JPS6056210B2 - Steel transformation structure control method - Google Patents

Description

[Detailed Description of the Invention] As is well known, the quality of steel materials generally depends on the chemical composition of the steel material, the transformation structures such as ferrite, pearlite, bainite, and martensite that are generated during cooling from high temperature (austenite state), and the temperature at which they are formed. do.

For this reason, in order to impart the desired material properties to the steel material, various heat treatments for the purpose of controlling the transformed structure or direct heat treatment after rolling are performed on the steel material. At this time, during cooling, depending on the type of steel during heat treatment, the temperature history of the steel material may deviate significantly from the planned temperature pattern due to heat generation due to transformation, and the desired material quality may not be obtained. For example, Figure 1 shows the CCT diagram and heat treatment (cooling) curve of O, 82C-O, 7Mn steel after heating to austenitize at 900°C for 1 minute. When cooled, it becomes a fine pearlite structure, which increases strength and
It is assumed that a material with excellent toughness can be obtained. However, when the steel material is cooled under certain cooling conditions (for example, cooling with gas at a certain pressure and flow rate), the cooling pattern becomes as shown in B due to the effect of transformation heat generation, resulting in a coarse pearlite structure that reduces strength and toughness. . Furthermore, if the material is rapidly cooled during transformation as in C to make the structure finer, a martensitic structure tends to occur in some parts and the toughness deteriorates. Therefore, in order to provide the steel material with a cooling curve such as A, it is necessary to perform delicate control to offset the transformation heat generation by changing the cooling conditions (coolant supply amount, etc.) during the transformation. In order to perform such optimal cooling control, it is necessary to quantitatively predict the effect of transformation heat on the cooling curve, and to determine the cooling conditions from time to time based on this. Furthermore, depending on the type of steel and the material to be obtained, it is often necessary to arbitrarily change the cooling (or heating) rate of the thermal history, but even in such cases, transformation can occur through any cooling path. Cooling conditions cannot be determined unless the progress of the process is predicted. When the temperature pattern deviates from constant cooling or constant temperature maintenance due to changes in the heat of transformation, cooling (temperature rise) rate, etc., conventional CCT and TTT diagrams show the temperature and time at which transformation begins, progresses, and ends. It becomes difficult to predict such factors, and it becomes difficult to find appropriate cooling conditions. Therefore, it is possible to apply a cooling pattern that involves changing the cooling rate, and by giving the steel type and cooling conditions, it is possible to calculate the cooling curve and transformation (transformation start, process, end, and transformed structure) including the effect of transformation heat. If a method for predicting this is developed, it will be possible to obtain cooling conditions for giving the steel material the desired cooling curve, and appropriate cooling control will be possible.

Several methods have been introduced in previous studies to calculate the start temperature of transformation, but no method has yet been proposed to quantitatively grasp the influence of transformation heat and predict the end of transformation by calculation. Not yet.

The present invention also formulates the quantitative influence of transformation heat, and controls heat treatment based on a transformation prediction method that can be applied to any innovative temperature pattern that calculates transformation and cooling curves from the start to the end of transformation. This paper proposes a method.

As a result of repeated basic high-temperature cooling experiments and field experiments using actual wire rods, etc., the present inventors have found that, in general, during the cooling process of steel materials, the transformation behavior of steel materials (transformation We found that it is possible to predict the onset, progression, termination, and changes in temperature patterns due to heat of transformation.

In other words, the progress from the start to the end of metamorphosis is the so-called TOhn
It has been clarified through many basic experiments that it can be approximately expressed by the sOn-Mehl equation.

Here, B and K are constants, but B in particular changes depending on the composition and the heating conditions during heat treatment before transformation. A method is used to determine the temperature at a certain amount of time for transformation. Using this formula, temperature TlCC), transformation rate Y
If the steel at temperature 1 reaches a temperature T2 (° C.) in the next stage and is maintained at this temperature for a minute time Δt, the transformation rate Y2 after Δt is expressed as follows.

Using this formula, the change in transformation rate is calculated along the temperature pattern, and the cumulative transformation rate is determined. Next, FIG. 2 shows the procedure for calculating the generation of transformation heat accompanying transformation and the accompanying change in the cooling curve. Considering a certain minute time Δt, assuming that there is no transformation, the cooling rate (this will be distinguished and referred to as cooling intensity) is V, and the steel material will be ΔTc=
Although the Vdt temperature is supposed to drop, the transformation actually progresses by ΔY=Y2-Y1 during this time, so there is heat generated by the transformation in proportion to this. Letting this be Δn, H is the total transformation latent heat (C
al/f), and the average specific heat during this period is C (Deg/y), and there is a relationship of H·ΔY=C·ΔTh, so the actual temperature drop ΔT (Deg) is calculated as follows.

If the computer continues to perform similar calculations from the temperature T2=T1+ΔT calculated in this way, the momentary temperature change of the steel material taking into account the heat of transformation will be calculated.

In addition, when performing calculation 1, the transformation can be calculated in principle with respect to the time from the start of cooling, but at this time, the following procedure can be adopted as a simple method for determining the values of B and R. can.

1. Determine the isothermal transformation curve section TT of a certain steel by experiment or calculation.

At this time, the metamorphosis start line is set as t=Z(T). (Figure 3 shows a TTT curve when 0.62C-0.7Mn steel is heated at 900°C for 10 minutes.) The transformation start temperature Ts of two consecutive coolings is determined according to the following Scheil equation.

(E. Schecl; Kch. Eiserlhutte
nW. (1935), P565) where Δ×(T
) is the constant temperature holding time TO at temperature T is the starting point temperature U (T) is the temperature gradient versus time at temperature T. From this Ts, the progress of transformation is calculated according to equation 1, but usually the same Basic research has shown that R is constant (approximately 2 to 3) in steel, so we can calculate B from Z(T) by B=BOln
It can be determined as Z(T) (BO is a constant).

With this prediction method, it is possible to determine not only the temperature at the start of transformation, but also the temperature during transformation.
Since the transformation rate and the like can be predicted, it is possible to predict the heat treatment conditions to provide a reasonable temperature pattern to obtain the desired steel material.

The present invention provides a heat treatment control method using the above method. The control procedure will be explained below using FIG. 4.

Although this figure describes the case where heat treatment is performed directly using the latent heat of the rolled steel material, the same applies even when the steel material is heat treated starting from a cold state. If the composition of the steel material is known, it is possible to specify TTT from the heat treatment conditions. In the case of direct heat treatment after rolling, it is generally possible to predict what conditions (austenite grain size) correspond to the heat treatment conditions by specifying the rolling conditions.

This varies depending on the rolling method, so it cannot be shown uniformly here, but for example, 1 Steel Research JNO. 289
(1976), pages 43-61, page 54, Figure 13. With this procedure, Z(T), B, and R are determined, so any cooling conditions (time course of cooling intensity) can be determined.
The temperature and time at which transformation starts, progresses, and ends can be calculated using Equations 1 to 4.

For example, the optimum structure for high carbon steel wire is 550.
The pearlite structure is transformed at ~600°C, but if the above-mentioned structure is obtained under certain cooling conditions, for example, at a certain constant rate like A in Figure 1, then the transformation starts at a certain level. If you cool with a cooling intensity (for example, water cooling with a fixed small amount) and increase the amount of water until the end of the transformation to cancel out the heat of transformation according to the amount of transformation calculated every moment after the start of transformation, the result shown in Figure 1A is obtained. It is possible to perform cooling along such a cooling curve. Next, if the temperature can be actually measured during cooling (or after cooling) as shown by the dotted line in FIG. 1 (dotted line in FIG. 4a), the following more precise control becomes possible.

When cooling is started with the above optimum cooling intensity set, the cooling curve associated with the momentary increase in the amount of transformation on the cooling line and the generation of transformation heat due to this can be calculated using the formula shown in the claims. If a thermometer installed on the line detects a difference between the calculated temperature and the calculated temperature, it is considered that the difference is caused by the cooling conditions or rolling conditions actually varying from the conditions used in the calculation. (As mentioned above, rolling conditions affect diffusion transformation through changes in austenite grain size before transformation). Therefore, if the measured temperature is higher than the predicted temperature, the cooling is strengthened by means such as increasing the cooling intensity so that the final average transformation temperature is lowered and the desired microstructure is obtained. Further, after this time, it is possible to make changes such as increasing the initially set cooling intensity so that the target material quality can be obtained without intermediately modifying the cooling conditions. In this way it is possible to precisely control the material to meet the target. In addition, if it is not possible to obtain the optimal transformed structure by cooling control alone, heating or heat retention equipment may be used as shown in Figure 4-b.
If necessary, it is also possible to control the temperature by controlling the operating conditions (furnace time, fuel supply amount, etc.).

Examples are shown below.

A 5.5Tnφ steel wire having the components shown in Table 1 was heat treated by applying the present invention.

The steel with the composition shown in Table 1 is made by lead patenting etc.
It is known that when it is transformed in a temperature range of about 550 to 60 CfC, it becomes a fine pearlite structure and a material with excellent strength and toughness can be obtained. In this example, a fine pearlite structure having the same excellent strength and toughness can be obtained by simple application of water spray without conducting lead patenting treatment or the like.

The cooling curve when steel 62B shown in Table 1 is continuously cooled by applying water spray is shown for each spray water level.
The transformation rate (Y2) and the true temperature change ΔT of steel 62B are calculated from equation (3), and the fifth
Cooling curves A, B, and C shown in Figure a were obtained.

In order to obtain a fine pearlite structure with excellent strength and toughness, it is necessary to undergo transformation in a temperature range of approximately 550 to 600°C.
So, when steel 62B is cooled, the transformation temperature range is 60
It is clear from the prediction results shown in FIG. 5a that the temperature will be 0° C. or higher, and it is therefore predicted that the transformed structure will be ferrite + coarse pearlite.

The structure of ferrite + coarse pearlite has low strength. Similarly, when steel 62B is cooled according to the predicted cooling curve B, the pearlite transformation does not complete, and some untransformed parts remain, which become martensitic structures upon cooling, so that the final It is predicted that the structure will be pearlite + martensite.

Such a structure has poor toughness. Similarly, when steel 62B is cooled using the predicted cooling curve C, 570 to 60
Since the transformation occurs in the temperature range of 0°C, it is predicted that a ferrite + fine pearlite structure will be obtained. Such a structure exhibits the desired properties of excellent strength and toughness.
Therefore, in this example, steel 6? The cooling was started from a temperature of 800°C and proceeded.

The results are shown in Table 2. As is clear from Table 2,
The predicted results (finally obtained tissue) match the actual results.

This shows that a desired material quality can be obtained by applying the steel transformation and structure control method according to the present invention. In the next example, steel 77B shown in Table 1 was subjected to heat treatment. Steel 77B also needs to be transformed in a temperature range of about 550 to 600°C in order to form a fine pearlite structure, but this steel type
It has a large amount of C and is easily hardened. In this example as well, the cooling curve shown in FIG. 5b was determined at a cooling water flow level that resulted in a cooling rate of 35 C/S. This is cooling curve D, and if cooling is performed using this cooling curve, it is predicted that a pearlite + martensite structure will be obtained. Such a structure lacks toughness.

Therefore, in this example, cooling was stopped in the middle of the transformation,
The material was placed in a heat retention furnace in order to have a temperature pattern of slow cooling, slight increase in temperature, or constant temperature. The cooling curve at that time was determined by the method described above, and a cooling curve E was obtained. Along cooling curve E, cooling was started from 800° C., cooling was stopped in the middle of transformation as described above, and heat retention was performed. The results are shown in Table 2. As is clear from Table 2, the predicted results (
The final tissue obtained) is consistent with the actual results.

As mentioned earlier, this can be achieved by controlling the temperature/time relationship of the steel material, including heat retention or heating, based on the prediction of the cooling curve that incorporates the effects of transformation heat according to the present invention. , it has been shown that desired materials can be obtained for various steels.

[Brief explanation of drawings]

Figure 1 is an example showing a transformation curve and cooling curve, Figure 2 is a diagram showing the concept of transformation heat, Figure 3 is a diagram showing the transformation start line in the isothermal transformation curve TTT, Figure 4 A and b are according to the present invention. 5A and 5B respectively show examples of heat treatment patterns according to the present invention.

Claims (1)

[Claims] 1. In heat treatment to achieve the desired material quality by causing diffusion transformation of steel at a desired temperature, temperature T_1 (°C), transformation rate Y_1
The transformation rate Y_2 after the steel is held at temperature T_2 (℃) for a short time Δt in the next stage is expressed as ▲There are mathematical formulas, chemical formulas, tables, etc.▼ Also, the cooling rate in the absence of transformation heat is expressed as V (deg / second)
The total heat of transformation is H (cal/g), and the specific heat is c (cal/de).
gg), the net temperature change of the steel due to the transformation heat generation and cooling during this time Δt, ΔT=T_2−T_1 (deg), can be expressed as an arbitrary cooling value using the formula ΔT=VΔt−(Y_2−Y_1)(H/C). Calculate the progress of transformation moment by moment under the conditions, predict the transformation temperature range from this, compare this with the desired transformation temperature range, and if there is a difference, modify the cooling conditions, and if necessary, A method for controlling the transformed structure of steel, which is characterized in that by adding means for stopping cooling, retaining heat, and increasing the temperature, the desired transformation is carried out in the transformation temperature range and the desired material quality is obtained.