JPS62158826A - Method for directly quenching and tempering hot rolled steel bar or wire rod - Google Patents

Abstract

PURPOSE:To utilize the sensible heat of a finish-rolled steel bar or wire rod and to considerably save energy by carrying out quenching and tempering between finish rolling and ring forming stages. CONSTITUTION:Four rapid cooling units 22 and a rapid heating unit 24 are arranged between a finish rolling mill 2 and a coiler 10. A wire rod 4 finish- rolled with the finish rolling mill 2 is quenched with the rapid cooling units 22 and tempered with the rapid heating unit 24. A reformer 20 is placed just behind the coiler 10 so as to avoid conventional conveyance in the form of a ring.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for directly quenching and tempering hot rolled steel bars and wire rods, and is used in the field of manufacturing steel bars and wire rods.

[Conventional technology]

Generally, on-line heat treatment for the purpose of strengthening the toughness of wire rods, etc. is carried out after rolling and winding and forming into a ring shape, as shown in FIG. That is, the wire rod 4 from the finishing mill 2
After being cooled by a water cooling device 6, the rings are formed into rings 12 by a winder 10 via pinch rolls 8, and then placed on a conveyor 14 at appropriate intervals to reduce overlapping of the rings, as shown in FIG. The air is forcibly cooled by cold air 18 from the fan 16 and stored in the ring convergence device reformer 20. Note that forced cooling may be water cooling or salt cooling.

Due to various technical constraints, the above method inevitably suffers from the following problems and has to be accepted.

(A) The overlap of the rings differs depending on the location, causing variations in the cooling rate in the longitudinal direction of the material, and the continuous cooling transformation (hereinafter referred to as OCT) starting temperature differs, which is the main cause of the tensile strength and yield strength. There were large variations in the properties of various phases such as elongation, elongation, and squeezing at room temperature.

(B) In forced air cooling, the wind pressure and air volume can be adjusted depending on the overlap density, but it has not been possible to sufficiently reduce variations in material quality.

(C) In order to increase the strength of steels that start OCT quickly and to harden them, it is necessary to use a coolant with a high heat transfer coefficient, such as water or a salt bath, and in this case, the overlap density of the wire rods must be It became practically impossible to adjust the amount of refrigerant supplied according to the cooling rate, and variations in material quality due to differences in cooling rates were inevitable.

(DJ As shown in FIGS. 3 and 4, the ring-shaped cooling and conveyance after winding requires an enlarged device, resulting in an increase in equipment costs and running costs.

(E) It is difficult to find an appropriate reheating method in the ring state, especially in the production of hardened and tempered materials used for PC steel bars used as concrete pile reinforcing materials.
It had not been put into practical use.

[Problem that the invention seeks to solve]

The purpose of the present invention is to solve the problems of the above-mentioned conventional technology, and to provide direct quenching and tempering of hot rolled steel bars and wires, which reduces fluctuations in material quality in the longitudinal direction of the steel bars and wires, and eliminates ring conveyance after winding. We are here to provide you with a method.

[Means and operations for solving the problems] The gist of the present invention is as follows.

That is, in a method for manufacturing a steel bar or wire rod that is formed into a ring shape and wound after hot finish rolling, the steel bar or wire rod is quenched in a quenching device between the finish rolling step and the ring shape forming step. A method for directly quenching and tempering hot rolled steel bars and wire rods, comprising the steps of: tempering using the rapid heating device after quenching.

The inventor of the present invention focused on the fact that the wire is straight until it is coiled after rolling, and that once the transformation is completed, it can be rapidly cooled to room temperature and bundled into a coil, and intended to perform quenching and tempering treatment between the end of rolling and coiling. did.

In the conventional technical sense, it was considered as follows.

(b) Keeping in mind the mechanical descaling at secondary processing manufacturers, the coiling temperature is set in the range of 800 to 900°C to obtain an appropriate scale composition and thickness, and the finishing mill to winding machine (laying Only a water cooling device capable of cooling the finish rolling temperature from 1000 to 1100°C was installed between the heads).

(b) There was a preconceived notion that heat treatment should be performed after winding.

(c) It was thought that troubles would easily occur if the winding temperature was as low as 700°C or less.

(d) Since the time required between the end of rolling and winding is short, there was no idea that the transformation could be completed during this time.

As a result of various studies on these concepts, the present inventor made a major change in the conventional thinking and was able to obtain the following findings.

(A) The technology for manufacturing wire rods that is nearly scaleless by rapidly cooling the wire after rolling to prevent the formation of secondary scales is noteworthy.

(B) Heat treatment is carried out as soon as possible after the end of rolling.It is effective to suppress fit recrystallization, (11) to suppress grain growth, (iii) to reduce disappearance of working strain, etc. The idea that heat treatment should be performed after winding requires consideration because it can create a fine grain structure that is essential for high strength and toughness.

(C) Even if the winding temperature is low, there is no need to worry about trouble during winding unless the workability is extremely poor and the structure is mainly composed of lower bainite or martensite.

(D) If the time required between the end of rolling and winding is too short, reduce the rolling speed to lower the line speed, or extend the distance between the finishing mill and winding machine as necessary. You can deal with it by doing this. However, this increased turnaround time requires consideration of material quality.

The present invention has been made based on these findings.

In the conventional technology, as shown in FIG. 3, 2 to 4 water cooling devices 6 are arranged between the finish rolling 812 and the winding machine 10 to control the winding temperature, with appropriate intervals for heat recovery.
In the process of shifting and transporting the ring 12 after winding, the ring 12 is reformed in a reformer 20 through a quenching tank, followed by a tempering tank.

In contrast, in the present invention, as shown in FIG. 1, four rapid cooling devices 22 and four rapid heating devices 24 are provided between the finishing rolling mill 2 and the winding machine 10.

The rapid cooling device 22 preferably uses a high-pressure water stream, and the rapid heating device 24 preferably uses an induction heating method, both of which are suitable for short-time processing and uniform processing.

The wire rod 4 that has been finish rolled in the finish rolling mill 2 is quenched in a rapid cooling device 22 and then tempered in a rapid heating device 24, and a reformer 20 is placed immediately after the winding machine 10, eliminating the conventional ring-shaped conveyance. .

The reformer 20 has a function as a heat insulating tank, and is designed to uniformly temper the untempered portion (inner layer) in the length direction at the time of winding.

The faster the steel is cooled after rolling, the more martensite nuclei will be generated, and as a result, the time required for quenching will be shorter, and the finer the quenched structure, the shorter the time required for tempering at the same temperature. Therefore, the distance between the finishing rolling mill 2 and the winding machine zor4 can be designed to be short.

Figure 2 shows the relationship between the size of crystal grains and the material quality after unwinding. Note that since the grain size is determined by the grain size d-, the toughness, yield strength, and tensile strength are high, so early cooling is a technology that achieves both goals at once.

The heating temperature for tempering is preferably in the range of 300 to 600°C. The lower the heating temperature, the shorter the heating time, but if it is lower than 300°C, the holding time will be longer and the amount of tempering required before winding will be shorter. Also, although the higher the heating temperature, the shorter the required holding time, the longer the time required for heating and the higher the thermal energy load for heating, which becomes uneconomical.
A range of 00 to 600°C is optimal.

Note that the cooling rate of wires with larger diameters is slower when compared under the same refrigerant conditions (although this is largely offset by the longer cooling time due to the lower rolling speed).

Therefore, in general, the necessary heat treatment can be applied to a wide range of dimensions using the same device without changing the distance between the finishing mill 2 and the winder 45110.

In the present invention, quenching and tempering is performed after finish rolling and before winding, to make the material quality of the steel bar/wire material uniform in the length direction, and to omit ring conveyance after winding.

Although the method of the present invention is a technology related to quenching and tempering, it can of course be applied without any problems to the production of directly quenched materials that are finished as quenched. Also, cooling to 450~6
It is also possible to produce a direct pearlite transformation treated wire by stopping at an arbitrary temperature in the range of 50° C. and then winding and coiling while under-cooling.

〔Example〕

Example 1 1030 steel with the components shown in Table 1 was prepared using 18 bar stands and 6 wire finishing stands to reduce the diameter to 12 m + ntj
The billet size was 150 mm, the extraction temperature was 1100°C, and the finishing speed was 30 m.
/s, finish rolling temperature: 1050°C. These wire rods were quenched by the method of the present invention in a rapid cooling device 22 with a water cooling zone length of 8 m x 4 tanks = 32 m/4 tanks as shown in Fig. 1 at a total water flow rate of 8000 J/min. And so.

Table 1 For comparison, wire rods under the same conditions as the examples of the present invention were wound up using the device shown in Figure 3, and then subjected to forced air cooling and online quenching.
This was taken as a conventional example.

The mechanical properties and scale thickness of these online fired materials were investigated, and the results are shown in Table 2.

Table 2 In Table 2, the examples of the present invention are excellent in both tensile strength, reduction of area, and 3σ in proportion to the conventional examples, and the scale thickness is also significantly smaller at 1/8.

Example 2 A material with exactly the same components as the present invention example shown in Table 2 was used, and the process was the same as in Example 1 until quenching, and then it was heated to 450°C with a rapid heating device 24 and tempered by cooling. This was used as an example of the present invention.

As a conventional example, a wire rod under the same conditions as the present invention example was reheated at a secondary processing manufacturer to 900°C to austenite, then quenched, and then tempered at 380°C for 6 seconds.

The mechanical properties of these wires were investigated and the results are shown in Table 3.

From Table 3, it can be seen that the examples of the present invention have excellent results that are almost equivalent to those obtained by quenching and tempering materials by reheating in secondary processing manufacturers.

Table 3 [Effects of the Invention] As is clear from the above examples, the present invention can achieve the following effects by performing quenching and tempering between the finish rolling process and the ring forming process of the steel bar/wire rod. Ta.

(b) Since products of the same quality as quenched and tempered materials obtained by reheating at conventional secondary processing manufacturers can be obtained online, significant energy savings can be achieved by utilizing the sensible heat of the rolling finish.

(b) Steel bars and wire rods with uniform material in the length direction can be obtained.

(c) Huge equipment for ring-shaped conveyance and forced air cooling after @ removal is not required.

[Brief explanation of drawings]

Figure 1 is a layout diagram of the equipment used in the process of the present invention after finish rolling, Figure 2 is a diagram showing the relationship between grain size and mechanical properties after tempering, and Figure 3 is a diagram used in the conventional method. Figure 4 is a front view showing the shape of the wire rod after ring-shaped forming using the conventional method.

Claims (1)

[Claims] (1) In a method of forming a steel bar or wire rod into a ring shape and winding it after finishing hot finish rolling, the step of quenching the steel bar or wire rod in a rapid cooling device between the finish rolling step and the step of forming into a ring shape. , a step of tempering using the rapid heating device after quenching.
Direct quenching and tempering method for wire rods.