JPS5839890B2 - Method for producing cold-rolled steel sheets for drawing with excellent aging resistance through continuous annealing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a cold-rolled steel sheet for drawing that has excellent aging resistance by continuous annealing, and particularly relates to a method for manufacturing a cold-rolled steel sheet for drawing that can impart aging resistance through short-time annealing. .

Recently, a method for manufacturing cold-rolled steel sheets for drawing, which is an excellent processing method using continuous annealing, is being developed.

For the manufacturing method of cold rolled steel sheets that emphasizes the processing method,
Since the disclosure in No. 116, it has become a well-known fact that a method of reducing the amount of solid solute carbon and improving the workability of a product by performing a rapid cooling overaging treatment after annealing is effective.

However, in actual application, it is not always possible to obtain a satisfactory material based on this known fact alone.

Many of the proposals that followed examined the thermal cycle, etc. in rapid cooling and overaging treatment, and specified effective conditions for improving product material quality, but the basis for all of them can be found in Japanese Patent Publication No. 33-2116. It is no exaggeration to say so.

By the way, the cold-rolled steel sheets manufactured by the continuous annealing method that have been disclosed up to now have properties that can be compared with batch annealed materials in terms of processing methods, but one of the important characteristics of cold-rolled steel sheets for drawing is However, the aging resistance was significantly inferior.

As a result of many experiments, in order to improve the aging properties, it is necessary to either make the quenching rate after annealing very fast like water cooling, or to maintain the overaging temperature for a very long time if the cooling rate is slow. It turned out to be necessary.

In the former case, the steel plate is cooled very rapidly, making it difficult to improve the surface quality, and in the latter case, a long overaging time is required, which negates the advantages of continuous annealing. It has the following disadvantages.

The present invention overcomes the drawbacks of the conventional methods for producing cold-rolled steel sheets for drawing with excellent aging resistance, and particularly provides a new annealing thermal cycle when producing by continuous annealing process, thereby processing the continuous annealing process. An object of the present invention is to provide an effective manufacturing method that can shorten time.

The gist of the present invention is as follows.

That is, a step of winding the steel strip into a coil at a temperature of 650° C. or higher after hot rolling, a step of recrystallizing and annealing the wound steel strip and then rapidly cooling it, and a step of overaging the rapidly cooled steel strip. A method for producing a cold-rolled steel sheet for drawing by continuous annealing, comprising: a step of rapidly cooling to a temperature of 300°C or less after the recrystallization annealing;
℃ or less, and an overaging treatment step of heating and then cooling, and during the overaging treatment, the holding time t from 3500C to the time when cooling starts after reaching the maximum heating temperature T is the maximum heating temperature. Temperature (T) Holding time t If less than 400°C 10 to 60 sec 400°C to 50
If the temperature is less than 0℃: 60 seconds or less 500'C to 600℃
In the following cases: When controlling to 203 ec or less and cooling from the maximum heating temperature T, if the maximum heating temperature T is 450 to 600°C, the temperature range of 450°C or more is 5°C/see or more 35
The humidity range from 0℃ to less than 450℃ is 10℃/see or less to 350'C, and the humidity range less than 2℃/see is 350 to less than 450℃, the temperature range is 350'C or more. Aging resistance achieved by continuous annealing characterized by controlling the cooling rate to 2°C/see or less in the temperature range of 10°C/see or less and less than 350°C, and forcing cooling in all cases from a temperature of 250°C or less. This is an excellent method for producing cold-rolled steel sheets for drawing.

In the conventional method, for example, Japanese Patent Publication No. 50-113411 states that the cooling rate after overaging is "3°C/sec~
Cool to room temperature at a cooling rate of 20°C/see.

” method, and in Japanese Patent Application Laid-open No. 51-32418, “
It goes without saying that the slower the cooling rate after overaging, the better, but considering the furnace length and annealing efficiency, it is set at 40°C/sec.
Forced cooling of about e is desirable.

'', it is characterized by a fairly fast cooling rate from the overaging temperature to room temperature.

However, this is a decisive drawback in achieving excellent aging resistance.

A simple example is when a batch annealed material of ordinary aluminum killed steel is heated and held at the normal overaging temperature of 350°C and then water cooled, the aging index AI, which is an important index of aging resistance, increases from 3.5 kg/mi to 3.5 kg/mi. It rises to about mu2.

Therefore, it is clear that in order to improve the aging resistance, it is necessary to slowly cool the material in a humidity range of 350°C or less.

However, in the conventional method, sufficient consideration was not given to how slow the cooling rate after overaging should be, as well as to the efficient thermal cycle of continuous annealing.

For example, in the above-mentioned Japanese Patent Publication No. 50-113411, the lower limit of the cooling rate is set at 3°C/see.
If it is less than C/sec, the cooling time will be too long and equipment costs will increase.
"This would be unrealistic in terms of production efficiency."

According to experiments conducted by the present inventors, satisfactory aging resistance cannot be obtained at a cooling rate faster than 3°C/sec.
It has been found that very excellent aging resistance can be obtained at a cooling rate of 2°C/see or less.

Furthermore, we discovered that it is necessary to regulate other thermal cycle conditions in order to shorten the annealing cycle time, and that there are materials that are suitable for this purpose.

The present invention will be explained in detail below.

First, we will discuss changes in the steel sheet structure due to rapid cooling after annealing and overaging treatment during continuous annealing.

As mentioned above, the pattern schematically shown in FIG. 1 can be drawn as a common cycle in the conventional continuous annealing process which aims at excellent workability.

Such thermal history consists of the following four stages.

That is, First stage: Room temperature T.

Time course from t to reaching the rapid cooling start temperature T2Vc via the maximum heating T1.

→t1→t2.

Here, the copper strip is heated above the recrystallization temperature and held until it reaches the quenching start temperature, or during the slow cooling process (t1 → t2
), the cold-rolled structure changes to a recrystallized structure, and a solid solution state with a high concentration of carbon is obtained due to the dissolution of carbides.

Second stage: Next, there is a stage t2→t3 in which the temperature is rapidly cooled from the above-mentioned quenching start temperature T2 to quenching T3.

The purpose of this is to freeze a large amount of solid solute carbon in the steel, thereby forming a large number of carbide precipitation nuclei.

This greatly affects the precipitation rate and amount of carbon in the next step.

Third stage: The steel strip is then heated again from the above-mentioned quenching end temperature T3,
After reaching the set temperature [T4, start secondary cooling T]
During time t3→t4→t5 leading to , the solid solution carbon is subjected to precipitation treatment.

However, at this stage, the amount of solid solute carbon in the steel has not been reduced sufficiently.

4th stage: After this, the solid solution carbon in the steel plate approaches the equilibrium carbon solid limit corresponding to the steel plate temperature at time t5 → t6 → t7 from the start of secondary cooling to the humidity T, passing through the intermediate temperature T6 and reaching the room temperature T7. decreases.

This reaction progresses during the period t5→t6 while the steel plate temperature is high, but this reaction substantially stops during the period t6→t7 because the temperature is low.

Therefore, it can be said that the time period from t to t6 in the fourth stage determines the aging resistance, which is determined by the amount of solid solute carbon in the final product.

The feature of the heat treatment technology according to the present invention is that in order to most efficiently reduce the amount of solid solute carbon, the time period from t2 to t7 is divided and an appropriate time allocation is determined.In particular, the time period from t5 to t6 is cooled according to the steel plate temperature. This provides a method for controlling speed and a method for limiting other time periods to a short period of time.

The steel type to which the present invention can be effectively applied may be any low carbon steel having a composition within a commonly used range.

However, regarding the hot rolling coiling temperature, it is necessary to coil at a high temperature of 650° C. or higher.

That is, in steel wound under such conditions, cementite is coarsely and sparsely distributed, and a high concentration of solid solute carbon can be frozen by rapid cooling treatment after recrystallization annealing.

The present inventor conducted many experiments, among which a representative example will be described below.

The chemical composition of the steel used in the experiment was ordinary low-carbon aluminum killed steel that was hot-rolled and then coiled at a high temperature of 650°C or higher.

First, FIG. 2 shows the results of an experiment conducted to examine the effects of changes in the quenching end humidity T3 in FIG. 1 on the yield stress, elongation, and aging index AI.

In this case, the cooling rate between temperature T2 and T3 is 50°C/sec.
All conditions other than the quenching end temperature were kept constant at σ), which is within the range of the present invention.

As is clear from FIG.

When the humidity at the end of quenching is higher than 300°C, the aging index increases rapidly.

This trend did not change even if the quenching rate was changed.

Therefore, it was found that the quenching end temperature should be set at 300° C. or lower.

FIG. 3 shows the results of an experiment conducted at the overaging temperature T4.

From the same figure, it can be seen that in order to increase the elongation and decrease the yield stress and aging index, the overaging humidity should be set in the temperature range of 350 to 600°C.

The overaging time needs to be longer if the overaging temperature is low, and shorter if the overaging temperature is high.

The reason for this is shown by the following experimental results.

Some examples are shown in FIG. 4, but when the overaging temperature, that is, the maximum heating temperature during overaging is as low as 350°C, the aging index becomes high when the holding time is less than 10 seconds.

However, as is clear from the figure, if the holding time is longer than about 60 seconds, the aging index will not decrease even if the holding time is increased, so in order to shorten the processing time, it should be set to 10 seconds or more and 60 seconds or less.

Further, when the overaging temperature is 450°C, a sufficiently low yield stress and aging index can be obtained even if the overaging time is 0 seconds, but no change occurs when the overaging time is 60 seconds or more, so no longer holding is necessary.

Furthermore, when the overaging humidity is set at 600'C, the aging index sharply increases for 20 seconds or more.

This is due to the re-solid solution of carbon.

When the experimental results obtained in this manner are summarized, the appropriate range of overaging time and overaging temperature is as shown in FIG.

Then a cooling control step t leading from temperature T5 to T6.

→We will describe an experiment conducted to examine t6.

The relationship between the cooling rate and the characteristic values when the overaging temperature T4-T5 is 500℃ and the holding time is 10 seconds is shown in the sixth section.
As shown in Figure 7.

FIG. 6 shows the influence of the cooling rate (1) between 450 and 350°C when the cooling rate between 500 and 450°C and below 350'C is constant.

From the same figure, it can be seen that when ■1 is 100c/see or less, the aging index is stable and low.

FIG. 7 shows the influence of cooling rate (2) below 350°C when the cooling rate between 500 and 350°C is kept constant within the range of the present invention.

It can be seen from the figure that a low aging index can be obtained in the range of 2°C/sec or less.

Further, FIG. 8 shows the influence of the slow cooling end humidity T8.

In the temperature range from T5 to T6, the cooling rate was controlled as described above, and from temperature T6 to T7, forced cooling was performed at 30'c/See.

As is clear from FIG. 8, the lower the slow cooling end temperature T6 is, the better, but it can be seen that sufficiently excellent aging index, yield stress, and elongation can be obtained if it is set to 250°C or less.

As described above, the necessary conditions for obtaining a drawing steel sheet with excellent aging resistance in the shortest possible thermal cycle, which is the objective of the present invention, are as follows: 1) After recrystallization annealing, quench cooling to a temperature of 3000C or less, and then Heating to a humidity range of 0350-600°C,
If the maximum heating temperature during that period is T, then the time t required to exceed 350° C. and reach the temperature at which cooling begins is controlled as follows.

First, when T is less than 400℃, T
is 400℃ or higher and lower than 500℃ for 60 seconds or less, T is 5
When the temperature is above 00℃ and below 600℃, limit it to 20 seconds or less,
For continued cooling, the maximum heating humidity T is 450~
In the case of 600°C, the humidity range below 600°C and above 450°C is 5°C/see or more, below 4500C and above 350°C it is 1000/see or below, and below 350°C it is 2°C/sec.
The cooling rate is controlled to be as follows: (1) Forced cooling is performed in all cases from temperatures below 250°C.

Next, an example using the method of the present invention described above will be shown.

The applied thermal cycle is as shown in FIG.

Curve A is according to the present invention, and curves B, C, and D are continuous annealing thermal cycles according to the conventional method.

In these thermal cycles, each condition was set so that the total annealing cycle time was the same.

The difference between cycle curve A of the present invention and comparative cycle curves B, C, and D is that in the latter, the cooling rate from 450 to 350°C is slow, and as a result, the holding time below 350°C is significantly shortened.

Example 1 First, steel having the chemical composition shown in Table 1 was subjected to the heat cycle of the present invention shown in FIG. 9 at +1/A.

Steels A1 and A2 are low-order aluminum killed steels, and copper metals A1 and A4 are low-order rimmed steels.

Among these, steels A3 and 4 are steels of the present invention having a high hot-rolling winding temperature, and steels A2 and 4 are comparative steels having a low hot-rolling winding temperature.

The materials of the steel plates treated in this manner are shown in Table 2.

'As is clear from Table 2, even though it is a low-order aluminium-killed steel, in the comparison @ A2 where the hot rolling winding temperature is low, the present invention steel A
In addition to having a higher aging index AI than l, it is hard and has a low r value.

In particular, steel A1 has a low aging index AI and can be used as an excellent aging-resistant steel plate.

Rimmed steel A3,40 Aging index AI is aluminum killed steel A
I.

Although inferior to Copper Metal No. 2, Copper Metal No. 3 rolled at high temperature shows a lower aging index AI than rimmed steel cold-rolled steel sheet manufactured by batch annealing, and it is clear that the aging resistance can be significantly improved by the method of the present invention. .

Example 2 Next, using steel A1 in Table 1, thermal cycles A, B, and
Table 3 shows the results of processing in C and D.

This is to show the difference in quality level obtained by the method of the present invention and the conventional method at the same heat treatment time, but as is clear from Table 3, the product of the method of the present invention has a lower aging index AI and all other characteristics. It is superior to the comparative method in terms of value.

Example 3 Next, an example will be shown in which the overaging time can be significantly shortened by the method of the present invention.

The steels listed in Table 1 were annealed using the thermal cycle shown in FIG.

The cycle A/A of the method of the present invention is the same as in Example 1.

Cycle E is a comparative method. Cycle E performs the same recrystallization annealing and rapid cooling treatment as in A, but the subsequent treatment is different as shown in the figure. It is rapidly cooled to room temperature at a cooling rate.

The aging index AI of the annealed plate is shown in Table 4 along with the overaging conditions.

(Note) *t is the time between PQ on the temperature one-hour curve in Figure 10.

Table 4 shows that depending on the comparative method, when the overaging humidity is 300°C and the holding time is 300 seconds, the aging index is still large;
By holding it for as long as 0 seconds, the aging index is finally reduced to the same level as the method of the present invention.

However, when the overaging temperature is 350° C. or higher, the aging index does not decrease much even if the holding time is considerably increased.

This is due to the fact that the equilibrium solidity C content above 350°C is large enough to cause aging, and it is not possible to improve aging resistance unless the cooling rate below 350°C is reduced. It shall be clearly stated.

As described above, according to the method of the present invention, a cold-rolled steel sheet for drawing having excellent aging resistance can be obtained without impairing the production efficiency of the continuous annealing method, so that the industrial benefits of the present invention are significant. be.

In the method of manufacturing a cold-rolled steel sheet for drawing with excellent aging resistance by continuous annealing according to the present invention, after hot-rolling, a copper strip wound into a carp and a coil is recrystallized at a temperature of 650°C or higher. After rapid cooling to a temperature of 300℃ or less, an overaging treatment is performed, but in this overaging treatment, the maximum heating humidity range is limited to 350℃ or higher and 600℃ or lower, and By limiting different holding times and different cooling rates depending on the temperature, and using a method of forced cooling for all humidity below 250°C, we were able to obtain the following effects.

(b) By subjecting steel sheets manufactured by continuous annealing to a long batch aging treatment at lower temperatures, the process of non-aging can be omitted, further reducing wasted time and making cooling time more effective. By utilizing this method, we were able to effectively obtain a cold-rolled steel sheet for drawing with extremely excellent aging resistance.

(b) It is possible to obtain the desired aging resistance without adding expensive elements such as Ti and Nb, which are added to conventional non-aging steel sheets, and without using ultra-low carbon steel. This made it possible to significantly reduce manufacturing costs.

[Brief explanation of drawings]

Figure 1 is a temperature one-hour curve showing the basic pattern of continuous annealing thermal cycles for processing steel sheets, Figure 2 is a graph showing changes in aging index, yield stress, and elongation depending on the humidity at the end of quenching, and Figure 3 is Figure 4 is a graph showing changes in aging index, yield stress, and elongation depending on overaging temperature. Figure 5 is a graph showing changes in aging index, yield stress, and elongation depending on overaging time. Figure 5 is a graph showing changes in aging index, yield stress, and elongation depending on overaging time. An explanatory diagram showing the relationship between overaging temperature. Figure 6 is a graph showing changes in aging index, yield stress, and elongation depending on the cooling rate in the cooling range of 350 to 450°C. Figure 7 is a graph showing the changes in aging index, yield stress, and elongation in the cooling range of 350 to 450°C. Graph showing changes in aging index, yield stress, and elongation depending on cooling rate,
Figure 8 is a graph showing changes in aging index, yield stress, and elongation depending on the slow cooling end temperature, Figure 9 is the humidity one-hour curve of the annealing thermal cycle in Examples 1 and 2, and Figure 10 is the annealing at Vc in Example 3. It is a temperature-hour curve of a thermal cycle.

Claims (1)

[Scope of Claims] 1. A step of winding the steel strip into a coil at a temperature of 650° C. or higher after hot rolling, a step of recrystallizing and annealing the wound steel strip and then rapidly cooling it, and passing the rapidly cooled steel strip through a A process of aging treatment;
A method for producing a cold rolled steel sheet for drawing by continuous annealing, which comprises a step of rapidly cooling to a temperature of 300°C or less after the recrystallization annealing, and a maximum heating temperature T of 350°C or more and 600°C or less after the rapid cooling treatment. and an over-aging treatment step of heating and then cooling, and during the over-aging treatment, a holding time t from 350°C to the time when cooling is started after reaching the maximum heating temperature T.
Maximum heating temperature T Holding time 1 If less than 400℃ 10-60 seconds 400-5
If the temperature is below 00°C, the heating time is 60 sec or less.If the temperature is less than 500-600°C, the heating time is 20 sec or less. The temperature range above is 5℃/see or more, the temperature range from 350℃ to less than 450℃ is below 10℃/see, the temperature range below 350℃ is 2℃/sec or less, and the maximum heating temperature T is 350 to less than 450℃. In each case, the temperature range above 350°C is controlled to a cooling rate of 1 o'c/see and the humidity range below 350°C is controlled to a cooling rate of 28C/sec or less, and in all cases, forced cooling is performed from the temperature below 250°C. A method for manufacturing a cold rolled steel sheet for drawing with excellent aging resistance through continuous annealing.