JPS5818405B2 - Continuous annealing method for low carbon steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention provides a soft low carbon steel (00,
10% or less).

It is well known that if temper-rolled mild steel is left at room temperature or a slightly higher temperature, it will increase its hardness and tensile strength, and cause a decrease in ductility and impact resistance, that is, it will become brittle. ing.

This invention proposes a continuous annealing method for obtaining a soft low carbon steel strip by removing the above-mentioned embrittlement caused by aging of a copper strip produced by hot working and then cold working. be.

That is, in this invention, a steel strip that has been hot rolled, wound into a coil at a high temperature of 550°C or higher, and then cold rolled at a reduction ratio of 60 or higher is reheated and heated to 650 to 750°C for 3
Hold for 0 seconds or more to perform recrystallization annealing, cool to 30-50℃ or less, immediately heat at a temperature increase rate of 15℃/sec or more, and hold at 350-450℃ for 30 seconds or more. In this process, the hot-rolled steel strip is coiled at high temperature to coarsen crystal grains and carbides, delay carbon diffusion during annealing, and recrystallize the steel strip. By appropriately selecting the cooling rate after annealing, the degree of supersaturation of carbon in the ferrite after cooling is made suitable for the overaging treatment effect, and by increasing the temperature increase rate during the overaging treatment, the overaging treatment effect is improved. It is characterized by improving aging resistance and obtaining a soft low carbon steel.

The coil winding temperature after hot rolling in this invention may be the usual 550 to 650°C, but in order to allow sufficient growth of crystal grains during recrystallization annealing,
Winding at a high temperature of ℃ or higher is desirable.

Further, the reduction ratio in the subsequent cold rolling is preferably 60 to 80 in order to obtain sufficient drawability.

The heating humidity during recrystallization annealing was set at 650 to 750° C., although high temperatures are good from the viewpoint of crystal grain growth, but heating above the ACl transformation point deteriorates ductility.

Furthermore, if the cooling rate after recrystallization annealing is too slow (for example, 10°C/sec or less), the degree of supersaturation of carbon in the ferrite after cooling will become small, weakening the effect of the subsequent overaging treatment, and vice versa. If the temperature is too high, the degree of supersaturation of carbon will increase, but the quench aging described below will be more likely to proceed during the temperature raising process of the subsequent over-aging treatment, and the degree of supersaturation during the over-aging treatment at around 400°C will become smaller, causing the overall From a general perspective, the effect of overaging treatment becomes weaker.

As a result of experiments, it was found that the most desirable setting is 30 to 10 degrees, which is a little faster than the usual rate of about 20 T/sec.

Next, an over-aging treatment is performed, but since the amount of supersaturated solid solute carbon is increased by high-temperature winding and rapid cooling after recrystallization annealing, the solute carbon becomes a compound (F e) The phenomenon of fine precipitation as 3C) tends to occur.

If the temperature increase rate for overaging treatment is too slow, quench aging will progress and the degree of carbon supersaturation will decrease, making it impossible to obtain a sufficient overaging treatment effect. A temperature increase rate of sec or more is required.

Note that this temperature increase rate may not be achieved by heating with a conventional heating furnace, and in such cases, high frequency heating may be used in combination.

desirable.

Heating at temperatures below 350°C takes a long time to obtain a sufficient effect, and heating at temperatures above 450°C increases the amount of equilibrium solid solution carbon, making it impossible to obtain a sufficient effect.

Next, embodiments of the invention will be described.

After heating a 10 ton slab of low carbon steel containing 0.05% carbon to 1250°C and rolling it to a thickness of 3.2 mm,
It was wound into a coil at temperatures of 550°C and 700°C and cooled to room temperature.

This hot-rolled coil was pickled and then cold-rolled at a rolling reduction of 75% to form a thin steel strip with a thickness of 0.8 mm, which was then continuously annealed under the following conditions.

In examining the beat cycle, we conducted detailed tests on the influence of the cooling rate after recrystallization annealing and the temperature increase rate during overaging treatment.

Temperature increase rate: 10°C 7°C Recrystallization annealing temperature = 700°C Soaking time: 1 minute Cooling rate ('C/5ec): 10, 20, 30
, 50.

70 100 500 After performing the recrystallization annealing described above, overaging treatment was performed under the following conditions.

Temperature increase rate ('C/5ec): 10, 15, 50
Heating depth: 400° C. Soaking time: 1 minute Cooling method: Cooling Each steel strip that had undergone the above continuous annealing was finished by temper rolling at a rolling reduction of 1%.

Coil winding temperature 550 at which the above continuous annealing was performed
Table 1 shows the results of yield stress (YP) and elongation (El) tests immediately after temper rolling and after being left at room temperature for one month.

From the above test results, looking at the relationship between the cooling rate after recrystallization annealing and the temperature increase rate in overaging treatment, it is found that when the temperature increase rate in overaging treatment is 10°G/sec and heat removal is performed, recrystallization The faster the cooling rate after annealing, the higher the yield stress, the lower the elongation, and the worse the mechanical properties.

However, the cooling rate after recrystallization annealing is 30°C//
Products that are rapidly heated at SeC or higher, 70°C/sec or lower, and a temperature increase rate of 15°G/sec or higher during overaging treatment have good properties immediately after temper rolling, and also have good properties after aging. .

However, a cooling rate of 100° C./sec or more is not preferable because the properties immediately after temper rolling are poor.

Further, those having a cooling rate of 20° C./sec or less are not preferable because the properties after aging are poor.

Next, Table 2 shows the results of testing the yield stress (YP) and elongation (El) after temper rolling at a coil winding temperature of 700° C. and after being left at room temperature for one month.

Looking at the above test results, it can be seen that the same tendency as in the case of a coil winding temperature of 500° C. is shown, but the mechanical properties are further improved.

From the above test results, the cooling rate after recrystallization annealing is 30~b℃/5eC, which is faster than the conventional method (about 20℃/sec).
) It can be seen that a soft steel sheet with good aging resistance can be obtained at a faster speed of 15° C./sec or more.

Claims (1)

[Claims] 1. After hot rolling and winding into a coil at 550°C or higher,
A steel strip that has been cold rolled at a reduction ratio of 60 or more is reheated and held at 650 to 750°C for 30 seconds or more to perform recrystallization annealing, and then cooled to 50°C or less at a cooling rate of 30 to 750°C. After cooling, immediately
Heating at a temperature increase rate of 5°C/sec or more, 350 to 450
A continuous annealing method for low carbon steel, characterized by carrying out overaging treatment by holding the temperature at ℃ for 30 seconds or more.