JPS58130224A - Manufacture of aluminum killed cold rolled steel plate which is excellent in deep drawing property - Google Patents

Abstract

PURPOSE:To economically manufacture an aluminum killed cold rolled steel plate which is excellent in its deep drawing property, by continuously hot-rolling an aluminum killed steel hot casting piece of a specified composition without executing reheating, winding it, and subsequently heating and annealing at a high speed after cold rolling. CONSTITUTION:As to an aluminum killed steel hot casting piece consisting of <=0.15% C, <=0.5% Mn, 0.015-0.100% Al, 0.003-0.008% N, and Fe and inevitable impurities as remains, which has been manufactured by a continuous casting machine, continuous rolling is started at >=900 deg.C surface temperature without executing reheating, finish rolling is executed within a range of under an Ar3 point and the Ar3 point -100 deg.C, said piece is wound at <=600 deg.C, and after that, or regular cold rolling is performed as it is, and after that, annealing is executed at a heating speed of >=20 deg.C/hr, by which a heating speed in case of annealing can be increased without cooling and reheating the hot casting piece, and by precipitating fine AlN at the stage of finish rolling, and an aluminum killed cold rolled steel plate is obtained economically and with high productivity.

Description

[Detailed description of the invention] A method for manufacturing steel sheets, in particular, by continuously hot rolling a slab produced by a continuous casting machine in a low temperature range without reheating, it is possible to improve the conventional hot rolled steel sheet manufacturing process. In addition to making it possible to omit the previously used reheating process, it also precipitates fine AIN in the hot-rolled steel sheet, making it possible to accelerate the heating rate in the subsequent annealing after cold rolling. The present invention relates to a method for manufacturing aluminium-killed cold-rolled steel sheets that saves energy, reduces costs, and increases productivity by increasing the heating rate.

Aluminum killed steel plate is a typical high-grade steel plate for law enforcement.

Hot-rolled steel sheets in the conventional method are supplied by billets made by blooming a steel ingot using the ingot-making method or by continuous casting, and these billets are once heated to room temperature. Alternatively, the temperature is lowered to 600 to 800°C, and then heated for a long time (about 3 hours) at a high temperature (1200°C or higher) in order to completely dissolve AIN in the heating furnace of a continuous hot rolling mill. It is put into a continuous hot rolling mill, and the final rolling is completed at a temperature above the Ara point, and then rolled up at a low temperature of 600° C. or below to form a hot rolled steel sheet.

AIN is not precipitated in the hot-rolled steel sheet produced in this manner, and A1 and N are in a completely solid solution state. At present, this hot-rolled steel sheet is not subjected to any ordinary cold rolling, but is subjected to batch annealing, in which the temperature is raised to a predetermined temperature at a heating rate of 20° C./hr or less, and recrystallization is performed.

For a cold-rolled steel sheet with excellent deep drawability, it is preferable that the crystal grains after recrystallization are arranged with the (1111) plane parallel to or slightly inclined to the surface of the steel sheet. This type of recrystallized texture is obtained by the action of fine AIN precipitated during the annealing process, so it is necessary to prevent the precipitation of AIN in the hot-rolled steel sheet and to keep AI and N in a solid solution during hot rolling. In post-annealing, AIN is precipitated before recrystallization.Usually, A
Since IN precipitation requires low temperature and time,
Prior to recrystallization, the heating rate is very slow, less than 20° C./hr, to sufficiently precipitate AIN and effect the desired recrystallization texture development. This is the basis of current cold rolling annealing conditions, and the conventional manufacturing method required a long annealing process.

The present inventors have conducted various experiments and studies on an economical and highly productive method for producing aluminum quilt 8 cold-rolled steel sheets as an alternative to the conventional method described above.
It has been discovered that a method for manufacturing aluminum quilt 8 cold-rolled steel sheets can be implemented, which is a departure from conventional methods.

The fourth invention was made based on this knowledge, and is characterized by continuous hot rolling of aluminum quilted steel slabs produced by continuous casting without reheating, and final finish rolling. By carrying out the annealing at less than 3 points of Ar and up to 100° C. of Ar3 and precipitating fine AIN at this point, it has become possible to increase the heating rate during annealing to 20°C/hr or more.

The present invention changes the conventional manufacturing basics, precipitates fine AIN in the hot rolled steel sheet, has the same effect as the AIN precipitated during annealing, and develops a recrystallized texture that is favorable for deep drawability. Therefore, the important point in carrying out the present invention is to cause the precipitation of fine AIN to occur at the time of final finish rolling, to suppress precipitation before that, and to suppress growth and coarsening after precipitation. Focusing on this point, the present invention will be described below in detail based on an example in which the present invention was implemented in manufacturing steps from continuous casting to continuous hot rolling to cold rolling to annealing.

(1) First, when the slab after continuous casting reaches a temperature of 1,100 to 1,150℃, it is heated in a heating furnace (the slab temperature reaches about 1,200℃ in about 35 minutes with heating to the level of bale heating). Thereafter, medium reduction and thickness reduction are performed in a sizing mill to obtain a steel billet of a predetermined width and thickness. This steel billet is transported to a continuous hot rolling mill in the shortest possible time and hot rolled without high temperature reheating. At this time, the surface temperature of the steel billet in the rough rolling mill and immediately before facing is:
The temperature is set at 900°C or higher to prevent precipitation of AIN. Thus, the precipitation of AIN up to this point has been completely suppressed.

(2) Temperature at the final finishing point of finishing rolling =＼r3 points=
When the temperature is in the range of 100°C, precipitation of AIN is promoted due to rolling strain induction.

The reason why the upper limit of the finishing temperature was set at Ars point is because strain-induced precipitation of AIN does not occur at temperatures higher than this, and the reason why the lower limit was set at Ar3 point - 100°C is because at temperatures lower than this, hot rolling A very strong grain structure (1001
This is because since there are crystal grains with planes, this texture is carried into the cold rolled steel sheet even during subsequent cold rolling and annealing, reducing deep drawability.

(3) It is continuously cold-rolled at a low temperature of 600° C. or less as soon as possible, or it is rolled up and subjected to cold-rolling.

The reason for this is that since coarsening of AIN does not contribute to the development of a recrystallized texture that is favorable for deep drawability, the growth and coarsening of AIN precipitated during finish rolling is suppressed before cold rolling. .

(4) Regarding the component range, if C exceeds 0.15%, the material of the cold rolled steel sheet becomes hard and the workability deteriorates, so 0.
．． It was set to 15% or less. The reason why the grain size is set to 0.5 or less is to prevent deterioration of workability. In order to obtain the steel sheet targeted by the present invention, the acid 5T bath A1 should be adjusted to 0.015-0.
100%, N is 0.003 to o, oos. AI
When N is less than 0.015% or N is less than 0.003%, precipitation of AIN is small and the effect to obtain the desired recrystallized texture is insufficient. AI is 0100% or higher or N
If fJ″-0,008% or more, on the contrary, the precipitation of AIN will increase, and the recrystallization suppressing effect will be too strong, making it impossible to obtain the desired recrystallized texture. did.

(5) The hot-rolled steel sheet manufactured under the conditions of (1) to (4) above is cold-rolled at a rolling reduction of 60-90%, and heated to a predetermined temperature at a heating rate of 20°C/hr or more. Then, annealing is performed.

The cold rolling rate was set to 60 to 90% because it is already known that the cold rolling rate affects the formation of a preferable recrystallized texture, and this is the most preferable range. The i value is used as an evaluation measure of workability (deep drawability). It is generally considered that it is sufficient to obtain a value of 7 of 14 or more for cold-rolled steel sheets for deep drawing.

The reason why the heating rate is set at 20°C/hr or higher in the present invention is that even at a heating rate higher than this, F≧14
While setting the heating rate to 20°C/hr or more has a significant effect in terms of energy saving and productivity, reducing the heating rate to less than 20°C/hr This is because not only is there no particular effect at all, but both energy saving and productivity are reduced, leading to increased costs, which is an unavoidable disadvantage.

The samples of the examples were aluminum killed rice cakes having the following components, which were made into slabs using a continuous casting machine.

C: 0.044%, Mn: 0.26%, A1: 0.
049%, N: 0.0055 The slab after casting was inserted into a heating furnace (recuperating furnace), extracted after 35 minutes (surface temperature of slab 1200℃), and reduced in thickness under medium pressure in a sizing mill. A steel billet of 250tX950wXt was used.

The steel billet after rolling by the sizing mill was started hot rolling within 25 minutes without reheating, and the steel billet was reduced to less than Ara, Ar3.
After finish rolling at a temperature of 800 to 700°C, which is in the range of 100°C, one was rapidly cooled and rolled at 530°C to obtain a hot rolled steel sheet with a thickness of 30 mm.

This hot-rolled steel sheet was cold rolled (reduction rate 73) until 2008 and then batch annealed at a heating rate of 20°C/'hrs4.
The temperature was raised at 0°C/hr and 100°C/hr, respectively, to 710
Retention was performed for 6 hours at ℃. Otherwise, following the rapid cooling to 530°C, continuous cold rolling was performed in the same manner as above, and then the temperature was raised at a heating rate of 10°C/sec as continuous annealing.
One that was held at 30°C for 40 seconds and over-aged at 400°C for 120 seconds, and one that was heated at a heating rate of 100°C/equation.
The temperature was maintained at 750°C for 5 seconds, and an overaging treatment was performed at 400°C for 120 seconds.

FIG. 1 shows a comparison of the temperature history and processing conditions of an example up to the production of a hot rolled steel sheet according to the present invention and a conventional method.

In the figure, A: Insertion of slab, B: Piece after reduction by sizing mill. C: Continuous hot rough rolling, D: Final finishing point, E,
Winding up, F: high temperature heating, a゛heating recuperation furnace), b: billet conveyance, respectively.

FIG. 2 shows the seven values of the aluminum quilt 8 cold-rolled steel sheet, arranged by heating rate, when the hot-rolled steel sheet was cold-fertilized and annealed in 1]. In FIG. 2, the horizontal axis shows the heating rate, and the vertical axis shows the seven cases which are the evaluation scale of workability. ○ in the diagram
The marks are the results of F value in the examples, (a) is batch annealing,
(b) is an example of continuous annealing, and the diagonally shaded area shows the actual F value obtained by the conventional method. (c) is the conventional manufacturing condition range.

In conventional hot rolled steel sheets, when the heating rate increases to 20°C/hr or higher during annealing after cold rolling, the j value decreases rapidly as shown by the hatching, making it difficult to make cold rolled steel sheets for deep drawing practical. The value of 1≧1.4, which is the lower limit of acceptable workability, was significantly lowered, and the workability deteriorated. On the other hand, in the hot-rolled steel sheet in the example of the present invention, the heating rate was set at 20°C/hr or more (
A steel sheet was obtained in which the value 〒≧1.4 was maintained and could be used satisfactorily as a cold-rolled steel sheet for deep drawing.

The present invention described above eliminates steps in the manufacturing process of hot-rolled steel sheets and utilizes them to densely precipitate fine AIN in the sheet thickness during the hot rolling stage, so the heating rate during annealing is significantly increased. Even if the speed is increased to 1111
It is now possible to obtain recrystallization that is favorable for deep drawability with parallel or slightly inclined planes, making it possible to produce cold-rolled steel sheets for deep drawing with high productivity and energy savings (and low cost), which has a very large effect. .

[Brief explanation of the drawing]

Fig. 1 is a graph of temperature and time during the manufacturing process up to the hot-rolled steel sheet in the example of the present invention, and Fig. 2 is a graph of the temperature and time of the manufacturing process up to the hot-rolled steel plate in the example of the present invention. It is a relationship diagram of a value and a heating rate.

Claims (1)

[Claims] C: 0.15% or less, Mn manufactured by Lianzizi
: 0.5% or less, AI: 0.015-0.10
0%, N: 0.003-0.008%, balance Fe
and aluminum killed steel hot slabs consisting of unavoidable impurities.
Continuous hot rolling is started at a surface temperature of 900°C or higher without reheating, and finish rolling is carried out at an Ar of less than 3 points, Ar3-100.
Excellent deep drawability characterized by annealing at a heating rate of 20°C hr or more after rolling at 600°C or less, or after being subjected to as-is or normal cold rolling. A method for producing cold-rolled aluminum killed steel sheets.