GB2050419A - A continuous annealing process for producing cold rolled steel strips - Google Patents

Abstract

A process for producing a cold rolled steel strip having excellent workability, utilises a continuous annealing step and comprises hot rolling a low carbon steel slab, followed by cold rolling, heating over a temperature range from 600 DEG C to the annealing temperature with a heating rate of not less than 40 DEG C/second, and then annealing at a temperature T in the range of from 700 DEG C to the A3 transformation temperature for a specific period of time determined from the carbon content and the annealing temperature. Thereafter, the strip is initially cooled slowly with a cooling rate of less than 50 DEG C/second and then rapidly from a temperature above 600 DEG C but not higher than [T - 0.027 (T - 680) ( 2ROOT t + 23.7)] DEG C with a cooling rate of greater than 50 DEG C/second, down to an overageing temperature. Overageing is performed at between 300 to 500 DEG C, for not less than 10 seconds.

Description

SPECIFICATION A continuous annealing process for producing cold rolled steel strips This invention relates to a process for producing cold rolled steel strips which process employs a continuous annealing step. Strip produced by the process of this invention can display excellent wo rkabil- ity properties.

Cold rolled steel strips are very often used in the manufacture of cold-formed articles, such as pressformed automobile parts, and the strips are thus required to have an excellent press-forming property.

In order to improve the general workability of steel strips, it is necessary to allow a full growth of grains in the steel and, on the other hand, to minimize the amount of solute carbon in the steel. Further, with respect to the deep-drawability of the steel strips, it is desirable that the average plastic-strain ratio r is large. The r value is related to the crystal orientation and the larger the amount of the(l 1 1) component, the larger is the r value.

Cold rolled steel strips are generally produced by a process which essentially comprises the steps of hot rolling, cold rolling and annealing. For satisfactory enlargement of the grain size and the r value, it is effective slowly to heat the steel strips to the annealing temperature and to hold the strips at that temperature for a longer period of time. To reduce the amount of solute carbon, it is effective to subject the steel strips to slow cooling after the annealing so as to precipitate substantially all of the carbon content at the grain boundaries.

Conventionally, a batch annealing process has widely been used for the production of cold rolled steel strips because the above annealing conditions can be easily achieved in a batch-type annealing furnace. Although the batch annealing process has been considered to be most suitable for obtaining steel strips displaying excellent workability, it has the critical disadvantage that the process takes a long period of time to perform, and hence considerably lowers the production efficiency.

Therefore, much attention has been paid to new arts such as continuous annealing processes, aiming at the production of cold rolled steel strips having excellent workability but only a short treatment time.

In recent years, some continuous annealing arts have been disclosed, for example, in Japanese Patent Publication Sho 42-11911, Japanese Laidopen Patent Applications Sho 50-72816, Sho 50-125918 and Sho 51-32418. However, these prior art processes suffer from the defects discussed below.

In conventional continuous annealing processes, the main consideration has been the achievement of a satisfactorily large grain size and it was considered that a longer holding time is therefore better in spite of the fact that the purpose of using continuous annealing is to shorten the treatment time. This is clearly illustrated by the above-mentioned prior art publications, in each of which only the lower limit of the annealing time is defined. Indeed, a longer annealing time promotes full growth of grains so that a large grain size can be obtained giving certain advantages. However, if the annealing time is excessively long, the carbides which have been precipitated in the hot rolled steel strip will be dissolved during the annealing process to increase the amount of solute carbon, thus deteriorating the workability ofthe cold rolled steel strip.According to the conventional continuous annealing arts, therefore, the cold rolled steel strip after annealing is subjected to an over-ageing treatment at about 400"C for a considerably long period of time so as again to precipitate the solute carbon as carbides.

In conclusion, conventional continuous annealing processes are susceptible to the following contradictory factors: (a) a longer annealing time produces a larger grain size, thus improving the workability of the resultant cold rolled steel strips (b) a shorter annealing time reduces the dissolution of carbides formed in the hot rolled steel strip, thus contributing to a shortening of a subsequent over-ageing treatment.

Nevertheless, conventionally only factor (a) has been taken into consideration, little or no account being taken of factor (b).

It is an object of the present invention to provide a process for the continuous annealing of steel strip which can result in an improvement in the deepdrawability (as represented by the r value in particular) of the resultant cold rolled steel strip.

According to this invention, there is provided a process for producing a cold rolled steel strip by employing a continuous annealing step, which process comprises: hot rolling a low carbon steel slab to form strip; cold rolling the hot rolled steel strip; heating the cold rolled steel strip to an annealing temperature with a heating rate of not less than 40"C/second over the temperature range of from 600 C to the annealing temperature; annealing the strip at a temperature Tin the range of from 700"C to the A3 transformation temperature for a period of time t ranging from [10-0.03 (T-680)] seconds to [50-0.15 (T-680)] seconds;; slowly cooling the thus-annealed strip initially at a cooling rate of less than SO"C/second and then rapidly cooling the strip from a temperature TQ yin above 600"C but not higherthan [T-0.027 (T-680) (Vt + 23.7)j0C, the rapid cooling being performed at a cooling rate of not less than SO"C/second down to an over-ageing temperature range; and subjecting the strip to an over-ageing treatment at a temperature ranging from 300 to 500"C for a period of not less than 10 seconds.

During the annealing, in order to prevent the dissolution of carbides which have precipitated in the hot rolled steel strip, it is desirable to heat the strip for annealing as rapidly as possible, and as the dis The drawing(s) originally filed was/were informal and the print here reproduced is taken from a later filed formal copy.

solution of carbides remarkably increases when the temperature is 600"C or higher, it is necessary rapidly to heat the strip with a heating rate of not less than 40"Clsecond over the temperature range of from 600"C to the annealing temperature. Below a heating rate of 40"C/second, the time that the strip is at temperatures above 600"C is lengthened so that the dissolution of carbides progresses, though on the other hand, strip displaying the desired properties can be obtained even when the strip is rapidly heated at a rate of not less than 40"C/second overthe temperature zone upto 600"C.

For obtaining grains which are large enough to improve the workability, it is necessary for the annealing temperature Tto be not lower than 700"C, but if the temperature T is higherthan the A3trnnsforma- tion temperature, the{111} component tends to decrease due to the transformation which will occur during the annealing. Therefore, the annealing temperature T must lie in the range from 700"C to the A3 transformation temperature.

Further, for promoting a full growth of the grains, the annealing time t (seconds) may be reduced the higher the annealing temperature T, and the critical range has been found to be [10-0.03 (Tr680J t C50-0.15 (T-680)1 If the annealing time t is shorter than 10-0.03 (T-680)j, no full grain growth can be expected, though on the other hand, if the annealing time t is excessively long, the dissolution of carbides becomes vigorous, as mentioned hereinbefore.

Because also a higher annealing temperature T can promote the dissolution of the carbides, the annealing time t should not exceed [50-0.15 (T-680)4. As discussed hereinbelow, for optimum improvements in the workability of the final product, the annealing time t should slightly be modified for carbon contents in the range of greater than 0.04% but not more than 0.08%. In any event, if the annealing temperature T gets above the A3 transformation temperature, the dissolution of carbides rapidly progresses. For this reason alone, the annealing temperature T should be below the A3 transformation temperature.

When annealing is performed under these conditions, it is possible to obtain satisfactorily large grains while minimising the carbide dissolution during the annealing process. It should be noted, however, that a very small amount of solute carbon can be present in the steel strip as hot rolled, and at least a small amount of dissolution of carbides is unavoidable during the annealing process. Therefore, consideration should be given to the precipitation of solute carbon originating in this way.

For precipitation of the solute carbon as carbides, it is desirable that a relatively slow cooling is effected during the initial stage of cooling following the annealing so as to secure the longest possible time in a higher temperature zone, because the diffusion of carbon constantly proceeds but proceeds more rapidly ata higher temperature. For this purpose, the initial cooling immediately after the annealing should not be at a rate greaterthan 50 C/sec- ond. A more rapid cooling rate does not provide enough time for full precipitation of the solute carbon.

On the other hand, it is not advantageous to con tinue such slow cooling right down to lowtempera- tures, because the overall treatment would require a longer time. Therefore it is recommended that the slow cooling is terminated after an appropriate period of time. The control of th is appropriate period for terminating the slow cooling is an important aspect of the present invention.

Supposing that the termination point of the slow cooling is TQ( C), the larger is the amount of carbides dissolved during the annealing, the greater should be the period of slow cooling, and hence the temperature difference T-TQ should be increased;; moreover, the amount of solute carbon produced during the annealing increases in proportion to Vt It has been established that an appropriate range for TQ is higher than 600"C but not higher than [T-0.027 (T-680) (vet+ 23.7)J C. At temperatures of 600"C or lower, the diffusion rate of carbon is significantly retarded, so that only a slight promotion in the precipitation of carbon could be achieved were TQ set at 600"C or lower. On the other hand, if T0 were set above [T-0.027 (T-680) (Vt+ 23.7)j0C, no efficient carbon precipitation could be effected at such high temperatures.

At this stage, the greater part of the solute carbon is converted into precipitates and thus the amount of remaining solute carbon decreases to a very small amount. It is important further to precipitate the remaining solute carbon to leave only a very small amount if a further improvement in the workability of the resultant product is to be obtained. However, at a temperature of 600"C or lower, the diffusion rate of carbon is retarded so that the carbide precipitation is considerably delayed.

Therefore, in the present invention, the degree of super-saturation of carbon is increased by rapid cooling from the temperature To so as to promote further carbide precipitation. Thus, the steel strip is rapidly cooled from the temperature TQ to the overageing temperature range, at a cooling rate of not less than 50 C/second.

It should be noted that when the steel strip is annealed and cooled to the temperature TQ under the conditions as defined hereinbefore, but slowly cooled from the temperature TQ at a cooling rate of less than SO"C/second, the degree of supersaturation of carbon cannot sufficiently be increased. On the other hand, when the steel strip is rapidly cooled to a temperature lower than the over-ageing temperature range, the degree of super-saturation of carbon becomes excessively high and the carbides are too finely and closely dispersed, so that precipitation hardening is caused.

This produces the disadvantage that reheating is required for over-ageing and this requires an additional energy input. When the carbide dissolution during the annealing process is inhibited, and the degree of carbon super-saturation is enhanced, it is possible markedly to shorten the time required by the over ageing treatment. Thus the overageing treatment can in some circumstances be as short as 10 sec onds, but an overageing time exceeding 2 minutes does not provide any additional effect, for carbon contents not greater than 0.04%.

In the process of the present invention, the overageing temperature is defined to be in the range from 300 to 500"C. Below 300"C, the diffusion rate of carbon is further retarded so that an overageing treatment for only about 10 seconds cannot produce any effect, and above 500"C, on the other hand, it is no longer possible to reduce the amount of solute carbon however much the overageing treatment may be lengthened, because the dissolution limit of carbon is so high.

The desired results of the present invention can be obtained evenif the steel strip is subjected to a surface treatment before, after or during the continuous annealing process, or even if the steel strip is subjected to temper rolling and slight plastic deformation for shape correction, after the continuous annealing process.

Preferable conditions for practising the present invention are set forth below.

(1) The present invention is preferably applied to steels containing 0.003 to 0.04% carbon. If the present invention is applied to steels containing less than 0.003% carbon only a slight improvement can be obtained due to the low level of carbon content, and when the present invention is applied to steels containing more than 0.04% carbon, the workability of the resultant product is hindered by the carbon content, despite the process of the invention. However, when the steel contains more than 0.04% carbon but not more than 0.08% carbon, a satisfactory improvement in the workability of the steel can be obtained if the steel is annealed for a period ranging from [30-0.03 (T-680)] to [90-0.15 (T-680)] second, and then over-aged for a period longer than 2 minutes.

(2) In orderto permit full grain growth in the hot rolled steel strip and to promote full precipitation of carbides so as to obtain soft final products, it is preferable that the slab heating for the hot rolling is maintained in a range of from 950 to 1200"C, the hot rolled steel strip is finished in a temperature range of from 680 to 950"C and coiled at a temperature not higherthan 760"C.

(3) If the initial cooling rate after the annealing is excessively high, the grains are finely divided due to the yto transformation, thus causing lowered r values. Therefore, it is particularly desirable that the initial cooling rate after the annealing is maintained at less than 35 C/second.

(4) In order to maintain a high degree of carbon super-saturation so as to improve the efficiency of the overageing treatment, it is desirable that the temperature To is not lower than 30 degrees below the upper limit specified hereinbefore forty.

(5) Also in order to maintain a high degree of carbon super-saturation so as to improve the efficiency of the over-ageing treatment, and further to prevent deformation of the steel strip due to the thermal strain resulting from the rapid cooling, it is desirable that the cooling rate from the temperature To is in the range of from SO"C/second to 650"Clsecond and more preferably from 80"Clsecond to 650 C/second.

(6) In order to prevent excessive super-saturation of carbon, and thereby to prevent the finely divided and close precipitation of carbides during the overageing treatment, it is desirable that the initial temperature of the over-ageing treatment is identical to the finishing temperature of the rapid cooling from the temperature TQ, but if the finishing temperature of the rapid cooling is lower than the initial temperature of the over-ageing treatment, the temperature difference should preferably not be larger than 50 degrees.

(7) If the temperature is slowly raised during the over-ageing treatment, the carbides are dissolved.

Therefore, it is desirable to maintain the temperature constant during the treatment, or to lower the temperature either slowly or stepwise, or to combine these procedures, so as to maintain the finishing temperature of the over-ageing treatment in a range of from 300 to 400do.

(8) In order to promote grain growth during the heating and holding steps of the annealing process, it is desirable intermittently to give 0.1% or more strain to the steel strip. Moreover, to prevent finelydivided carbide precipitation during the over-ageing treatment it is desirable to ensure any strain given to the steel strip during the over-ageing treatment is not greater than 1.2%.

(9) For the purpose of softening the product by utilising the carbon precipitation during cooling to or near room temperature after the over-ageing treatment, it is desirable to cool the steel strip to a temperature near the room temperature at a cooling rate of not higher than 30 C/second after the over-ageing treatment.

(10) Regarding steels containing solute nitrogen, it is desirable rapidly to cool the steel strip afterthe over-ageing treatment to a temperature not higher than 1 00"C at a cooling rate of not less than 30 C/second, and then slowly to cool the sheet to a temperature near the room temperature at a cooling rate not higher than 10"Cisecond.

(11) In orderto prevent the strain ageing hardening during or immediately after a temper rolling step, it is desirable to cool the steel strip to a temperature not higher than 45"C before temper rolling or shape correction.

In order that the present invention may better be understood, it will now be described in greater detail and certain specific Examples thereof given, reference being made to the accompanying drawings, in which: Figure 1 shows a continuous annealing cycle as employed in Example 1; Figure 2 is a graph showing the influence of the heating rate up to the annealing temperature (referred to hereinafter as 'HR') on the rupture elongation of a steel strip treated according to Example 1; Figure 3 shows a continuous annealing cycle as employed in Example 2; Figure 4 is a graph showing the relation between combinations of the annealing temperatures (T) and the annealing times (t) and the rupture elongation of steel strips treated according to Example 2; Figure 5 shows the continuous annealing cycle as employed in Example 3; and Figure 6 is a graph showing the influence of the termination temperature (TQ) of the slow cooling on the rupture elongation of a steel strip treated accord ing to Example 3.

Example 1.

Al-killed steel containing 0.025% carbon and 0.21% manganese was prepared in a converter and made into slabs by continuous casting. The slabs were hot rolled to a thickness of 2.8 mm under the following conditions: Heating temperature: 11 00"C Finishing temperature: 905"C Coiling temperature: 650"C After acid pickling, the hot rolled strips were cold rolled to form 0.8 mm thick cold rolled strips. The cold rolled strips thus obtained were subjected to the continuous annealing cycles as illustrated in Figure 1.

The heating rate (HR) through the range of from 350 to 800"C was varied from 1 to 1200C/second, and tension test pieces (in accordance with JIS B7702 No. 5) were prepared from the steel strips and estimated for their rupture elongation, as an index ofthe workability.

The results are shown in Figure 2. When the heating rate (HR) was not less than 40"C/second, excellent rupture elongation values were obtained.

Example 2.

Al-killed steel containing 0.021% carbon and 0.18% manganese was prepared in a converter and made into slabs by continuous casting. The slabs were hot rolled to form strips 3.2 mm thick under the following conditions: Heating temperature: 1050"C Finishing temperature: 880"C Coiling temperature: 700"C After acid pickling, the hot rolled strips were cold rolled to form 1.0 mm thick cold rolled strips. The A3 transformation temperature of these cold rolled strips was 875"C. These strips were subjected to the continuous annealing cycles as illustrated in Figure 3.

The annealing temperature T was varied from 650 to 1 0000C and the annealing time t was varied from 0 to 120 seconds, to provide various combinations of T and t.

After the annealing, the cold rolled strips were given a 0.8% temper rolling, and tension test pieces (JIS B7702 No. 5) were prepared therefrom and evaluated for their rupture elongation.

The results are shown in Figure 4, from which it is clear that a high level of rupture elongation can be obtained under the annealing conditions as defined by the present invention. The recrystallisation temperature of the strip used in this example was 560"C.

Example 3.

The same cold rolled strips as were made in Example 2 were subjected to the continuous annealing cycles as illustrated in Figure 5, at 700 C for 20 seconds and 850"C for 10 seconds, while the terminal temperature of the slow cooling TQ was varied from 500 to 8000C.

The rupture elongation was evaluated in the same manner as in Example 2. The results are shown in Figure 6, from which it is clear that cold rolled steel strips having excellent workability can be obtained by the processes of the present invention.

Example 4.

A capped steel containing 0.055% carbon and 0.20% manganese was prepared in a converter, made into slabs by an ingot-making method, and hot rolled to a thickness of 3.2 mm under the following conditions: Heating temperature: 1150"C Finishing hot rolling temperature: 885"C Coiling temperature: 675 C After acid pickling and cold rolling to 1.0 mm thick strips, samples were taken from the cold rolled strips. The A3 transformation point of the samples was 850"C.

The samples were subjected to the continuous annealing cycle as shown in Figure 3 with the annealing temperature varying over a range of from 580 to 890"C and with the annealing time varying over a range of from 5 to 120 seconds. An overageing treatment was performed for 120 seconds.

Test pieces according to JIS B7702 No. 5 were prepared from the strip, and the rupture elongation and r value thereof were determined by tension tests. The results showed that both the rupture elongation and the r value varied depending on the annealing temperature and time, and a rupture elongation not less than 44% with an r value of not less than 1.30 can be obtained when the temperature is within the range of from 700 to 840"C and the time is within a range of from [30-0.03 (T-680)1 to [90-0.15 (T-680)t seconds.

Example 5.

The same samples obtained as in Example 4 were subjected to the annealing cycle shown in Figure 3, with an annealing temperature of 740"C and an annealing time of 35 seconds. Subsequently, an overageing treatment was performed at 400"C with the overageing time varying over a range of from 5 to 300 seconds. The rupture elongation and the r value were determined in the same way as in Example 4. The results revealed that both the rupture elongation and the r value improved as the overageing time was increased, and with an overageing time of 120 seconds, 44.5% rupture elongation and 1.30 r value were obtained. With the overageing time exceeding 120 seconds, the rupture elongation and the r value gradually improved, but saturated at an overageing time of about 200 seconds.

Claims (6)

1. A process for producing a cold rolled steel strip by employing a continuous annealing step, which process comprises: hot rolling a low carbon steel slab to form strip; cold rolling the hot rolled steel strip; heating the cold rolled steel strip to an annealing temperature with a heating rate of not less than 40"C/second over the temperature range of from 600"C to the annealing temperature; annealing the strip at a temperature Tin the range of from 700"C to the A3 transformation temperature for a period oftimet ranging from [10-0.03 (T-680)g seconds to [150-0.15 (T-680) seconds;; slowly cooling the thus-annealed strip initially at a cooling rate of less than S00Clsecond and then rapidly cooling the strip from a temperature TQ lying above 600"C but not higher than [T-0.027 (T-680) ( < + 23.7)g C, the rapid cooling being performed at a cooling rate of not less than 50 C/second down to an overageing temperature range; and subjecting the strip to an overageing treatment at a temperature ranging from 300 to 500"C for a period of not less than 10 seconds.

2. A process according to claim 1, in which the steel contains 0.003 to 0.04% carbon.

3. A process according to claim 1 or claim 2, in which the overageing treatment is performed for not more than 2 minutes.

4. A process according to claim 1, in which the steel contains more than 0.04% but not more than 0.08% carbon, and the annealing time t is modified so as to lie in the range of from [30-0.03 (T-680)j seconds to [90-0.15 (T-680)g seconds.

5. A process according to claim 4, in which the overageing treatment is performed for not less than 2 minutes.

6. A process according to any of the preceding claims, in which the hot rolling is performed with a slab heating temperature in the range of from 950 to 1200"C, a finishing temperature in the range of from 680 to 950 C, and a coiling temperature of not higher than 760 C.

6. A process according to any of the preceding claims, in which the hot rolling is performed with a slab heating temperature in the range of from 950 to 1200 C, a finishing temperature in the range of from 680 to 950"C, and a coiling temperature of not lower than 760"C.

7. A process according to any of the preceding claims, in which the initial cooling rate afterthe annealing is less than 35"C/second.

8. A process according to any of the preceding claims, in which the temperature TQ is not lower than 30 degrees below the upper limit thereof of [T-0.027 (T-680) (f+ 23.7)] C.

9. A process according to any of the preceding claims, in which the cooling rate from the temperature TQ ranges from SO"C/second to 650"C/second.

10. A process according to claim 9, in which the cooling rate from the temperature To is not less than 80"C/second.

11. A process according to any of the preceding claims, in which the cooling from the temperature To is finished at the initial temperature of the overageing treatment.

12. A process according to any of claims 1 to 10, in which the cooling from the temperature TQ is finished at a temperature not more than 50 degrees below the initial temperature of the overageing treatment.

13. A process according to any of the preceding claims, in which the overageing treatment is performed within a temperature range of from 300 to 400"C.

14. A process according to any of the preceding claims, in which the steel strip is given not less than 0.1% strain during the annealing, and is given not more than 1.2% strain during the overageing treatment.

15. A process according to any of the preceding claims, in which the steel strip is cooled after the overageing treatment to a temperature near the room temperature with a cooling rate not greater than 30"C/second.

16. A process according to any of the preceding claims, in which the steel strip is rapidly cooled after the overageing treatment to a temperature not higherthan 100 C with a cooling rate of not less than 30"C/second, and then slowly cooled to a temperature near the room temperature with a cooling rate of not larger than 10 C/second.

17. A process according to any of the preceding claims, in which a temper rolling step is employed, the steel strip being cooled to a temperature not higher than 45"C before being subjected to temper rolling.

18. A process according to any of the preceding claims and substantially as hereinbefore described in the Examples.

19. Cold rolled steel strip whenever produced by a method according to any of the preceding claims.

Amendments to claims filed on 17 July 1980.

Amended claims: