JPH06145809A - Manufacture of cold rolled steel sheet having excellent aging resistance and workability - Google Patents

Abstract

PURPOSE:To manufacture a cold rolled steel sheet having excellent aging resistance and workability by slowly cooling to the specific temp. range after executing the soaking treatment to the low carbon cold rolled steel sheet, rapidly cooling and immediately executing over-aging treatment. CONSTITUTION:A steel slab containing, by wt%, 0.008-0.05% C, <=0.20% Si, 7XS-0.5% Mn, <=0.10% P, <=0.020% S, 0.025-0.120% Al, <=0.005% N, <=0.0050% O or further, <=0.0080% B is hot-rolled to make the hot rolled steel sheet and this steel sheet is coiled at >=650 deg.C. Then, pickling is executed to this sheet, and after removing the surface scale, the cold rolling is executed to make the finish sheet thickness, and after executing soaking treatment to this sheet by holding at the recrystallizing temp. to AC3 point temp. for 30sec, the sheet is slowly cooled to the temp. range of 760-530 deg.C by <=20 deg.C/sec cooling rate. Successively, this sheet is rapidly cooled by >=70 deg.C/sec cooling rate, and in the case of being >=630 deg.C of the starting temp. in the rapid cooling, the finish temp. of the rapid cooling is made to be <=280 deg.C, and in the case of being <=630 deg.C of the starting temp. in the rapid cooling, the finish temp. of the rapid cooling is made to be the starting temp. of the rapid cooling -350 deg.C, and thereafter, after immediately reheating to 360-430 deg.C, the over-aging treatment is executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low carbon cold-rolled steel sheet having good aging resistance and workability, and in particular, it is possible to realize such characteristics by continuous annealing.

[0002]

2. Description of the Related Art A method for producing a low carbon cold-rolled steel sheet having excellent workability and aging resistance by continuous annealing has been proposed from an early stage, for example, JP-A-47-26313. Japanese Patent Publication No. 51-5335 and Japanese Patent Publication No. 51-5335. These are a method of rapidly heating a low carbon steel, then cooling it to a temperature close to room temperature at a very fast rate such as water quenching, and immediately reheating it to subject it to overaging treatment to precipitate carbides, or directly. This is a method of performing cooling to the overaging treatment temperature by a method such as a gas jet, which has a relatively slow cooling rate, but is easy to control the temperature of the cooling end point, and performs the overaging treatment.

However, the former method has an advantage that the overaging time can be shortened because the cooling rate is high, but if C is high, the ductility is deteriorated by a large amount of carbides precipitated in the grains. There is a problem on the production side, and in the case of water quenching, there is a production problem that it is difficult to pass the plate due to the shape of the steel plate during cooling.

On the other hand, in the latter method, since the cooling rate is slow, the carbides are mainly precipitated at the grain boundaries, which is advantageous for ductility, but from the viewpoint of aging resistance, it takes a long time to sufficiently precipitate the carbides. It has the drawback of requiring overaging treatment.

[0005]

Further, the quenching end temperature is regulated within a certain range and reheated, and then overaged to produce a low carbon cold-rolled steel sheet for working having excellent aging resistance by continuous annealing. The method is, for example, JP-B-2-5803, JP-A-60-165321, and JP-A-2-93023.
It is proposed in the Gazette and the like.

[0006] However, in these methods, after quenching, it is basically necessary to hold at the quenching end temperature for a time of about 5 to 30 seconds for nucleation of carbides, and cooling that allows strict quenching end temperature control. It is disadvantageous in terms of equipment costs, such as equipment and a furnace that can hold steel plates at a constant temperature for a certain period of time.

The present invention solves the above-mentioned problems of the prior art, and when manufacturing a low carbon cold-rolled steel sheet by continuous annealing, a low carbon excellent in aging resistance and workability even with a short time overaging treatment. It is an object of the present invention to provide a method for easily manufacturing a cold rolled steel sheet.

[0008]

In order to solve the above-mentioned problems, the present inventors have proposed rapid cooling to near room temperature and reheating as in the former method described above, which requires a short overaging treatment time. Based on the heat cycle of the post-aging treatment, the heat cycle that does not impair the aging resistance and the deterioration of the steel sheet shape due to the high cooling rate, especially the rapid cooling condition was examined.As a result, the rapid cooling end temperature was determined by the rapid cooling start temperature. By controlling the temperature below the specified temperature, the cooling rate as high as water quenching (≥10
It was found that substantially sufficient aging resistance can be obtained without employing (00 ° C / sec). According to this method, the quenching end temperature may be equal to or lower than the temperature determined by the quenching start temperature, and after the quenching, it may be reheated to the overaging treatment start temperature without holding the constant temperature immediately. Does not occur. The present invention has been completed based on such knowledge.

That is, according to the present invention, 0.008% ≦ C ≦
0.05%, Si ≦ 0.20%, 7 × S ≦ Mn ≦ 0.5%,
P ≦ 0.10%, S ≦ 0.020%, 0.025% ≦ Al ≦
A steel containing 0.120%, N ≦ 0.005%, O ≦ 0.0050% and, if necessary, B ≦ 0.0080%, the balance being iron and inevitable impurities, is heated by a conventional method. After hot rolling, coiling at 650 ° C or higher, pickling, cold rolling, soaking for 30 seconds or more at a recrystallization temperature or higher and an Ac ₃ transformation point or lower, and subsequently to a temperature range of 760 to 530 ° C. After slow cooling at a cooling rate of 20 ° C / sec or less, 70
When quenching at a cooling rate of ℃ / sec or more, when the quenching start temperature is 630 ° C or more: quenching end temperature 28
0 ° C or less, when the quenching start temperature is 630 ° C or less: quenching end temperature ≤ quenching start temperature -350 ° C, quenching to the end temperature determined by
Immediately after reheating to 360-430 ° C, total overaging time 90 sec or more, and average cooling rate 0.5-2.0 ° C / sec
The gist is a method for producing a cold-rolled steel sheet having excellent aging resistance and workability, which is characterized by performing an overaging treatment of cooling to 200 to 280 ° C.

The present invention will be described in more detail below.

[0011]

[Action]

First, the reasons for limiting the chemical composition of steel in the present invention will be explained.

C: The smaller the content of C, the better for improving the ductility, and the upper limit is 0.05%. On the other hand, if it is less than 0.008%, a sufficient degree of supersaturation cannot be obtained after quenching in continuous annealing, and the precipitation of carbides is small in the subsequent overaging treatment, so that the aging resistance cannot be improved. To do.

Si: If Si is added in excess of 0.20%, the number of surface defects due to scale increases, so this is the upper limit.

Mn: Mn forms MnS in steel and is added to prevent hot brittleness due to S. For that purpose, it is necessary to add at least 7 times the amount of S (7 x
S ≦ Mn). On the other hand, excessive addition of Mn in excess of 0.5% is disadvantageous for workability due to refinement of crystal grains and deterioration of recrystallized texture, so this is the upper limit.

P: P has a high strengthening ability, and can be added as an effective strengthening element when strengthening steel. But,
If the addition exceeds 0.10%, the weldability deteriorates, so
This is the upper limit.

S: S is fixed in steel by Mn as a precipitate and rendered harmless, but an increase in the amount of Mn to be added not only increases cost but also lowers workability. It must be 0.020% or less.

Al: Al is necessary not only for deoxidation but also for precipitation-fixing N harmful to aging as AlN at the stage of hot rolling. To do that, at least 0.025%
Is required. On the other hand, addition of more than 0.120% causes deterioration of workability due to an increase in inclusions in the steel, so this is the upper limit.

N: N deteriorates aging resistance, suppresses grain growth of crystals, and lowers ductility, so the smaller the better, the better, and is 0.005% or less.

O: If O (oxygen) is present in the steel in an amount of 0.0050% or more, the amount of inclusions increases and the ductility decreases, so
It should be 0.0050% or less.

B: In the present invention, 0.0080% or less of B can be added if necessary. B has a function of fixing N as a precipitate and detoxifying N similarly to Al.

Next, the reasons for limiting the manufacturing conditions in the present invention will be described.

The steel slab having the above chemical composition is hot-rolled after casting. Basically, hot-rolling may be carried out according to a conventional method. The steel slab may be once cooled to room temperature and then heated in a heating furnace, or may be charged into the heating furnace without being cooled to room temperature. Furthermore, hot rolling may be performed directly after casting without heating in a heating furnace. When heating, the heating temperature is 1000-
A normal heating temperature of 1300 ° C. may be used, but 1000 to 11
Heating at a low temperature of about 00 ° C is advantageous for deep drawability after annealing. If the hot rolling finishing temperature is lower than the Ar ₃ transformation point, a texture, which is disadvantageous to the deep drawability after annealing, develops. Therefore, it is desirable to finish at the Ar ₃ transformation point or higher.

However, it is preferable for the deep drawability after annealing to fix N in the steel as AlN at the stage of hot rolling at a coiling temperature of 650 ° C. or higher. However, if it is too high, the pickling property is deteriorated, so it is desirable to wind at 780 ° C or lower.

After the hot rolling, pickling, cold rolling and annealing are carried out in a conventional manner.

From the viewpoint of workability, it is better to anneal at a temperature higher than the recrystallization temperature. However, heating to a temperature above the Ac ₃ transformation point adversely deteriorates workability, which is not preferable. The soaking time needs to be 30 seconds or more. 30s
If it is less than ec, sufficient grain growth cannot be achieved and workability is deteriorated. It should be noted that it is desirable to set the upper limit to about 200 seconds because the equipment will only become huge even if soaking is carried out for too long.

The cooling conditions after soaking are important in the present invention. First, it is necessary to quench C to a sufficiently supersaturated state of C and then to carry out overaging treatment to efficiently precipitate carbides.
To do so, set the quenching start temperature to 760 to 530 ° C,
C is dissolved in ferritic iron as much as possible. Fe-
In the C equilibrium diagram, at the Ac ₁ point, C is at most 0 in the ferritic iron.
Although it can form a solid solution up to 02%, in an actual thermal cycle such as continuous annealing, the solid solution amount of C in the ferrite iron becomes maximum at a temperature lower than Ac ₁ point because the cooling from the higher soaking temperature is faster. Become. Above 760 ° C or below 530 ° C, the solid solution amount of C in ferritic iron is small and C does not become sufficiently supersaturated after quenching. Therefore, carbide precipitation does not occur sufficiently even during long-term overaging treatment, and aging resistance Sex deteriorates.

Further, it is preferable that the cooling from the soaking temperature to the quenching start temperature is performed by slow cooling at a cooling rate of 20 ° C./sec or less. At a cooling rate of 20 ° C./sec or more, the supersaturation degree of C is insufficient when the quenching start temperature is reached, so that the aging resistance after overaging treatment deteriorates. If the cooling rate is too slow, the line length becomes long, which is disadvantageous in terms of equipment cost. Therefore, it is desirable to set it to about 5 ° C./sec or more.

The material is rapidly cooled after gradual cooling. The higher the cooling rate, the higher the degree of supersaturation of C and the shorter the overaging treatment time. Therefore, the present invention requires a cooling rate of at least 70 ° C./sec or more. If the quenching rate is slow, the quenching end temperature can be controlled, but in the present invention, it is only necessary to cool to a predetermined temperature or lower, and since temperature control in a strict range is not required, rather than overaging treatment time High-speed cooling is used to shorten the time. The upper limit of the cooling rate is not specified,
A high cooling rate (≧ 1000 ° C./sec) as high as water quenching is not necessarily required to improve the ductility, and 70 ° C./sec or more can sufficiently exert the effect.

The quenching end temperature is particularly important. In the present invention, when the quenching start temperature is 630 ° C. or higher, the quenching end temperature is 280 ° C. or lower, and the quenching start temperature is 63.
When the temperature is 0 ° C or less, the quenching end temperature is the quenching start temperature -350 ° C.
Below. This is based on the following experimental results.

The present inventors have found that Mn: 0.15%, P: 0.
010%, S: 0.005% as a basic component, using a cold-rolled steel sheet in which the amount of C was changed in the range of 0.003 to 0.052%, a heat treatment equivalent to continuous annealing was carried out. Rapid cooling was performed by changing the starting temperature and the ending temperature, and the aging index (AI) after the overaging treatment was measured. The heat treatment conditions corresponding to the continuous annealing are soaking condition: 820 ° C. × 45 sec, cooling rate up to the quenching start temperature: 10 ° C./sec, quenching rate: 100 ° C./sec,
Overaging condition: 200 seconds from 400 ℃ to 220 ℃
Cooled in.

An example of the result is shown in FIG. Aging index
(AI) is stable when the quenching start temperature is 630 ° C. or higher and the quenching end temperature is 280 ° C. or less, AI ≦ 3.0 kgf /
mm ² has been achieved. On the other hand, when the quenching start temperature is 630 ° C. or less, AI ≦ 3.0 kgf / mm ² cannot be achieved unless the quenching end temperature is also lowered in accordance with the decrease in the quenching start temperature. The relationship between the quenching start temperature and the quenching end temperature (within the range of the solid line in the figure) is (quenching end temperature) ≤ (quenching start temperature-
350 ° C). However, when the amount of C is too low (eg, C: 0.007% in the upper part of the figure), the degree of supersaturation of C before the overaging treatment is not sufficient, and therefore, no matter what the rapid cooling condition is, AI It is impossible to achieve ≤3.0 kgf / mm ² .

Immediately after the rapid cooling, it is reheated to the overaging treatment start temperature. Since carbides have already started to precipitate in the reheating process, there is no need to keep the temperature constant after the rapid cooling, and the rapid cooling in the cooling zone may be followed by immediate heating in the reheating zone. Reheating,
Any method such as direct heating, indirect heating or induction heating may be used, and the heating rate may be 2 to 100 ° C./sec.

The overaging treatment starting temperature is 360 to 430 ° C. In this temperature range, the reduction of solute C and the growth of carbide occur most efficiently. However, if the temperature is lower than 360 ° C., the precipitation of carbide is slow, and the overaging treatment time becomes long. On the other hand, 43
When the temperature exceeds 0 ° C, the amount of equilibrium solid solution C at that temperature is high, so the precipitation of carbides at the initial stage of overaging is not sufficient, and the temperature range of 360 to 430 ° C during cooling is maintained in the subsequent overaging process. Although some carbide precipitation occurs during passage, it is not efficient. Moreover, it is not economical considering the heat energy required for heating.

The overaging end temperature is lower than the overaging treatment start temperature and is specified to 200 to 280 ° C. Since the amount of solid solution C after the overaging treatment approaches the equilibrium solid solution C amount at the overaging end temperature, it is necessary to set it to 280 ° C. or lower in order to achieve AI ≦ 3 kgf / mm ² . On the other hand, since C hardly diffuses in the temperature range of less than 200 ° C., this is the lower limit.

The overaging treatment time is required to be at least 90 seconds or more, and if it is shorter than this, AI ≦ 3 kgf / mm ² cannot be obtained. Further, although the end temperature of the overaging treatment is lower than the start temperature, it is cooled monotonously without heating during this period, and its cooling rate is specified to be 0.5 to 2.0 ° C./sec. The cooling rate here means an average cooling rate obtained by simply dividing the temperature difference between the overaging treatment start temperature and the overaging treatment time by the overaging treatment time. If the cooling rate is less than 0.5 ° C / sec, the overaging treatment time is too long, causing a problem in terms of equipment. On the other hand, if it exceeds 2.0 ° C./sec, the cooling is too fast, the amount of solid solution C decreases and the accompanying growth of carbide is insufficient, and as a result, a steel sheet having excellent aging resistance cannot be obtained.

After annealing, temper rolling is usually carried out.
The rolling reduction may be about 0.6 to 2.0% according to a conventional method.

Next, examples of the present invention will be described.

[0039]

【Example】

Steel having the chemical composition shown in Table 1 was melted into a steel slab. This slab is hot rolled to a thickness of 3.6 mm at a slab heating temperature of 1100 ° C. and a finishing temperature of 880 to 920 ° C.,
The winding was performed under the conditions shown in Table 2, respectively. Then
After pickling, cold rolling was performed to a thickness of 0.8 mm (cold rolling rate 78%), and continuous annealing was performed under the conditions shown in Table 2. After further temper rolling with an elongation of 1.0%, the mechanical properties were investigated. The results are shown in Table 3.

As is clear from Table 3, the invention examples (No.
1, No. 3, No. 6, No. 9, No. 10) are low YP,
It has excellent mechanical properties such as high El and high r value, and the AI value, which is an index of aging resistance, is 3 kgf / mm ² or less, which is a good value that can be said to be substantially non-aging.

On the other hand, in Comparative Examples No. 2 and No. 14, either the winding temperature after hot rolling or the chemical composition is outside the scope of the present invention, so that the AI value is 3 kgf / mm ² or less. However, the YP is high, the El and r values are low, and the workability is poor. Further, Comparative Examples No. 4 to No. 5, No. 7 to No. 8, N
No. 11 to No. 13 and No. 15 to No. 16 have an AI value of more than 3 kgf / mm ² because either the thermal cycle of continuous annealing or the chemical composition is out of the range of the present invention, and the aging resistance is high. The sex is inferior to the inventive examples.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

[0046]

As described in detail above, according to the present invention,
A low carbon cold-rolled steel sheet excellent in aging resistance and workability can be obtained even by a short-time overaging treatment, and since it is not necessary to maintain a constant temperature immediately after quenching, it can be easily manufactured without a problem in terms of equipment cost.

[Brief description of drawings]

FIG. 1 shows the quenching start temperature and quenching temperature and C on the AI value.
It is the figure which showed the relationship of quantity.

Claims (2)

[Claims] 1. In% by weight (hereinafter the same), 0.008% ≦
C ≦ 0.05%, Si ≦ 0.20%, 7 × S ≦ Mn ≦ 0.5
%, P ≦ 0.10%, S ≦ 0.020%, 0.025% ≦
Al ≦ 0.120%, N ≦ 0.005%, O ≦ 0.0050
%, With the balance being iron and unavoidable impurities, hot-rolled by a conventional method, then wound at a temperature of 650 ° C. or higher, pickled and cold-rolled, and then recrystallized temperature or higher and Ac ₃ transformation point or lower. Soaking for 30 seconds or more, and then 760-
When gradually cooling to a temperature range of 530 ° C at a cooling rate of 20 ° C / sec or less and then rapidly cooling at a cooling rate of 70 ° C / sec or more, when the rapid cooling start temperature is 630 ° C or higher: the rapid cooling end temperature 28
0 ° C or less, when the quenching start temperature is 630 ° C or less: quenching end temperature ≤ quenching start temperature -350 ° C, quenching to the end temperature determined by
Immediately after reheating to 360-430 ° C, total overaging time 90 sec or more, and average cooling rate 0.5-2.0 ° C / sec
Then, a method for producing a cold-rolled steel sheet excellent in aging resistance and workability, which comprises performing an overaging treatment of cooling to 200 to 280 ° C. 2. The method according to claim 1, further comprising B ≦ 0.0080%.
The method described in.