NL8006403A - METHOD FOR MANUFACTURING STEEL SHEET, PREPARATIONS - Google Patents

Description

4 »V.O.1250

Method of manufacturing steel sheet; products.

The invention relates to methods for manufacturing cold-rolled high-strength steel plates which are excellent for painting, welding and machining. Steel strips and cut steel plates are here commonly referred to as plates.

5 In recent years, more and more cold-rolled high-strength steel sheets have been used for automotive bodies.

For the manufacture of high-strength cold-rolled steel sheets, methods have been predominantly employed using a continuous annealing treatment. However, in these production processes, the addition of a large amount of elements such as carbon, manganese, silicon, phosphorus and chromium is required to improve strength.

When these elements, for example carbon, manganese and silicon, are present in the steel in large quantities, the weldability of the steel is adversely affected by these elements, so that the strength of the welded part deteriorates or fatigue phenomena of the welded part increase. Furthermore, excessive levels of these elements also have an undesirable effect on the paintability (paint adhesion) of the steel when a paint coating is applied to finished products made from the steel.

For all these reasons, there is an increasing demand for new techniques which can produce high-strength cold-rolled steel sheets that are highly workable with lower contents of these elements.

The object of the invention is therefore to provide a method for the production of high-strength cold-rolled steel plates that meet the above-outlined demand, and the main technical feature of the invention lies in the addition of the strength-improving elements which adversely affect weldability and dyeability, is reduced to the minimum possible amount, and primary cooling is effected after the soaking step in a final continuous annealing treatment at a controlled cooling rate, in particular using a vapor-liquid cooling method.

According to the present method, various drawbacks of known techniques can be eliminated in an advantageous manner and at the same time it is possible to produce cold-rolled steel plates with high strength, which have a strength of 50 -80 kg / mm and yet have a good balance between strength and ductility.

The invention will be explained in more detail below.

Carbon, silicon and manganese are each effective in imparting strength to the steel, especially carbon and manganese must be present in an amount of not less than 0.05% carbon and not less than 0.7% manganese.

However, excessive levels of these elements adversely affect the weldability and paintability of the obtained steel, and for this reason the upper limits for these elements are set at 0.12% carbon, 1.2% silicon and 1.5% manganese, respectively. .

Phosphorus is the most desirable element for imparting strength to the steel because it can effectively impart strength to the steel 15 without exerting substantial adverse effects on weldability and paintability, unlike carbon, manganese and silicon. However, phosphorus levels of less than 0.04% will not give the desired strength. Therefore, the phosphorus content should be at least 0.04%. On the other hand, when the phosphorus content is excessive, the balance between strength and ductility deteriorates and at the same time the weldability is reduced, although the detrimental effect is not so great.

The upper limit for the phosphorus content is therefore set at 0.15%.

Aluminum is necessary for the deoxidation of the steel, and less than 0.01% aluminum is not sufficient for this purpose, but it is not necessary to add aluminum in amounts greater than 0.10%.

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added.

As for the other elements, a smaller amount is more preferable, but they can be added if desired.

Steel plates ("slabs") of the above defined chemical composition are produced by the conventional continuous casting or casting crushing process, and these steel plates are hot rolled, cold rolled and then subjected to the continuous annealing treatment of the invention. With respect to the hot-roll wrapping temperature, a higher temperature is more suitable, and preferably a wrapping temperature of, for example, 650 ° C or higher is used to achieve the desired strength-ductility balance. - to get.

A detailed description of the conditions of the continuous annealing treatment according to the invention follows.

The continuous annealing treatment of the invention includes one of the following two heating cycles. The first heating cycle consists of heating-soak-fast cooling, and the second heating cycle consists of heating-soak-fast cooling to the over-aging treatment temperature over-aging treatment.

In the first heating cycle, rapid heating is desired for reasons of productivity, etc., and a jet stream heating system is most preferred for this.

As for soaking, this is done in a temperature range of 730-850 ° C for 20 sec. to 2 min., because when the temperature is below 730 ° C and the time period is less than 20 sec. 15, satisfactory recrystallization and grain growth cannot be achieved, and therefore the desired ductility cannot be ensured. On the other hand, when the softening temperature is above 850 ° C, it is excessively high and the stronger conductivity balance is disturbed. The soaking duration may be longer, but an excessively long soaking time requires a long oven, which has economic disadvantages. Therefore, the upper limit of the week duration is set at 2 min.

At the end of the soaking, the steel sheet is subjected to rapid cooling. The optimum cooling rate for rapid cooling ranges from 30 to 300 ° C / sec, and with a cooling rate of less than 30 ° C per 25 sec. the amount of the aforementioned alloying elements must be increased in order to obtain the desired strength of the steel obtained.

However, the increased amount of the alloying elements leads to deterioration of the weldability and paintability. On the other hand, when the cooling rate is greater than 300 ° C / sec. the amount of martensite formed by the rapid cooling increases, and the strength / ductility balance deteriorates. For all these reasons, the cooling rate for the rapid cooling following the completion of the soaking treatment is limited to a range of 30-300 ° C / sec. In this way, in combination with the specific steel composition, a cold-rolled steel sheet of high strength, well balanced between strength and ductility, and with excellent weldability and paintability can be obtained in a most economical manner.

800 6 40 3 -4-

The cooling rate as defined above is difficult to achieve with a conventional cooling method such as water immersion, gas shielding and water jet treatment, but can be easily achieved by blowing a vapor-liquid mixture on the steel sheet. The advantage of this cooling method is that a uniform cooling effect across the width of the steel strip can be achieved despite the rapid cooling, so that the shape of the steel strip can be improved and a uniform quality of the material can be ensured. A further advantage of this cooling method is that it is possible to control the end-point of the cooling at a desired temperature of the strip. This is very advantageous for carrying out the second heating cycle.

When performing the heating cycle of the first type, when the steel sheet is slowly cooled after the soaking step from the softening temperature, which ranges from 730-850 ° C, to a temperature also ranging from 720-650 ° C with a cooling rate of no more than 20 C / o sec. and then cooled at a cooling rate of 30-300 C / sec, a further improved strength-ductility balance is obtained, although the strength is slightly reduced.

The over-aging treatment in the temperature range of 500-300 ° C in the second heating cycle serves to precipitate the carbon solid solution in the steel and improve the ductility of the steel. According to the invention, rapid cooling is performed at a cooling rate of 30-300 ° C / sec. following the termination of. the soaking stage, terminated at the over-aging temperature, so that the steel sheet is subsequently subjected to the over-aging treatment without the need for re-heating.

In this way, the heating cycle from cooling to room temperature reheating can be avoided so that the distribution of the carbon precipitates is significantly improved and fine carbon precipitates do not appear in the ferrite grains, but relatively large carbon precipitates appear in the grain boundary areas. Under this condition, satisfactory ductility can be obtained from the over-aging treatment.

35 When applying the second type heating cycle, when the steel sheet is slowly cooled from the softening temperature which varies from 830 6 40 3 -5- from 730-850 ° C, to a temperature ranging from 720-650 ° C at a cooling rate of not more than 20 ° C / sec. and then cooled to the over-aging temperature ranging from 500 to 300 ° C with the cooling rate of 30-300 ° C / sec., a further improved strength-ductility balance is obtained, although the strength is slightly reduced.

As described above, the two types of heating cycles according to the invention can be used. When more importance is attached to a low failure ratio (failure point / tensile strength), the first type of heating cycle is applied, and when / more importance is attached to the strength-ductility balance, the second type of heating cycle is performed.

As for the over-aging treatment duration, it can be equal to the usual treatment duration and a range of 30 sec. up to 15 5 min. is suitable.

The invention will be further elucidated on the basis of the preferred embodiments described below.

Steel plates with compositions according to Table A were manufactured and hot rolled to hot rolled steel strips with a thickness of 2.5 mm, and then cold rolled to cold rolled steel strips with a thickness of 0.7 mm. These cold-rolled steel strips thus obtained were subjected to continuous annealing under conditions of varying cooling rates with or without the over-aging treatment as shown in Table B.

In cases where the over-aging treatment was carried out in a group, the cooling was done after the soaking to room temperature, and after reheating the over-aging treatment was carried out, and in the other group, the cooling after the soaking was finished at the over-aging temperature and the over-aging treatment without prior reheating executed. The weldability and paintability of the obtained steel plates are shown in Table A.

In Table A, samples A and B have the same chemical composition, but the cooling end point is controlled and the over-aging treatment (.e) is carried out in steel A and the over-aging treatment (d) is carried out after reheating in steel B.

As shown by comparison of steel A with steel B, steel A has a lower point of failure and a greater elongation than steel B. This indicates that control of the cooling end point is more favorable.

In the cases of the steels C and D, the composition is adjusted so that a tensile strength of 60 kg / mm, respectively, when cooling at a speed of 30 ° C / sec. (b) and cooling by immersion in water (h), is obtained, showing that steel C has a better yield point and elongation than steel D.

In the cases of samples E and F, the composition is adjusted to obtain a tensile strength of 70 kg / mm or when slowly at 10 ° C / sec. is cooled (a) and quickly at 100 ° C / 10 sec. is cooled (c).

Since a large amount of alloying elements is required under the continuous annealing conditions (a), both paintability and weldability are poor.

Finally, in the cases of samples G and H, sample G quickly became 15 at 300 ° C / sec. cooled and steel H quickly at 350 ° C / sec. (g) cooled. The elongation in steel H is shown to be low due to the excessively high cooling rate.

I and J represent the invention, wherein the steel plates after the soaking step are slowly at a cooling rate of 10 ° C / sec. are cooled to the initial temperature of 690 ° C for the rapid cooling, and the final steel plates of I and J show better ductility and a better strength-ductility balance than A and E which also represent the invention, although the strength has slightly decreased.

As shown in the embodiments A, C, E, G, I and J. 25 according to the invention, according to the invention a cold-rolled steel sheet with a good balance between strength and ductility and excellent paintability and weldability can be obtained .

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Claims (6)

A method of manufacturing a cold-rolled high-strength steel sheet which is excellent paintable and weldable, characterized in that a steel containing 0.05-0.12% carbon, not more than 1.2% silicon, 0.7 -1.5% manganese, 0.04-0.15% phosphorus, 0.01-0.10% sol. Al and the remainder contains 5 iron and impurities, hot rolled; the hot-rolled steel sheet is cold-rolled; and the cold-rolled steel sheet is subjected to a continuous annealing treatment, soaking in a temperature range of 730-850 ° C for 20 sec. to 2 min., and cooled rapidly at a cooling rate of 30-300 ° C / sec. Method according to claim 1, characterized in that an over-aging treatment is further carried out in a temperature range of 300-500 ° C after the rapid cooling. A method according to claim 2, characterized in that the rapid cooling is carried out to a temperature in a range of 500-300 ° C. The method according to claim 1, characterized in that the soaked steel sheet is further slow at a cooling rate of no more than 20 C / sec. is cooled to the initial temperature of the rapid cooling which ranges from 720 to 650 ° C. The method according to claim 1, characterized in that the soaked steel sheet is further slow at a cooling rate of no more than 20 ° C / 20 sec. is cooled to the initial temperature of the rapid cooling which ranges from 720 to 650 ° C, and the slowly cooled steel sheet is rapidly cooled to a temperature of 500-300 ° C with a cooling rate of 30-300 ° C / sec. and the rapidly cooled steel sheet is subjected to an over-aging treatment in a temperature range of 300-500 ° C. Articles of manufacture obtained using a steel produced by the method according to one or more of claims 1 to 5. 800 6 40 3