JPH0826409B2 - Manufacturing method of cold rolled steel sheet with excellent press formability by continuous annealing - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a cold-rolled steel sheet having excellent press formability by using continuous annealing equipment in the steel industry.

(Prior Art) Cold-rolled steel sheets used for automobile steel sheets are required to have good press formability.

These were conventionally manufactured by box annealing, but continuous annealing equipment has come into operation, and most of them have been manufactured by continuous annealing. In recent years, at a steelworks, a typical process for manufacturing a cold-rolled steel sheet is to first slab molten steel with predetermined components in a steelmaking factory in a continuous casting process. The thickness of this slab is 1.6m by a continuous hot rolling mill.
It is rolled into a hot-rolled coil of about m to 6.0 mm, after which the surface scale is removed by pickling, and then rolled by a continuous cold rolling mill to a predetermined plate thickness. After that, recrystallization annealing is performed by continuous annealing to soften it, and then temper rolling is performed to obtain a steel sheet. Regarding the hot rolling during the above process, the temperature at which the steel sheet passes through the final stage rolling stand, that is, the hot rolling finishing temperature (hereinafter FT
And the temperature at which the steel plate is wound into a coil, that is,
The hot rolling coiling temperature (hereinafter referred to as CT) has a significant effect on the properties of the steel sheet.

Generally, in cold-rolled steel sheet for press, FT is the temperature at which transformation from austenite to ferrite in the steel sheet starts, that is, Ar ₃
It is known that if the temperature is not higher than the temperature, the press formability of the steel sheet is significantly deteriorated. In the case of a normal cold-rolled steel sheet for press, this FT needs to be 850 ° C or higher.

On the other hand, with regard to CT, as far as continuous annealing is concerned, the method of winding at high temperature is generally used.

The reason why this high temperature winding is required is to improve the deep drawability by enlarging the ferrite crystal grains, completely precipitating and enlarging AlN, and largely precipitating the carbide.

However, in the coiling step of hot rolling, since the cooling rate is high in the outer peripheral portion of the coil, the above effect is not exhibited, resulting in non-uniformity of the material of the cold rolled steel sheet. The non-uniformity of the material in the continuous annealing process is not due to the continuous annealing itself, but is due to the hot rolling process which is the preceding stage process. Generally, in a normal hot rolling process, the steel strip that has been finished by rolling is usually cooled by cooling water on a subsequent run-out table and wound up at a predetermined temperature.
At this time, it is well known that the steel sheet is not uniformly cooled on the run-out table over the entire length of the steel strip (especially inside and outside the strip coil). To solve this problem, if the hot rolling coiling temperature is raised, the above-mentioned drawbacks are eliminated, but the increase of the coiling temperature causes the generation of coarse grains in the surface layer, and significantly deteriorates the ductility of the cold rolled steel sheet. Various proposals have been made to improve these problems. For example, JP-B-55-22533 discloses a method in which aluminum killed steel is continuously annealed after being wound at a high temperature of 730 ° C. or higher in a hot rolling winding process,
Also, Japanese Patent Publication No. 55-36051 discloses a method of winding both ends of a hot-rolled strip without water injection when winding the hot-rolled strip. Thus, it can be seen that the effects of FT and CT, especially CT, in the hot rolling process when manufacturing aluminum-killed steel by continuous annealing are significant. Certainly, the continuous annealing process for cold-rolled steel strip has a great advantage that cannot be obtained by the normal batch annealing process, that is, high productivity, labor saving, and uniform material quality (high yield). . However, it is well recognized for those engaged in the steel industry that the continuous annealing process which is commonly used, and that the above-mentioned improvement process cannot obtain a uniform material over the entire length of the strip.

(Problems to be Solved by the Invention) An object of the present invention is to prevent deterioration of the material of the coil front end and the rear end of aluminum killed steel in a continuous annealing step, and to provide a continuous coil having uniform characteristics in the coil length direction. It is intended to provide a method for manufacturing a cold rolled steel sheet having excellent press formability by annealing.

Uniformity here means the YP, value of the mechanical properties of the coil front and rear ends, and the YP of the mechanical properties of the coil center,
We assume that there is no difference from the value, but we
If the YP is 2 kg / mm ² or less and the Δ value is 0.20 or less, it is considered that there is no problem in the actual press work, so this value was used as an index of uniformity.

(Means for Solving the Problems) The present invention was developed to solve the above problems, and relates to the uniformity of the material quality of aluminum-killed steel produced in a continuous annealing process, and is its gist. However, (1) C: 0.08% or less, Mn: 0.30% or less, S: 0.030% or less, A
l: 0.03 to 0.08% or less, N: 0.0040% or less, Al-killed steel consisting of the balance iron and unavoidable impurity elements is melted into a slab, and then stripped in a hot rolling step, and then cold rolling, In the method of producing a cold-rolled steel sheet for press forming through each step of continuous annealing and continuous annealing, on the run-out table in the hot rolling step, the sheet temperature in the strip length direction intermediate portion is set to 65%.
The temperature of the strip is set to 0 to 700 ° C., the plate temperature of the front and rear ends of the strip is set to be 50 to 80 ° C. higher than the plate temperature of the middle part, and the set length range is set to 5 to the total length of each coil. A cold-rolled steel sheet with excellent press formability by continuous annealing characterized by cooling with cooling water at 25% and then winding (2) The conditions of the continuous annealing step are annealing temperature 750-850 ℃, heating Time 10 to 60 seconds, then cooling rate at 60 to 150 ° C / second 2
Cool to below 70 ° C, then heat to 350 ° C, 350-27
A method for producing a cold-rolled steel sheet excellent in press formability by continuous annealing according to the above (1), which is characterized by overaging treatment for 120 to 180 seconds at 0 ° C.

(Function) The reason for limiting the components of the present invention will be described. (1) C hardens the steel and impairs the formability of the steel sheet, so the upper limit is made 0.08%. When a material with better press formability is required, it is effective to perform vacuum degassing treatment to reduce the C content to 0.015% or less.

(2) The reason why Mn is 0.3% or less is that the material becomes hard when Mn is 0.3% or more, and it is preferable that Mn is lower.

(3) The upper limit of S is set to 0.030% because cracks caused by S occur during hot rolling when S is high. It is considered that Mn / S ≧ 10 to 15 is a measure for preventing hot cracking, and the upper limit of S is regulated from this viewpoint. (4) As for Al, as a result of various studies,
This is because this range was a necessary and sufficient condition for precipitating N in the steel as AlN after hot rolling. (5) N is 0.00
The reason why the content is set to 40% or less is that if it is more than this, it combines with the above Al to precipitate fine AlN, which deteriorates the mechanical properties of the steel strip. (6) The reason why the coiling temperature of the coil middle part in the hot rolling process is set to 650 to 700 ° C is that predetermined material properties cannot be obtained if the coiling temperature is less than 650 ° C.
This is because the surface layer coarse particles are generated, and further, the oxide scale becomes thicker, the pickling time becomes longer, and the productivity is deteriorated. Further, when winding in the hot rolling step, the set length of the temperature at the leading and trailing ends of the strip is set to be 5% or more and 25% or less when the winding length is 5% or less, the winding pattern of FIG. 1 (C). Therefore, the uniformity of the coil length direction, which is the essence of the present invention, cannot be achieved. The temperature set length at the front and rear ends of the strip is set to 25% or less, because if it is 25% or more, the thickness of the oxide scale after coiling becomes thicker and the pickling time during pickling is longer. This is because the productivity will deteriorate.

The annealing temperature in continuous annealing was 750 to 850 ° C, which is 750.
If the temperature is less than ℃, sufficient material properties cannot be obtained. On the other hand, if the annealing temperature exceeds 850 ° C, annealing will occur in the γ region, and the texture will be randomized, leading to a decrease in the value. In addition, a heat buckle will occur during operation, causing breakage of the plate and impairing workability. If the cooling rate and overaging temperature after annealing are within this method range (cooling rate: 60 to 150 ° C, overaging temperature: 350 → 270 ° C), the solid content C in the steel decreases and the natural aging of the steel sheet Is improved.

(Example) C: 0.03%, Mn: 0.20%, P: 0.010%, S: 0.010%, Al: 0.050
%, N: 0.0025%, other unavoidable components of aluminum-killed steel were smelted in a converter, and a slab manufactured by a normal method was hot-rolled to a 4.0 mm strip at a hot-rolling finishing temperature of 920 ° C. It was wound in the cooling pattern shown in. First
Figure (B) shows a cooling pattern of a type I coil when wound normally at 730 ° C, and (C) shows a cooling pattern of a type II coil when the portion corresponding to the inside and outside of the coil is wound without water injection. .

FIG. 1 (A) is a cooling pattern of the present invention according to type III. As shown in FIG. 1 (A), the plate temperature of the coil middle part is set to 650 ° C. to 700 ° C., and the temperature of the leading end and the rear end part of the strip is set 50 to 80 ° C. higher than the plate temperature of the middle part. That is, when the plate temperature in the middle part is 700 ° C, the temperature of the front and rear ends becomes maximum 780 ° C, and in the case of 650 ° C, the minimum temperature is 700 ° C. The hot-rolled steel sheet wound by these three types of cooling patterns is cold-rolled (cold rolling reduction 80%), continuously annealed at 800 ° C for 30 seconds, and then at a cooling rate of 80 ° C / sec at 250 ° C. After cooling to 330 ~ 270 ℃ 120 seconds overaging treatment,
Table 1 shows the mechanical properties of the product that was subjected to 1.0% temper rolling.

As a result, Type I is the result of winding at a winding temperature of 735 ° C or higher, but the material inside and outside the coil has a higher YP and a lower value compared to the center of the coil, as shown in the right end of Table 1. From the viewpoint of coil uniformity, this is a problematic level. Next, Type II is a method of winding the portion corresponding to the outer periphery of the steel strip on the run-out table in the hot rolling winding step without water injection (without controlling the cooling water). This result shows that the difference between the material inside and outside the coil and the material at the center of the coil is slightly improved as compared with Type I, but the uniformity in the length direction is still poor.

Type III is the result of performing the method of the present invention. As a result, the difference in material between the inside and outside of the coil and the center of the coil is reduced, which is clearly improved. In this method, the homogenization of the material in the coil is sufficiently achieved, and its significance can be seen.

(Effects of the Invention) As described above, the present invention is able to solve the non-uniformity of the material caused by the temperature decrease of the outer circumference during the hot rolling and winding of the material for continuous annealing, which cannot be solved by the conventional method. Temperature of both ends of coil and intermediate temperature of strip during winding (650-700 ℃)
The temperature difference between the strip and the strip is 50 ° C or more and 80 ° C or less, and both ends of the strip, that is, the inner and outer windings have a temperature difference of 50 ° C with the middle part of the strip within the range of 5 to 25% of the total strip length.
By controlling the amount of cooling water so as to be 80 ° C. or less and winding up, it becomes possible to manufacture a cold-rolled steel sheet with excellent press formability that has uniform material properties as shown in the examples, and thus in the steel industry. In addition, it has become possible to fully exhibit the advantages of continuous annealing such as high productivity and material uniformity (high yield).

[Brief description of drawings]

FIGS. 1 (A), (B), and (C) are views showing winding patterns on a hot run table during hot rolling.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Nobuhiko Matsuzu 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Co., Ltd. Kimitsu Steel Co., Ltd. (72) Inventor Akira Sunada Kimitsu, Chiba Shin-Nihon Steel Stock company Kimitsu Works (72) Inventor Nobuaki Ito 1 Kimitsu, Kimitsu City, Chiba Nippon Steel Works Stock Company Kimitsu Works

Claims (2)

[Claims] 1. C: 0.08% or less, Mn: 0.30% or less, S: 0.030%
Hereinafter, Al: 0.03 to 0.08% or less, N: 0.0040% or less, Al-killed steel consisting of the balance iron and unavoidable impurity elements is melted to form a slab, which is then stripped in a hot rolling step, and then cold-rolled. Rolling, and in the method of producing a cold-rolled steel sheet for press forming through each step of continuous annealing, on the runout table of the hot rolling step, the strip temperature in the strip length direction intermediate portion is set to 650 to 700 ° C., and Set the plate temperature at the leading and trailing ends of the strip 50 to 80 ° C higher than the plate temperature in the middle part, and set the range of the set length to 5 to 25% of the total length of each coil, and cool with cooling water. A method for producing a cold-rolled steel sheet having excellent press formability by continuous annealing, which is characterized by rolling and rolling. 2. The continuous annealing process is performed under the conditions of an annealing temperature of 750 to 850.
℃, heating time 10 ~ 60 seconds, then cooling rate 60 ~ 150 ℃
Per second, cooling to below 270 ° C, then heating to 350 ° C,
The method for producing a cold-rolled steel sheet having excellent press formability by continuous annealing according to claim 1, which is characterized by performing an overaging treatment at 350 to 270 ° C for 120 to 180 seconds.