JPH0137457B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing an aluminium-killed cold-rolled steel sheet with excellent deep drawability, and in particular, a method for producing an aluminium-killed cold-rolled steel sheet with excellent deep drawability, in particular a method for continuously hot rolling a cast slab produced by a continuous casting machine in a low temperature range without reheating. By doing this, it is possible to omit the reheating process that was conventionally used in the manufacturing process of hot rolled steel sheets, and since fine AlN is precipitated within the hot rolled steel sheets, the heating rate in the subsequent annealing after cold rolling can be reduced. The present invention relates to a method for manufacturing an aluminium-killed cold-rolled steel sheet that can be manufactured quickly, saves energy through process abbreviations, reduces costs, and improves productivity through accelerating the heating rate.

Aluminum killed steel sheet is a typical high-grade steel sheet for deep drawing. Hot-rolled steel sheets in the conventional method are supplied by billets made by blooming a steel ingot by an ingot-forming method or by billets made by a continuous casting method.
These steel slabs are once lowered to room temperature or a temperature of 600 to 800°C, and then heated for a long time (3 hours) at high temperature (1200°C or higher) in order to completely dissolve AlN in the heating furnace of a continuous hot rolling mill. The steel sheet is heated to a temperature of 300°C or higher, then rolled into a continuous hot rolling mill, and the final rolling is completed at a temperature of Ar ₃ or higher, and then rolled at a low temperature of 600°C or lower to form a hot rolled steel sheet.

There is AlN in the hot-rolled steel sheet made in this way.
is not precipitated, and Al and N are in a completely solid solution state. This hot-rolled steel sheet is subjected to normal cold rolling and batch annealed to a specified temperature at 20℃/
Currently, the temperature is raised and recrystallization is performed at a heating rate of less than hr.

As a cold-rolled steel sheet with excellent deep drawability, the crystal grains after recrystallization are {111} crystals against the steel sheet surface.
It is preferable that the surfaces are arranged parallel to each other or slightly inclined. In aluminium-killed steel sheets, such recrystallized textures are caused by fine particles that precipitate during the annealing process.
Because it is obtained by the action of AlN, it is necessary to prevent AlN from precipitating in the hot-rolled steel sheet before recrystallization during post-hot rolling annealing where Al and N are in a solid solution state.
AlN is precipitated. Usually compared to recrystallization
Since the precipitation of AlN requires a low temperature and takes time, the heating rate is set very slowly at less than 20°C/hr before recrystallization to sufficiently precipitate AlN and develop the desired recrystallized texture described above. It has an effect on This is the basis of current cold rolling annealing conditions, and the conventional method therefore requires a long annealing process.

The present inventors have conducted various experiments on an economical and highly productive method for producing aluminium-killed cold-rolled steel sheets as an alternative to the conventional method described above.
We have discovered that by performing AlN precipitation, it is possible to produce aluminium-killed cold-rolled steel sheets, which is a departure from conventional methods.

The present invention was made based on this knowledge, and is characterized by continuous hot rolling of aluminum killed steel slabs manufactured by continuous casting without reheating, and final finishing rolling performed using Ar ₃ By performing the annealing at temperatures below Ar ₃ -100°C and precipitating fine AlN at this point, it has become possible to increase the heating rate during annealing to 20°C/hr or more.

The present invention completely renews the conventional manufacturing basics, precipitates fine AlN within the hot rolled steel sheet, has the same effect as AlN precipitated during annealing, and develops a recrystallized texture favorable for deep drawability. The important point in carrying out the present invention is that fine AlN
The purpose is to cause precipitation 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 recuperation heating)
Thereafter, the steel piece is subjected to width reduction and thickness reduction using 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 just before it is bitten by the rough rolling mill is
The temperature should be 900℃ or higher to prevent precipitation. Thus, the precipitation of AlN up to this point has been completely suppressed.

(2) Temperature at the final finishing point of finishing rolling at Ar ₃ points −
In the range of 100℃, AlN due to rolling strain induced
Acceleration of precipitation is observed.

The reason why the upper limit of the finishing temperature was set at Ar ₃ points is that strain-induced precipitation of AlN does not occur at temperatures higher than this, so the lower limit was set at Ar ₃ points - 100
The reason for this is that at temperatures below this temperature, there are crystal grains with very strong texture {100} planes in the hot-rolled steel sheet, so even after cold rolling and annealing, this texture is not cooled. This is because they are carried into the rolled steel sheet and reduce deep drawability.

(3) Continuously cold-roll the material as quickly as possible at a low temperature of 600°C or less, or roll it up and subject it to cold-rolling. The reason for this is that since coarsening of AlN does not contribute to the development of a recrystallized texture that is favorable for deep drawability, the growth and coarsening of AlN precipitated during finish rolling is suppressed before cold rolling.

(4) As for the range of components, the C content was set to 0.15% or less because if it exceeded 0.15%, the material of the cold-rolled steel sheet would become hard and the workability would deteriorate. The reason for setting Mn 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-soluble Al content is 0.015-0.100% and the N content is 0.003-0.008%. When Al is less than 0.015% or N is less than 0.003%, precipitation of AlN is small and the effect is insufficient to obtain the desired recrystallized texture. Al is
When N is 0.100% or more or N is 0.008% or more,
On the other hand, AlN and N were set within these ranges because the precipitation of AlN increased and the recrystallization suppressing effect was too strong, making it impossible to obtain the desired recrystallized texture.

(5) Hot-rolled steel sheets manufactured under the conditions of (1) to (4) above,
Cold rolling is carried out at a rolling reduction ratio of 60 to 90%, the temperature is raised to a predetermined temperature at a heating rate of 20° C./hr or more, and 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 the cold rolling rate was set as the most preferable range. The r value is used as an evaluation measure of workability (deep drawability). It is generally considered that a value of 1.4 or more is sufficient for cold-rolled steel sheets for deep drawing.

The reason why the heating rate is set at 20°C/hr or more in the present invention is as shown in Figure 2. Even at a heating rate higher than this, ≧1.4 can be sufficiently ensured, and setting the heating rate at 20°C/hr or more saves energy. However, reducing the heating rate to less than 20°C/hr not only has no specific effects, but also reduces both energy saving and productivity, leading to increased costs. This is due to the inability to avoid being disadvantaged.

EXAMPLE The sample was made of aluminum killed steel having the following components and made into a slab using a continuous casting machine.

C: 0.044%, Mn: 0.26%, Al: 0.049%, N:
0.0055% The slab after slab is inserted into a heating furnace (recuperation furnace), extracted after 35 minutes (surface temperature of slab 1200℃), and reduced in width and thickness in a sizing mill to 250t x 950w.
A steel piece of xl was used.

This steel billet after sizing mill rolling is started to be hot rolled within 25 minutes without reheating, and finish rolling is completed at a temperature of 800 to 700℃, which is less than Ar ₃ and in the range of Ar ₃ -100℃. After that, one was rapidly cooled and rolled at 530°C to form a 3.0 mm thick hot rolled steel plate. This hot rolled steel plate was cold rolled to 0.8 mm (reduction ratio 73%).

After that, batch annealing was performed at a heating rate of 20℃/hr.
The temperature was raised at 40℃/hr and 100℃/hr, respectively, to 710℃×
Retention was performed for 6 hours. Otherwise, following the rapid cooling to 530°C, continuous cold rolling was performed in the same manner as above, and then continuous annealing was carried out at a heating rate of 10°C/sec.
One that was held at 730°C for 40 seconds and over-aged at 400°C for 120 seconds.The other was heated at a heating rate of 100°C/sec, held at 750°C for 5 seconds, and over-aged at 400°C for 120 seconds. I processed it.

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: slab inserted, B: slab rolled by sizing mill. C: Continuous hot rough rolling, D: Finishing final point, E: Rolling, F: High temperature heating, a: Heating (recuperation furnace), b: Billet conveyance.

FIG. 2 shows the values of the aluminum-killed cold-rolled steel sheet obtained by cold-rolling and annealing this hot-rolled steel sheet in Examples, as determined by the heating rate. In FIG. 2, the horizontal axis shows the heating rate, and the vertical axis shows the value serving as the evaluation scale of workability. In the figure, the circles indicate actual values in the examples, a indicates the batch annealing, b indicates the continuous annealing, and the hatched area indicates the actual values obtained by the conventional method. c is the conventional manufacturing condition range.

For hot-rolled steel sheets using conventional methods, when the heating rate is high at 20°C/hr or more during annealing after cold rolling,
The r value suddenly decreased as shown by the hatching,
The value was significantly lower than 1.4, which is the lower limit of practical workability for cold-rolled steel sheets for deep drawing, and workability deteriorated. On the other hand, in the hot rolled steel sheet in the example of the present invention,
Even if the heating rate is increased by 20℃/hr or more, the value ≧
1.4 was maintained, and a cold-rolled steel sheet that could be used satisfactorily for deep drawing was obtained.

The present invention described above eliminates steps in the manufacturing process of hot-rolled steel sheets and utilizes them to densely precipitate fine AlN within the sheet during the hot-rolling stage, thereby significantly increasing the heating rate during annealing. Even if the speed is increased, it is possible to obtain recrystallization in which the {111} plane of the crystal grains is parallel or slightly inclined to the steel sheet surface, which is favorable for deep drawability. It has high properties, can be manufactured at low cost, and its effects are very large.

[Brief explanation of drawings]

FIG. 1 is a graph of temperature and time in the manufacturing process up to the hot-rolled steel sheet in an example of the present invention, and FIG. 2 is a graph of the relationship between values and heating rate of the cold-rolled steel sheet in an example of the present invention.

Claims (1)

[Claims] 1 C manufactured using a continuous casting machine: 0.15% or less,
Mn: 0.5% or less, Al: 0.015-0.100%, N: 0.003
Continuous hot rolling of aluminum killed steel hot slabs consisting of ~0.008%, balance Fe and unavoidable impurities is started at a surface temperature of 900℃ or higher without reheating, and finish rolling is carried out at less than ₃ Ar points and Ar ₃ −100℃. carried out within the range of
An aluminium-killed cold-rolled steel sheet with excellent deep drawability characterized by being rolled at 600°C or less or subjected to normal cold rolling and then annealed at a heating rate of 20°C hr or more. Production method.