JPH0699744B2 - Method for manufacturing softened rods / wires - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method of manufacturing a softened wire rod and a softened steel bar.

[Prior Art] As is well known, among cold-forged wire rods and steel bars, those that are hard and have poor formability are spheroidized as a softening heat treatment in order to lower their hardness in advance and improve their deformability. It is generally annealed. As well known, there are three types of spheroidizing annealing. That is, the sixth
As shown in Figure (a), after heating to a temperature of A1 point or higher, then gradually cooling to a temperature of A1 point or lower, as shown in Figure 6 (b), heating to a temperature of A1 point or higher After doing
There are three types, a constant temperature transformation type that is rapidly cooled to a temperature of A1 point or lower and kept at that temperature for a long time, and an isothermal holding type that is kept at a temperature of A1 point or lower for a long time, as shown in FIG. 6 (C). However, each of them takes 10 hours or more, and in some cases, 20 hours or more, which is a very disadvantageous heat treatment from the viewpoint of energy saving.

Therefore, in order to shorten the spheroidizing annealing time, JP-A-62-62
As disclosed in JP-A-139817 and JP-A-58-3919, a method of adjusting rolling conditions has been proposed.

[Problems to be Solved by the Invention] However, in each of the above-mentioned publications, although the time is slightly shortened compared to before, basically, the heat pattern of the conventional spheroidizing annealing is directly adopted, It is not very effective in terms of significantly shortening the processing time.

In view of this point, the present inventor has diligently studied, by appropriately combining finish rolling conditions and subsequent cooling conditions,
The present invention has been completed based on the finding that the softening heat treatment in the subsequent step has a sufficient softening effect not only by spheroidizing annealing but also by simply performing heating-cooling in the air, and it is possible to significantly shorten the heat treatment time.

That is, the main purpose of the present invention is a heat treatment for an extremely short time,
It is an object of the present invention to provide a method for manufacturing a softened rod / wire that can obtain a spheroidized structure.

[Means for Solving the Problem] The present invention for solving the above-described problem is to provide a finish rolling start temperature of 750 ° C. or less and a finish rolling end temperature when hot rolling a steel material containing 1.5% or less of C. Finish rolling while controlling to 850 ℃ or less, and after completion, 50 at a cooling rate of less than 4 ℃ / sec.
After cooling to 0 ° C and then to room temperature, the scale on the surface of the product was removed, and (A1 point-
A spheroidized structure is obtained by heating to a temperature of (50 ° C) or higher and (A1 point + 50 ° C) or lower, maintaining this temperature range for 10 minutes or longer, and then allowing it to cool in the atmosphere. It is a thing.

[Operation] In the present invention, by specifying finish rolling conditions, subsequent cooling conditions, and spheroidizing annealing conditions as described above, spherical carbides can be formed by heat treatment for an extremely short time without decarburization. Thus, a sufficiently softened wire or the like can be obtained. Further, by this, it is possible to prevent the occurrence of breakage or dislocation cracking during handling after annealing.

[Specific Configuration of the Invention] Next, the reasons for limiting each numerical value in the present invention will be described in detail. First, the amount of C contained in steel is limited to 1.5% or less. Generally, when so-called pro-eutectoid cementite which precipitates in a mesh-like shape is present in steel, surface cracks are generated from this pro-eutectoid cementite as an origin during cold forging. If the C content exceeds 1.5%, it is impossible to prevent the formation of the above pro-eutectoid cementite under any conditions of rolling and cooling, and it is impossible to eliminate it by subsequent annealing. Because there is.

Next, the finish rolling start temperature was controlled to 750 ° C or lower, the finish rolling end temperature was controlled to 850 ° C or lower, and then the cooling rate up to 500 ° C was specified to be less than 4 ° C / sec.
In the structure of the cooled steel, since the carbides are partly spheroidized, the softening heat treatment of the next step is not a so-called spheroidizing annealing that requires a long time, but a very short time heat treatment of heating-cooling to the atmosphere. This is because the carbide can be spheroidized. The metallurgical reason is explained below.

First, when the structure before the start of finish rolling is austenite or austenite-ferrite, finish rolling at a temperature of 750 ° C or lower causes work-induced dispersion precipitation of extremely fine carbides in the austenite and ferrite grain boundaries, As a result, spherical carbides are formed during the subsequent cooling at less than 4 ° C./sec. Further, if the rolled material exceeds 850 ° C, the fine carbides induced by work-induction completely dissolve into solid solution, making it impossible to form spherical carbides.Therefore, the finish rolling end temperature must be 850 ° C or lower. There is.

Further, when the cooling rate after finish rolling is 4 ° C./sec or more, there is not enough time for the carbide to grow with the fine carbide as the nucleus, so that it does not become spherical and becomes entirely layered or extremely Becomes a quenched structure such as bainite and martensite. Therefore, the cooling rate after finish rolling needs to be less than 4 ° C / sec. At this time, the precipitation of carbides is completed up to 500 ° C, so cooling at less than 4 ° C / sec is specified to be up to 500 ° C. is there.

Next, if austenite has already undergone phase transformation to ferrite / pearlite, bainite, or martensite before finish rolling, carbides in the structure are finely crushed due to plastic deformation accompanying finish rolling, and as a result, The carbides are dispersed. Then, during the subsequent cooling at less than 4 ° C./sec, it grows spherically. Further, the effect of such finish rolling becomes particularly large when the finish rolling start temperature is lower than 650 ° C. Therefore, the starting temperature is preferably 650 ° C or lower. The lower limit of the finish rolling start temperature is not particularly specified, but if it becomes extremely low, the load on the rolling mill increases and it becomes substantially impossible to roll, so it is necessary to set the lower limit value according to the ability of the rolling mill. There is.

Next, the reason for softening heat treatment in a reducing gas atmosphere is to prevent decarburization of the steel surface. Before that, it was decided to remove the scale on the product surface.
This is because when the scale is present in the reducing gas atmosphere, the reducing gas and the scale react with each other to generate an oxidizing gas, and decarburization rather proceeds. This scale removal is generally performed by heat treatment such as spheroidizing annealing, and the method thereof may be chemical removal such as pickling or mechanical removal such as shot blasting and repeated bending.

Next, as a softening heat treatment for obtaining a spheroidized structure, heating to a temperature of (A1 point −50 ° C.) or higher and (A1 point + 50 ° C.) or lower, and maintaining at this temperature range for 10 minutes or longer is This is because if the material is heated and maintained at a temperature outside this range and then left to cool in the atmosphere, the spheroidization of the carbide will not be sufficiently carried out. That is, as described above, in the present invention, since the steel wire rod or the steel bar is rolled at a low temperature of 750 ° C. or less, a large amount of plastic working energy is accumulated in the matrix structure, so it is only necessary to add some heat energy. , The accumulated plastic working energy is released, and accordingly, the layered carbide is divided into rod-like or spherical particles, and the solid-solved carbide is already precipitated during the air cooling after finish rolling and cooling. The spherical carbide is re-precipitated with the nucleus as a nucleus, and the growth of the spherical carbide proceeds further. To do this, at least (A1 point −
It is necessary to keep the temperature above 50 ℃ for 10 minutes or more,
When the temperature exceeds (A1 point + 50 ° C), the already-precipitated spherical carbide completely dissolves in solution, and when re-precipitated, it becomes a layered carbide. For the above reasons, heating within the above temperature range
It was decided to hold.

[Example] Next, an example will be described.

18-mm wire rods were manufactured by changing the finish rolling start temperature and finish rolling end temperature of 2 ton steel pieces of S45C and SCM435, respectively. After rolling, the cooling rate up to 500 ° C was 2 ° C / sec.

Next, the wire rod rolled under these various conditions was cut at A1 point (S45C: 730
C., SCM435: 745.degree. C.), held for 18 minutes, then allowed to cool in the atmosphere, and the structure of each wire was observed to measure the degree of spheroidization of the carbide (spheroidization rate).

The results are shown in FIGS. 1 and 2.

From both figures, S45C, SCM435 both finish rolling start temperature and finish rolling end temperature are both within the scope of the present invention are all 80
It has a spheroidization rate of at least%.

Next, for S45C, the finishing rolling start temperature and end temperature are 665 ° C and 805 ° C, respectively, and for SCM435, the same 675 ° C and 805 ° C wire rods, the temperature and time of subsequent softening heat treatment are variously changed to change the carbide content. The spheroidization rate was measured. The results are shown in FIGS. 3 and 4.

From both figures, it can be seen that if S45C and SCM435 are treated under the conditions within the range of the present invention, a spheroidization rate of 80% or more can be obtained.

Furthermore, Fig. 5 shows the above S45C wire after finishing rolling.
The spheroidization rate is shown when the cooling rate up to 500 ° C was changed and then the softening heat treatment was performed at A1 point for 18 minutes, but when the cooling rate was 4 ° C / sec or more, the spheroidization rate was It can be seen that it drops sharply.

Next, in FIG. 3 and FIG. 4, after performing normal rolling with S45C and SCM435 held for 18 minutes at point A1, FIG. 6 (a)
S45C that has been subjected to conventional spheroidizing annealing (slow cooling type),
Table 1 shows each characteristic of SCM435 and the softening heat treatment time.

As is clear from Table 1, according to the method of the present invention, although the heat treatment time was shortened to 1/4 to 1/5 of that of the conventional method,
The characteristics are rather improved.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide a method for manufacturing a softened rod / wire having a spheroidized structure by heat treatment for an extremely short time.

[Brief description of drawings]

1 and 2 are for S45C and SCM435, respectively.
Figures showing the effect of the finish rolling start temperature and finish rolling end temperature on the spheroidization rate, Fig. 3 and Fig. 4, respectively are S45.
FIG. 6 is a diagram showing the effects of heating temperature and holding time on the spheroidizing rate in C and SCM435, FIG. 5 is a diagram showing the relationship between the cooling rate and the spheroidizing rate in S45C, and FIGS. 6 (a) and 6 (b). ,
(C) is a figure which shows the heat pattern of spheroidization annealing in the conventional method.

Claims (1)

[Claims] 1. When hot rolling a steel material containing 1.5% or less of C, finish rolling is performed while controlling the finish rolling start temperature to 750 ° C. or lower and the finish rolling end temperature to 850 ° C. or lower, and after finishing, 4 After cooling to 500 ° C at a cooling rate of less than ° C / sec and then to room temperature, the product surface scale is removed,
In a reducing gas atmosphere, (A1 point-50 ° C) or higher, (A1 point +
50 ° C) and heat it to a temperature below 10 ° C.
A method for producing a softened rod / wire, comprising obtaining a spheroidized structure by holding for at least minutes and then cooling in the atmosphere.