichi, Toyota Central Research Institute Co., Ltd. (72) Inventor Masatoshi Sawamura, Nagakute-cho, Aichi-gun, Aichi Prefecture, long letter No. 41 Yokomichi Toyota Central Research Institute, No. 1 (72) Inventor Yoshiki Fujita Minami, Osaka, Osaka (2) Koyo Seiko Co., Ltd., Nishinomachi, Nishinoya-ku, Ku (72) Masamichi Shibata, Nishinocho-cho, Unagiya, Minami-ku, Osaka-shi, Osaka (56) Reference (Japanese Patent Publication No. 50-31530) JP, B1) JP-B-50-31529 (JP, B1)

Claims (3)

(57) [Claims] 1. A method for producing a high carbon steel tough component, which comprises treating a high carbon steel material through the following steps (a), (b) and (c). (A) The base material is heated to austenite. (B) Subsequently, plastic working is performed on the material in each of the hot temperature range in which the matrix structure is austenite and the warm temperature range in which the matrix structure is substantially ferrite and pearlite or pearlite. (C) Subsequently, the material is reheated to just above the austenitizing temperature of the base structure to austenite the base structure, followed by quenching and then tempering. 2. The method according to claim 1, wherein the high carbon steel is hypereutectoid steel. 3. The manufacturing method according to claim 1, wherein in the step (b), the plastic working is either rolling or forging.