JPS59126720A - Direct spheroidizing treatment of steel material - Google Patents

Abstract

PURPOSE:To obtain a hot-rolled wire rod and steel bar having the characteristic equivalent to those by ordinary spheroidizing annealing by combining a specific hot rolling condition and cooling condition. CONSTITUTION:A steel material is subjected to finish rolling continuously under the condition of >=80% total draft in a temp. region from the Ar1 transformation point temp. to the temp. which is not as low as 50 deg.C or below in the hot rolling stage of the steel material. Spheroidization is thus advanced fairly in the finish rolling stage. The steel material is treated at the finishing temp. in the range of the Ac1 transformation point temp. or above and the Ac3 transformation point temp. or below by the working heat thereof and thereafter the steel material is slowly cooled. A cementite nucleus glows quickly and the spheroidization advances quickly in the process when the steel material is slowly cooled.

Description

[Detailed Description of the Invention] (Technical Field) The present invention relates to a method for direct spheroidization of wire rods and steel bars among steel materials, and in particular, enables various cold working processes while still being hot rolled, which is indispensable for conventional wire rods and steel bars. The present invention provides a new method for manufacturing steel products that simplifies the process and saves energy by omitting the spheroidizing process in the secondary processing step.

(Prior art) Usually, hot-rolled wire rods and bars of medium-high carbon steel or alloy steel are subjected to wire drawing and cold forging processes in the secondary and eighth processing processes to form various types of bolts and nuts. It is processed into various mechanical parts.

During hot rolling, bainite, reticular cementite, and layered pearlite structures are formed, so cold workability is poor and strong processing cannot be performed as is, and deformation and cracking occur during the quenching process. It is easy to use and the material is also brittle.

Therefore, in order to improve these defects, so-called spheroidizing arm annealing is usually performed to convert cementite into stable spheroidized cementite. Various methods have been developed and put into practical use for spheroidizing and sintering, but all methods require long-term high-temperature heating and precise temperature control.

Therefore, this process generally requires expensive heat treatment equipment, and not only is it difficult to increase productivity due to long-term temperature control, but it also consumes a tremendous amount of thermal energy, which becomes a major bottleneck in reducing costs. ing.

There are still many problems in terms of material quality and cost, such as decarburization and scale formation during heat treatment, no surface roughening, and increased pickling time in the post-process.

(Motivation for the Invention) From this point of view, the inventors conducted a detailed study of hot rolling conditions and cooling conditions, and found that by combining specific hot rolling conditions and cooling conditions, it is possible to They have discovered a method that can advantageously produce wire rods and steel bars with similar properties as they are hot-rolled.

(Structure of the Invention) In other words, as a result of numerous experiments and studies regarding hot rolling of wire rods and steel bars, it was discovered that finish rolling can be carried out between the lr□ transformation point temperature and a temperature not lower than 50°C. In the temperature range, the process is carried out continuously under the conditions of a total reduction rate of 80% or more, and the processing heat is used to bring the finishing temperature to a range of υAC transformation point temperature or higher and AC8 transformation point temperature or lower, and then the above is achieved by slow cooling. It has been determined that the characteristics can be advantageously realized.

In this invention, finish rolling is started in Ar, from the transformation point 1 to 50℃ below it, at a temperature 1, that is, below the Ar□ point and above (Ar□-50℃), and continuously. First of all, it is necessary to perform rolling at a total reduction rate of 80% or more so that the finishing temperature is increased from the AO work point to the Ac8 point using processing heat, and the reason for this is as follows.

By a conventional method, hot-rolled wire rods and steel bars are brought to a temperature below the LR transformation point temperature, thereby forming a sybarite structure. When rolling is applied in this state, the cementite plates constituting the pearlite are divided due to processing strain. In addition, in hypereutectoid steel, the pearlite structure and reticulated cementite at grain boundaries are similarly divided. At the same time, the ferrite phase and pearlite phase themselves are also processed, causing an increase in dislocation density and an increase in interfacial energy.

In other words, finely divided cementite has high interfacial energy and is easily self-agglomerated because it is divided.

Furthermore, by continuously rolling, there is less processing heat dissipation, and the temperature of the material increases and cementation) [
Parting and accumulation of machining strain progresses significantly. For this reason, the diffusion rate of carbon atoms becomes much faster than in the case where rolling is applied too much.

Normally, by reheating the pearlite structure to a range of AC□ point or higher and Ac8 point or lower, a part of the plate-like cementite is melted and divided, but especially when the material temperature is reduced to a range of ACC point or higher and 108 point or lower due to processing heat. Due to the synergistic effect of processing strain, the operation to increase the
It was found that by dramatically increasing the frequency of fragmentation and increasing the diffusion rate of carbon atoms, the self-cohesive force of cementite increases significantly.

It has also been found that the morphology of the cementite nuclei is extremely fine and uniform, resulting in force uniformity that cannot be achieved by conventional spheroidizing annealing methods.

If the rolling start temperature is below Ar, the rolling start temperature is too low and the deformation resistance is significant, and unless this is disadvantageous in terms of the load on the rolling mill and the productivity, the above-mentioned cementite plate Considering that the accumulation of breaking and strain energy increases at low temperatures, the lower limit of the rolling temperature was set to (Ar□-50°C).

Next, the reason why rolling is applied continuously at a total reduction rate of 80% or more in a temperature range of Ar, below the point, and above (Ar□-50°C) is as follows.

FIG. 1 shows QO, 85%, Si 0.22%, Mn 0.
75%, p 0.010%, So, 008%,
A steel having a composition containing 1.02% of Or and 20% of MOo is hot-rolled in a conventional manner, and at the finish rolling stage, the temperature is 670°C, which is in the range of below Ar□ point and above (Ar□-50°C).
Cementite spheroidization rate and rolling reduction between 690 and 750℃ after continuous rolling and slow cooling by changing the rolling reduction rate from ℃ to keep the wire temperature above AC point and below AC8 point due to processing heat This shows the relationship with the rate.

The degree of spheroidization was evaluated based on the spheroidization test specified in JIS-G 8589.

As is clear from the figure, when the total rolling reduction exceeds 80%, spheroidization is significantly promoted.

In other words, in order to finely divide the cementite plate and sufficiently accumulate processing strain in the ferrite phase and pearlite phase, a continuous total reduction rate of 80% or more is required.

Furthermore, the calorific value due to processing depends largely on the total reduction rate. Therefore, in order to keep the finishing temperature within the range of 108 points or less on the AC point plate, which is a constituent requirement of this invention, it is necessary to suppress heat dissipation during rolling as much as possible and to increase the number of heat generators. The lower limit of the rolling reduction required to achieve this by normal linear phase or bar steel rolling is 80%.

Next, the reason for limiting the finishing temperature to the range below 108 points on the AC point plate is that below the AO work point, the fragmentation of the cementite plate progresses sufficiently, but the melting is insufficient, and the diffusion of carbon atoms. Since the speed does not increase, it is necessary to set it to AC□C star point.

However, when the Ac point reaches 8 or higher, it becomes completely austenitic and the cementite, which is the core of spheroidization, disappears.
On the other hand, it is difficult to make it spherical, so it is necessary to set it to 108 points or less.

As described above, the finely and uniformly dispersed cementite nuclei have a higher self-cohesive force due to the higher interfacial energy and the remarkable increase in the diffusion rate of carbon atoms due to processing compared to the conventional method. This increases significantly, and spheroidization is already progressing at the finish rolling stage.

In the process of gradual cooling through this state, the cementite core grows rapidly, and spheroidization progresses rapidly.

In this invention, since processing strain energy plays a central role in promoting spheroidization, the morphology of cementite can be controlled by setting the rolling conditions as described above.

Of course, the present invention does not particularly limit the chemical composition of the steel, but the most preferred steels for the intended purpose include carbon steel for machine structures, alloy steel for machine structures, case hardening steel, and bearing steel.

(Example) Examples of the present invention will be described below. Steels (A) and (B) having the chemical composition shown in Table 1 were melted by a conventional method, with a final wire diameter of l Ornmφ, under the conditions shown in Table 2, and at a finishing temperature of 750'C. After continuous rolling with force, slow cooling was performed. Table 2 shows the spheroidization state of these hot rolled materials. As a reference example, Table 2 shows an example in which a conventional method of spheroidizing annealing was performed.

Table 1 (℃) Table 2 *1: 710℃X 8 hr -+ 20℃/hr
-+ 650℃X 4hr, -+Air cooling*2: 7
40℃×8hr-+20℃/hr-+650℃×
6t1r, -) The measurement and display method of the degree of air-cooling spheroidization was based on the spheroidization structure test of JIS-08589.

In Table 2, sample materials A1 and /I62 have a rolling reduction ratio of 8
This is a comparative example when the filling capacity is 0%, and in both cases, spheroidization has progressed only slightly.

Also, sample material 45. A6, 49, and AIO are reference examples in which the spheroidized portion was annealed by the conventional method, and although the degree of spheroidization differs depending on the conditions, the heat treatment requires an extremely long time in all cases.

On the other hand, the sample material shop 8゜Ji 4 according to the present invention
, A 7 and A 8 have significantly progressed into spheroidization,
``In both cases where the rolling reduction ratio is 80% and 60%, the spheroidized portion of the conventional method shows exactly the same spheroidization as the tempered material.

Although this invention has been described above as being applied to wire rods and steel bars, it can also be widely applied to other types of steel that require spheroidization, such as hot-rolled materials and thick steel plates.

Therefore, according to the present invention, it is possible to perform spheroidization during hot rolling without the need for long-time heat treatment as in the conventional method, resulting in improvements in energy saving, productivity, and economic efficiency. Not only does it make a big contribution, but it also has a great effect on improving the quality of the material.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the rolling reduction rate and the spheroidization rate of cementite between 690 and 750°C. Patent applicant: Kawasaki Steel Corporation

Claims (1)

[Claims] L When hot rolling steel materials, finishing is carried out under conditions of a total reduction rate of 80% or more continuously in the warm region from the Ar□ transformation point temperature to a temperature as low as 50°C below that temperature. Rolling is performed and due to the processing heat, the temperature is higher than the QAC transformation point temperature, A
A direct spheroidizing method for steel material, which comprises bringing the finishing temperature to a range below the c8 transformation point temperature and then slow cooling.