JPS6254375B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a continuous annealing method for cold-rolled steel strip for drawing, and aims to produce a steel sheet with excellent aging resistance through short-time overaging treatment by optimizing the thermal cycle. It is well known that the conventional continuous annealing method for cold-rolled steel strip for drawing consists of three successive steps: a short-time heating/soaking step, a rapid cooling step, and an overaging step. In the process of developing this continuous annealing method,
Although various proposals have been made for each process, the process that can demonstrate the most distinctive characteristics is the treatment process from the rapid cooling process to the overaging process, which is both hard (equipment) and soft (thermal cycle). Many proposals have been made regarding this process. The two basic methods are listed below. (i) In the method proposed in Japanese Patent Publication No. 49-1969, after annealing, the steel strip is quenched in flowing water at an appropriate temperature to bring the solid solute C in the steel into a highly supersaturated state, and then heated to a predetermined temperature ( By reheating and soaking to the temperature (overaging treatment temperature), solid solute C is precipitated in a short time using the supersaturation degree of solid solute C as a basis (starting source). (ii) The method proposed in Japanese Patent Publication No. 51-5335 attempts to precipitate solid solution C by cooling to a predetermined overaging temperature by gas jet cooling after annealing and then soaking at that temperature. . In this method, cooling from the annealing temperature is slower than in (i) above, so the degree of supersaturation of solid solute C at the start of overaging treatment is low, making it difficult to use as a starting point for C precipitation. Since it is too low, it is necessary to take a longer overaging treatment time than in the method (i) above. In addition to the two conventional basic methods mentioned above, a method using hot water as the cooling medium, a method using alcohol, a method of cooling by contacting with a water-cooled roll, and a method using mist, which is a mixture of gas and water, as the refrigerant. Although various cooling methods such as The problem is whether to continue the aging process and to precipitate solid solution C through short-term over-aging treatment.The heat cycles and heat treatment patterns in the two basic conventional methods mentioned above are ultimately based on the same idea. . Conventionally, techniques for improving the above two basic thermal cycles have been proposed from the viewpoints of material, steel composition, process, and energy. Although most of these have been made for the purpose of optimizing overaging treatment, no attempt has been made to optimize them in a unified manner based on the technical idea of precipitation of solid solution C from a supersaturated solid solution C state. The current situation is that nothing has been done. In view of the above-mentioned problems of the prior art, the present invention has been developed in which the precipitation site of solid solution C changes during the overaging process depending on the cooling rate from the annealing soaking temperature. There is a lot of solid solution C)
Now, we will consider the fundamental difference that carbides precipitate within ferrite grains as fine carbides, and precipitate at ferrite grain boundaries under slow cooling rates such as gas jet cooling (where the amount of supersaturated solid solute C is small). This article specifies the optimum overaging cycle for the fastest precipitation of solid solution C under these conditions. The present invention relates to a continuous annealing method for cold-rolled steel strip for drawing, and in particular to a method for continuously annealing a cold-rolled steel strip for drawing, and in particular to a method for continuously annealing cold-rolled steel strip for drawing, and in particular to a method for continuously annealing a cold-rolled steel strip for drawing.
The main objective is to sufficiently precipitate the In the present invention, after heating and annealing, the cooling end point temperature T _s is determined at the primary average cooling rate v (°C/sec) after the soaking zone.
After cooling to (℃), the time t (minutes) from the overaging start temperature T ₁ (℃) to the overaging end temperature T ₂ (℃) is shown below in the thermal cycle of the cold rolled steel strip that is overaged. This is a continuous annealing method for a steel strip that is continuously annealed under one condition selected from conditions 1) to (3), and the conditions are: (1) v300°C/sec T _s < T ₁ ; T ₁ 500°C; T ₂ 200℃T ₁ −T ₂
50℃; 1 minute t5 minutes; -0.5T ₂ +500℃T ₁
−0.5T ₂ +650℃ (2) v＜300℃/sec 0T ₁ −T _s ＜50℃; T ₁ 450℃; T ₂ 320
℃; 0T ₁ -T ₂ 50℃; 2 minutes t5 minutes; (3) 50℃/sec v<300℃/sec T ₁ -T _s 50℃; T ₁ 450℃; T ₂ 300℃; 0
T ₁ −T ₂ 70°C; 1 minute t5 minutes; Next, the requirements constituting the present invention will be explained in detail based on examples. As already explained, the present invention provides an overaging start temperature, an overaging end temperature, and an overaging time under a combination of a predetermined cooling rate and cooling end point temperature before overaging treatment in a continuous annealing thermal cycle. The heating and soaking conditions before heating and soaking conditions do not essentially impair the effects of the present invention, and the normal continuous annealing thermal cycle, that is, heating and annealing at a temperature higher than the recrystallization temperature. There is no difference from the conventional method in that respect. Now, in continuous annealing, it is well known that there is a close relationship between the cooling rate after annealing soaking and the overaging effect. In other words, increasing the cooling rate prevents the precipitation of solid solute C in ferrite during cooling and increases the degree of supersaturation of solid solute C before overaging. Solid solution C precipitates in a short time. Figure 1 shows the above relationship using Steel 1 in Table 1, which will be shown later, as a raw material.The steel strip was hot-rolled at 200°C and then cold-pressed to 0.8 mm, and then rapidly rolled. In a heating annealing furnace
After heating and annealing to 700℃, the average cooling rate is about 20 to about 1500
After cooling to room temperature at ℃/sec cooling condition, 350℃
In the case of overaging treatment by reheating and holding until
The primary cooling rate when cooling to 350℃ and soaking overaging treatment, and the aging index obtained when soaking and air cooling at 350℃ (100℃ x 1 after 8% tensile deformation)
Relationship between the soaking time required for the stress increase (evaluated by the amount of stress increase when tensile deformation is applied again after time aging) to be less than 4 Kgf/mm ² and the number of fine precipitated carbides observed in the ferrite grains at the end of over-aging. This is what is shown. As is clear from FIG. 1, the precipitation rate of solid solution C during the overaging treatment process largely depends on the primary cooling rate and the supercooling before overaging. This is nothing but the fact that the average precipitation path of solid solute C became shorter due to the precipitation of carbides within the ferrite grains as the amount of supersaturated solid solute C increased. Based on the findings of the above experiments, the present invention is based on the findings of the present invention that, when the primary cooling rate is slow such that no fine carbides are observed and solute C is mainly precipitated at grain boundaries, most of the solute C is finely precipitated within the grain boundaries. When the primary cooling rate is high, and when the primary cooling rate is intermediate between the two, 3.
The purpose of this study is to optimize the overaging thermal cycle for two conditions. Figures 2 to 5 are diagrams for determining the optimal combination of overaging start temperature _T1 and overaging end temperature _T2 . These figures all show equal aging index curves when steel 1 hot- and cold-rolled under the same conditions as in FIG. 1 is annealed in the thermal cycles shown in FIGS. 11A to 11D, respectively. From the results in Figures 2 to 5, it is clear that when there are intragranular precipitated carbides, it is better to have a temperature gradient from T ₁ to T ₂ (Figure 2), and when there are no intragranular precipitated carbides (T 3 (Fig. 4) and when there are few intragranular precipitated carbides (Fig. 4), it is preferable to overage in the state of T ₁ T ₂ . Furthermore, under the conditions shown in Figure 4, T _s before T ₁
When supercooling is performed to (250℃) (Figure 5), a decrease in the aging index is observed. The main point of the present invention is basically to define the optimum combination of T ₁ and T ₂ in addition to the overaging time and supercooling conditions described later. Figure 6 shows T ₁ where the above-mentioned aging index is the lowest.
This figure shows the aging index change when changing the time from T ₁ to T ₂ under the combination of and T ₂ . Originally, it is preferable that the aging index is low, and from that point of view, it is preferable to carry out the aging treatment for as long as possible. However, for a continuous annealing line, excessive aging treatment is not preferable because it increases the length of the line. On the other hand, in continuous annealing, it is impossible to reduce the aging index to 0 just by thermal cycling, so it is necessary to divide the aging index below the level at which strain aging is considered not to be a problem in practice. In the present invention, from the above point of view, the aging index is 4Kgf/mm ²
From Figure 6, assuming that the primary cooling rate is less than 300
If the primary cooling rate is 300°C/sec or more, set the overaging time t to 1 to 5 minutes, and if the primary cooling rate is less than 300°C/sec, set the overaging time t to 2 to 5 minutes.
In the case of less than 50° C./second and more than 50° C./second, one condition is selected from among the respective prescribed conditions, with the overaging time t being 1 minute to 5 minutes. The effect of supercooling on the aging index reduction shown in FIGS. 1, 5, and 6 is as shown in FIG. 7. This figure shows the same pretreatment conditions as in Figure 1, with cooling rates before overaging of 50°C/sec and 200°C/sec.
This graph shows the relationship between the degree of supercooling (the difference ΔT between the primary cooling end point temperature T _s and T ₁ when T ₁ = 350 to 400°C) and the number of precipitated carbides in the grains. It can be seen that the number of intragranular precipitated carbides increases when This is due to the increase in the number of precipitation sites for solute C due to supercooling, and with supercooling of 50°C/sec or more, it is possible to accelerate the precipitation rate of solute C even if the primary cooling rate is slow. It is shown that. Therefore, in the present invention, the conditions for thermal cycling of cold rolled steel strip are: when the average primary cooling rate is 300°C/second or more, when the average primary cooling rate is less than 300°C/second and the degree of subcooling is less than 50°C, Average cooling rate is 50℃/
For the three cases in which the degree of supercooling is 50°C or more at a rate of 300°C/second or more and less than 300°C/second, the combination areas of T ₁ and T _{2 shown in Figures 8 to 10 are defined, and the combinations of T 1 and T 2} are determined from among these thermal cycle conditions. One condition is selected. Table 1 shows the chemical compositions of the steels used in the examples of the present invention, and Tables 2 to 5 show the chemical compositions of steels 1 and 2 in Table 1, with respect to T _s , T ₁ , T ₂ , t specified in the present invention. What kind of material can be obtained by continuous annealing through thermal cycles of , is shown in comparison with the conventional method. C is obtained by the method of the present invention, and C is obtained by the conventional method.
The primary cooling rate in Table 2 is approximately 1500°C/sec (560°C water cooling), the primary cooling rate in Table 3 is approximately 30°C/sec (gas jet), and the primary cooling rate in Table 4 is Approx. 200℃/sec (mist)
The primary cooling rate in Table 5 is approximately 200
°C/sec (mist). Moreover, the tensile test values in Tables 2 to 5 are based on JIS No. 5 test pieces. As is clear from Tables 2 to 5, in all tables, a low aging index and excellent material quality were obtained only when all of T _s , T ₁ , T ₂ , and t specified by the present invention were satisfied. I understand that.

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【table】 [Brief explanation of the drawing]

Figure 1 shows the cooling rate before overaging and the aging index <4Kg
This is a graph showing the relationship between the overaging time at 350°C resulting in f/mm ² and the number of intragranular precipitated carbides observed at that time.
FIG. 6 is a graph showing the relationship between overaging time and aging index for the combination of T ₁ and T ₂ based on the present invention, and FIG. Speed is 200℃/sec
It is a graph showing the relationship between the degree of supercooling before overaging (ΔT) and the number of intragranular precipitated carbides after overaging at 50°C/sec, and FIGS. 8 to 10 are graphs showing the relationship between T ₁ and T ₂ in the present invention.
11 is a graph showing the combination range of FIG.
D is a graph showing the thermal cycles of FIGS. 2 to 5, respectively.

Claims (1)

[Claims] 1. After heating and annealing, after cooling to the cooling end point temperature T _s (°C) at the primary average cooling rate v (°C/sec) after the soaking zone, the temperature is increased from the overaging start temperature T ₁ (°C). Time t (minutes) to reach the aging end temperature T ₂ (°C) In the thermal cycle of a cold rolled steel strip to be overaged, the following (1) to
(3) A continuous annealing method for steel strip, characterized by continuous annealing under one condition selected from conditions. (1) v300℃/sec T _s <T ₁ ; T ₁ 500℃; T ₂ 200℃T ₁ −T ₂
50℃; 1 minute t5 minutes; -0.5T ₂ +500℃T ₁
−0.5T ₂ +650℃ (2) v＜300℃/sec 0T ₁ −T _s ＜50℃; T ₁ 450℃; T ₂ 320
℃; 0T ₁ -T ₂ 50℃; 2 minutes t5 minutes; (3) 50℃/sec v<300℃/sec T ₁ -T _s 50℃; T ₁ 450℃; T ₂ 300℃; 0
T ₁ −T ₂ 70℃; 1 minute t5 minutes;