JPH06336620A - Direct heat treatment method of wire rod - Google Patents

Abstract

PURPOSE:To prevent quench crack and season cracking of a wire rod by forming a carbon steel for machine structural use into the wire rod by hot-rolling in a specific temp. condition and applying cooling and annealing after winding to ring-state of the wire. CONSTITUTION:A billet of the carbon steel or the alloy steel for machine structural use is heated to the optimum temp. of austenite in a heating furnace 1, and after hot-rolling with rougher-intermediate rolling mill 2 in the hot-rolling process, finish- rolling is executed in the temp. range of the Ar3 transformation temp. to (the Ar3 transformation temp. +200 deg.C) to form into the wire rod S and this austenitic grain size is made to be the gamma crystal grain of grain size No.8 or higher. After forming the wire rod S to non-concentrical circular ring-state at the Ar3 temp. or higher with a coiler 4, the wire rod is cooled at least at the Ms temp. or lower with warm water spraying in a cooling adjusting device 5, and after changing the whole cross section of the wire rod S to martensitic structure, the wire rod is collected with a collecting device 6. Then the wire rod is charged into a tempering furnace 9 with a loading machine 7 and hook conveyor 8, and after executing the tempering treatment by holding at the furnace atmospheric temp. of > 450 deg.C to 650 deg.C for >= 1 hr, the wire rod is formed to a bundled product 11 with a bundling machine 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of directly heat treating a carbon steel or alloy steel based wire for machine structural use, which is particularly capable of preventing quenching cracks and cracks during placement.

[0002]

2. Description of the Related Art Conventionally, wire rods made of steel components for machine structures, which are processed into bolts and stabilizers, are hot-rolled and formed into a desired size, and then heat-treated to cool them into a predetermined structure. After that, they are bundled and shipped, and the customer has further straightened or wire-drawn and carried out quenching and tempering to impart mechanical properties. This quenching and tempering process imposes a considerable burden on consumers, and therefore many direct heat treatment methods for rolled wire rods have already been proposed which can omit these steps. For example, in Japanese Patent Application Laid-Open No. 58-164731, cooling is performed in a cooling zone between a hot rolling finish and a winding device (loop layer) in a range of a martensite transformation start (Ms) temperature to Ms + 100 ° C., and then a loop ( There is disclosed a method in which a wire material developed into a non-concentric ring) is subjected to martensitic transformation by blast cooling at a cooling rate of 20 ° C./s or more, and is directly heat-treated by passing through a heat retaining device and tempering.

Further, in Japanese Unexamined Patent Publication No. 61-102726, a hot-rolled wire formed into a non-concentric ring is cooled with a gas-water mixed phase fluid refrigerant containing oxidizing bubbles at 95 ° C. or lower, and the temperature of 350-
Direct quenching and tempering method in which after cooling at 500 ° C., the conveyor (wire material conveying) speed is reduced to 1/3 to 1/20, the material is passed through a heat holding tank to undergo martensite transformation, and is then immediately annealed by self-heating. Is proposed.

As will be described later, a hot-rolled wire material that has been directly quenched from a high temperature to a martensitic transformation structure has a localized increase in residual stress and quenching cracks due to the addition of stress such as binding due to the contained hydrogen. Or, cracks will occur when placed. The above-mentioned proposal of Japanese Patent Application Laid-Open No. 164731/1983 does not disclose the technology for quenching cracks or misplacement cracks. That is, the rolling end temperature only describes general conditions, and the processing in the heat retention device is a ring-shaped continuous running processing, so the processing time is short (54 seconds in the example), so stress Release and hydrogen removal are not performed sufficiently. Moreover, because it is difficult to take sufficient heat retention residence time due to equipment, it is difficult to raise the wire temperature to the optimum tempering temperature and to secure the time required for tempering martensite, resulting in sufficient characteristics. I feel uncomfortable. Further, there is a problem that it is difficult to form a loop when the film is wound right above the Ms point because of its high rigidity.

The disclosure of Japanese Patent Application Laid-Open No. 61-102726 is also the same, and it is the slow cooling by self-heating that requires tempering, which makes temperature control difficult, and even if the slow cooling is a dense non-concentric ring. Since it is conveyed by a conveyor, the processing time is short (10 to several hundreds of seconds), so that stress release and hydrogen removal cannot be sufficiently performed. Further, it is expected that the martensite transformation by gentle cooling of the heat retaining layer will result in incomplete quenching (mixed structure of martensite + bainite) and that the mechanical properties, especially the variation of the structure, will increase in the ring. Furthermore, although it is tempered by self heat,
The temperature is low and the time is not long enough for tempering.

[0006]

SUMMARY OF THE INVENTION The present invention is based on the conventional direct heat treatment method (in-line quenching, tempering) for a hot-rolled wire in order to omit the quenching and tempering treatments performed in separate steps (offline). It is a solution to the problem, and by optimizing the processing conditions in each process, it is possible to prevent grain boundary destruction and quench cracking caused by quenching and to perform stable treatment, while at the same time providing a uniform tempered structure. An object of the present invention is to provide a direct heat treatment method for a wire rod, which secures and imparts good mechanical properties and realizes high productivity without energy and cost.

[0007]

In the quenching from the austenite region using the heat retained by the hot-rolled wire, the hardenability depends on the grain size. That is, as the austenite grain size is larger, the hardenability is improved, but as the grain size is coarsened and the hardenability is improved, grain boundary fracture easily occurs, and the delayed fracture susceptibility also becomes sensitive. Due to the stress applied at 1, quenching cracks easily occur. The same phenomenon occurs when coarse particles (mixed particles) are locally formed. Therefore, in order to suppress these phenomena, it is extremely important to maintain the conditions of hot rolling and cooling and the conditions of the post process appropriately.

The present invention is based on such knowledge, and manufactures a wire rod including the steps of hot-rolling carbon steel or alloy steel for machine structure, winding, cooling, then bundling, and further bundling. In the method, Ar ₃ transformation point to Ar ₃ transformation point +
A wire rod which is finish-rolled in a temperature range of 200 ° C. and formed into a predetermined shape is wound at a temperature not lower than the Ar ₃ transformation point to form a continuous non-concentric ring shape, and after winding, at least the wire M
It is cooled to the s point and the whole cross-section is made into a martensitic structure and then bundled.
The gist is a direct heat treatment method for a wire, which is characterized by performing tempering by holding the furnace atmosphere temperature range of more than 0 ° C to 650 ° C for 1 hour or more. In the present invention, the crystal grains of the finish-rolled wire are preferably fine grains having an austenite (γ) grain size number of 8 or more. Further, it is preferable to cool the wire rod after winding by continuously immersing it in warm water, and particularly to cool it by further injecting warm water in the warm water. Further, a method of cooling the wound wire rod in a foam can also be adopted.

[0009]

According to the present invention, in the finish rolling for finishing the wire rod into a predetermined shape, the temperature immediately after the finish rolling should be the Ar ₃ transformation point or higher.
By processing so that the temperature is within the range of ₃ transformation points + 200 ° C or less, it is possible to obtain fine particles with a γ grain size number (JIS G 0551) of 8 or more. As a result, the grain boundary fracture and delayed fracture (deposition cracking) seen immediately after quenching of coarse grains are not caused. That is, the coarsening and local coarsening of the γ crystal grains cause imbalance in quenching to induce quenching strain, and as a result, local increase in residual stress causes grain boundary fracture or stress addition. Cause burning cracks. This is also the case when non-uniform cooling is performed, and for that reason, quenching of wire rods with a fine-grained structure formed in a non-concentric ring shape by winding at an Ar ₃ transformation point or higher is performed continuously in warm water, especially in boiling water. Method of removing heat and cooling by immersing and boiling the film, or 20 to 99 ° C in warm water
Using the method of spraying warm water of 5m / sec or more,
Uniform rapid cooling is carried out by these methods to form a martensite structure having a uniform cross section, and the destruction is suppressed. Also,
A method of injecting foam from a supply nozzle onto a non-concentric ring-shaped wire at a high temperature, adjusting the foam supply amount according to the ring density, and uniformly cooling can also be adopted for the same purpose.

On the other hand, the hardened wire is bundled into a coil and further bundled before shipping, but a considerable pressure is applied from both ends of the bundled coil when bundled. Hydrogen is inevitably mixed in the treated wire, and there is residual stress generated during quenching. Especially, if there is a locally increased residual stress such as a defect, there is a diffusivity in this part. Hydrogen intrudes in a concentrated manner, and the stress during binding makes it easier for quench cracks to occur. Further, in the case of a hardened structure, the mechanical properties, particularly the drawing, elongation, impact value, etc., are not practically sufficient. Therefore, before binding, a tempering treatment is performed in which the temperature is maintained in a furnace atmosphere temperature range of more than 450 ° C. and less than 650 ° C. for 1 hour or more to form a tempered martensite structure and release residual stress inside the wire, and Diffusible hydrogen is removed.

The present invention will be described in more detail below. FIG. 1 shows an outline of an equipment line for carrying out the present invention. In the figure, reference numeral 1 denotes a heating furnace, which heats a carbon steel for machine structural use or an alloy steel billet to an optimum temperature of austenite, and, for example, typically treats it in the furnace at 950 ° C to 1250 ° C for 60 to 180 minutes. The billet extracted after the treatment is formed into a wire rod S shaped into a predetermined size and shape by a rough-intermediate rolling mill 2 and a finishing rolling mill 3 in a hot rolling hot step. The crystal grain size of the steel is finally determined by the temperature at the exit side of the finish rolling mill 3. In the present invention, the grain size is set to the Ar ₃ transformation point or higher and the Ar ₃ transformation point + 200 ° C. or lower to suppress crystal coarsening. Specifically, by controlling the temperature in the range of Ar ₃ transformation point to 1000 ° C. depending on the steel type, γ crystal grains with a γ grain size number of 8 or more can be obtained. By carrying out such control, it is possible to prevent stress release and occurrence of disposition crack (delayed fracture) before the diffusible hydrogen release treatment.

The hot-rolled wire S is continuously formed into a non-concentric ring shape by the winder 4 at the Ar ₃ transformation point or higher, and then introduced into the cooling adjusting device 5, where the martensite is selected from the austenite region depending on the steel type. Cool to a temperature below the Ms point at a cooling rate that can secure the site. For example, in SCM435, it is necessary to cool to a Ms point of 374 ° C. at a cooling rate of 5 ° C./sec or more. If this cooling is insufficient, not only the mechanical properties of a completely quenched structure cannot be obtained, but also the residual stress caused by the non-uniformity of the microstructure of the cross section causes quench cracking before the retention treatment. The reason why the coil is wound above the Ar ₃ transformation point is to secure an austenite structure in the wire during cooling.

2 (a) and 2 (b) show an example of the cooling adjusting device 5, in which the hot-rolled wire S formed in a non-concentric ring shape is transferred to the conveyor 51 and dropped into the cooling tank 52 for cooling. It is processed. The heat treatment apparatus 52 stores a cooling medium 53 that circulates as appropriate, and is provided with a conveyor 54 on which the ring-shaped wire rod S is placed and conveyed.
4 is continuously quenched while being transferred to the cooling tank 52, and is carried out of the tank. Further, the cooling tank 52 is provided with a hot water jet nozzle 55 for increasing the cooling efficiency. In the figure, reference numeral 56 is a support for supporting the conveyor 52, and 57 is an outlet.

According to the present invention, quenching can be carried out by such a cooling adjusting device 5. In this case, by using boiling water as a cooling medium, film boiling is caused on the surface of the immersed high temperature wire to remove heat, Allows uniform cooling. Alternatively, hot water may be jetted from the jet nozzle 55 to cool the stored hot water while stirring or spraying the hot water onto the wire. In this case, hot water of 20 to 99 ° C. is used,
By injecting at a speed of 5 m / sec or more, instability of the boiling film, which is a problem at this temperature, can be eliminated and stable uniform cooling can be performed. As an alternative method, it is possible to use a foam cooling method.
It is preferable to directly supply the foam at a jet speed of 0 m / sec and pass it between the rings, and at this time, adjust the supply amount according to the overlapping density of the rings, and to achieve uniform cooling by such means. You can

A ring-shaped wire S hardened by the cooling adjusting device 5
Is guided to the focusing device 6 and focused in a coil shape. Next, this coil is transferred to a transfer machine 7, transferred to a moving hook conveyor 8 and loaded into a tempering furnace 9 provided up to the bundling device 10. Over 450-650 in tempering furnace 9
A tempering treatment is performed in which the temperature is held in the furnace atmosphere temperature range of ℃ for 1 hour or more. In the carbon steel / alloy steel for machine structures having a C content of 0.3% or more, which is the target range of the present invention, a stable equilibrium phase metal state is obtained by tempering at 450 ° C. or higher,
The structure is an extremely fine mixture of ferrite and cementite, and the structure has an excellent balance of strength and toughness in terms of mechanical properties. On the other hand, when tempered at over 650 ° C, the strength rapidly decreases, and the meaning of securing the strength by quenching becomes less. On the other hand, in order to release the residual stress generated inside the steel material during quenching, a minimum temperature of 100 ° C. or higher is required, and it is advantageous that the temperature becomes higher. Further, diffusible hydrogen which affects delayed fracture is released in a temperature range of 100 to 450 ° C., but is maintained at this temperature during the temperature rising process of the tempering treatment and is sufficiently performed. Therefore, the tempering treatment of the present invention can simultaneously achieve stress release and removal of diffusible hydrogen. It should be noted that the time from the completion of cooling to the start of tempering treatment is preferably set to 8 hours or less because if this is too long, diffusible hydrogen aggregates in the stress concentrating portion, and cracking may occur. More preferably, it is within 30 minutes.

The coil wire thus treated is pressed from both sides of the coil by the bundling device 10 and bound, but the cause of delayed fracture is eliminated by the series of processes described above, and therefore stress is added during bundling. The product 11 can be stably supplied with almost no defects caused by the above.

[0017]

EXAMPLES A 162 mm square steel slab made of the components (carbon steel for machine structure and alloy steel) shown in Table 1 was hot-rolled and cooled under the conditions shown in Table 2 according to the steps shown in FIGS. Quenching)
Then, the restraint treatment was carried out, and a full length inspection was carried out while rewinding the coil for the crack generation state before restraint and after binding. For comparison, the samples treated under the conditions deviating from the present invention are also shown. The results are shown in Table 3 together with the mechanical properties.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

Comparative examples Nos. 7 and 11 have high γ-grains because of high hot rolling finishing temperature, and Nos. 8 and 12 are materials having a long time between focusing and retaining, and therefore both are retained. There is a burning crack before it. In addition, the sample No. 9 has a low tempering temperature and a high hydrogen content, and the sample No. 10 has not been sufficiently dehydrogenated because the tempering time is short, and any sample has a quench crack after binding. Sample No. 13 has a tempering temperature outside the lower limit of the present invention, high strength, and low drawing and elongation. Sample No. 14 has a tempering temperature outside the upper limit of the present invention, and is excellent in drawing and elongation but low in strength.

On the other hand, the samples of the present invention did not have any burning cracks before and after binding, and had excellent mechanical properties, showing good results.

[0023]

As described above, according to the present invention, a steel wire rod that has been directly quenched using the hot rolling heat in the wire rod hot rolling step (in-line) is combined with the subsequent tempering treatment to obtain a machine. Characteristics, and can be manufactured without any problems such as quenching cracks or placement cracks, and compared to the conventional offline quenching and tempering process,
It is extremely advantageous in terms of energy, cost, and equipment, and because it can realize high productivity, its industrial effect is extremely large.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an outline of an example of a device array for carrying out the method of the present invention.

FIG. 2 (a) is an explanatory view showing an example of a cooling adjusting device for carrying out the method of the present invention. (B) is the sectional view on the AA line of (a) figure.

[Explanation of symbols]

 1 Heating Furnace 2 Rough / Intermediate Rolling Mill 3 Finishing Rolling Mill 4 Winding Machine 5 Cooling Adjustment Device 51 Conveyor 52 Cooling Tank 53 Cooling Medium 54 Conveyor 55 Injection Nozzle 56 Support 57 Discharge Port 6 Focusing Device 7 Transfer Machine 8 Hook Conveyor 9 Tempering furnace 10 Bundling machine 11 Products

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryuichi Seki 12 Nakamachi, Muroran, Hokkaido Inside Nippon Steel Co., Ltd. Muroran Works (72) Inventor Takeshi Takahashi 12 Nakamachi, Muroran-shi, Hokkaido New Japan Steel Co., Ltd. Muroran Co., Ltd. Inside the steelworks

Claims (5)

[Claims] 1. A method for manufacturing a wire rod, comprising the steps of hot rolling carbon steel for machine structural use or alloy steel, winding, cooling, then bundling, and further bundling, from the Ar ₃ transformation point to Ar.
A wire rod which is finish-rolled in a temperature range of ₃ transformation point + 200 ° C and has a predetermined shape is wound at a temperature of Ar ₃ transformation point or higher to form a continuous non-concentric ring shape, and after winding, at least Ms of the wire material is formed. It is rapidly cooled to the point and the whole cross-section is made into a martensitic structure and then bundled, and until the bundled wire is bundled, it is 1 at a furnace ambient temperature range of more than 450 ° C to 650 ° C.
A direct heat treatment method for a wire, which comprises performing tempering for holding for a period of time or more. 2. The direct heat treatment method for a wire rod according to claim 1, wherein the wire rod after finish rolling has an austenite grain size of 8 or more. 3. The direct heat treatment method for a wire rod according to claim 1, wherein the wound wire rod is immersed in warm water to be cooled. 4. The direct heat treatment method for a wire rod according to claim 3, further comprising injecting warm water into the warm water to cool it. 5. The method for direct heat treatment of a wire rod according to claim 1, wherein the wound wire rod is cooled in a foam.