JPH0765112B2 - Method for manufacturing base material of cold rolled steel sheet for continuous annealing - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a cold rolled steel sheet base material for continuous annealing.

More specifically, the present invention can produce a cold-rolled steel sheet product having a good deep drawability similar to the case of performing high temperature winding in a hot rolling step, and also having a good surface texture. The present invention relates to a method for manufacturing a base material of a cold rolled steel sheet for continuous annealing.

[Prior Art] Conventionally, box annealing was generally used for annealing cold-rolled steel sheets for deep drawing, but continuous annealing is now widely used. The reason is that continuous annealing of a cold-rolled steel sheet provides a high-strength product with a short treatment time.

However, the continuously annealed cold rolled steel sheet tends to be inferior in formability, particularly deep drawability.

By the way, conventionally, the following technique is known as a manufacturing method of the base material of the cold-rolled steel sheet for continuous annealing.

That is, a slab made by a continuous casting method or a slab obtained by slab forging after casting is 22 to 30 ° C / m.
Once cooled to room temperature at a cooling rate of in, then heated at a high temperature for a long time (1200 to 1300 ° C x 1 hour) in a heating furnace, then hot rolled to a coiling temperature of 700 ° C or higher. It is a method of taking in.

In the above technique, the coiling at a high temperature of 700 ° C. or higher is for obtaining deep drawability, that is, r-value, in the cold-rolled steel sheet that is continuously annealed and comparable to the cold-rolled steel sheet by the conventional box annealing. That is, in the above method, during cooling of the slab, AlN
The deposited AlN re-dissolves during heating and soaking during hot rolling, so that high-temperature winding at 700 ° C causes coarse deposition of AlN. This is because as the winding temperature is higher, coarse precipitation of AlN proceeds, and it is desirable to obtain a cold rolled steel sheet having a high r value by continuous annealing.

[Problems to be Solved by the Invention] However, the conventional method for producing a base material of a cold-rolled steel sheet for continuous annealing has the following problems.

In the conventional method, 1200 ~ 13 prior to continuous hot rolling
The slab is heated to a high temperature of 00 ° C for a long time of 1 hour or more, but this heating amount is enormous.

Further, when heated to a high temperature of 1200 to 1300 ° C. for 1 hour or more, AlN precipitated during cooling is decomposed and re-dissolved, and finely re-precipitated during subsequent hot rolling. This finely reprecipitated AlN reduces the deep drawability. If the heating temperature is lowered to avoid it, the finishing temperature in hot rolling will be low in the slab once cooled to room temperature.

In the above method, the coiling temperature is high, and when the coiling temperature is high, surface scale is generated, and decarburization causes coarsening of particles (coarsening of ferrite crystals), which significantly impairs the appearance of the cold rolled steel sheet. If the coiling temperature is suppressed to 700 ° C. or lower to prevent this, continuous annealing cannot produce a product having an r value comparable to the product obtained by box annealing.

[Means for Solving Problems] The above problems are in terms of weight%, C; 0.01 to 0.06% Mn; 0.02 to 0.3% S; <0.01% sol Al; 0.01 to 0.06% O; <0.01% N; In a method for producing a base material of a cold-rolled steel sheet for continuous annealing containing <0.006% and the balance being iron and unavoidable impurities, a slab is obtained by solidifying the molten metal after casting the molten metal, and the slab after solidification is obtained. Temperature of the slab so that the temperature does not fall below 550 ° C., heating is performed at 105-1180 ° C. at a heating rate of 8 ° C./min or more, and then hot rolling is performed to precipitate AlN, and Rolling temperature ends at Ar ₃ transformation point or higher, 650
This is solved by a method for producing a cold-rolled steel sheet for continuous annealing, which is characterized in that winding is performed at a temperature of not higher than 0 ° C.

The present invention will be described in more detail below.

(Reason for Limiting Components) C C is a component necessary for obtaining the strength of the cold rolled steel sheet product.
Especially when less than 0.01%, not only the strength of the product is insufficient,
It is not preferable because the strain aging deterioration after continuous annealing is large. If it exceeds 0.06%, the deep drawability deteriorates. Therefore, the amount of C is limited to 0.01 to 0.6%.

Mn Mn has an effect of preventing hot embrittlement due to S, and therefore, it is preferable that 0.02% or more is present. However, the presence of a large amount generally causes deterioration of moldability. A steel type having a small amount of C, such as the steel of the present invention, has a slight adverse effect, but is not preferable from the viewpoint of increasing the recrystallization temperature. For this reason 0.
The upper limit is 3%.

N N lowers the elongation property value and therefore deteriorates the tensile properties in press working and the like, so the smaller the better, the better the upper limit is 0.006%.

S, [O] S and [O] are improved in moldability as the existing amount of each is smaller. Therefore, S is 0.01% and [O] is 0.01% as the upper limit, and the smaller the upper limit, the better.

sol Al solAl has a role as a deoxidizing agent during steel melting, and is also effective for fixing and detoxifying N as AlN. Therefore, 0.01% or more is added. However, if the amount is too large, not only the effect is saturated, but also the surface property is deteriorated due to the increase of non-metallic inclusions, or the recrystallized grains are miniaturized, which is not preferable. Therefore, the upper limit is 0.06%.

It should be noted that Si is added for deoxidation, but in this case the upper limit is
0.05%. If it exceeds 0.05%, the surface oxide scale may deteriorate.

Further, in the present invention, the temperature of the slab is maintained so that the slab after solidification does not fall below 550 ° C.

After the slab is melted, the slab is kept at 550 ° C. or higher for the following reason. The present invention is intended to precipitate AlN and the like during heating before hot rolling in order to prevent harmful fine precipitates that precipitate in the ferrite region after hot rolling. 550
When cooled to below ℃, these precipitates will be exceeded in the slab cooling process, decomposition and re-solid solution will occur during reheating soaking, and fine re-precipitation will occur after hot rolling. On the other hand, when the lower limit of the cooling temperature is set to 550 ° C or higher, AlN and the like do not precipitate during the slab cooling, and the state becomes supersaturated.

In the present invention, heating is performed at a heating rate of 8 ° C./min or more in order to release the supersaturated state and precipitate AlN and the like. In other words, by reheating at 8 ° C./min or more, the supersaturated state is released during hot rolling or during reheating / soaking, and AlN or the like precipitates.

However, in the present invention, since hot rolling is started at a temperature of 1050-1180 ° C or higher, the slab does not reheat at 8 ° C / min or higher.
Perform when the temperature falls below 1050 ℃.

By reheating at 8 ° C / min or more, the supersaturated state is released during hot rolling or during reheating / soaking, and AlN or the like precipitates.

Precipitation of AlN, etc. during soaking should be promoted by not precooling the slab below 550 ℃ and reheating to 1050 ～ 1180 ℃ at a heating rate of 8 ℃ / min and hot rolling. The present inventor has found out. The mechanism is not always clear, but during slab cooling AlN
AlN is not dissolved in supersaturated solid solution and AlN dissolved in supersaturated solution becomes a driving force for precipitation in the soaking process by heating at the above heating rate. Conceivable.

In the present invention, winding is performed at 650 ° C or lower.

As described above, the continuous annealed cold-rolled steel sheet needs to be wound at 700 ° C or higher in order to obtain excellent deep drawability in the conventional process. As described above, this is because coarse precipitation of AlN occurs in the winding process at such a high temperature. On the other hand, in the present invention, since this is done in the step before winding, it is not necessary to deposit in the winding step. Therefore,
It can be wound at a low temperature of 650 ° C or less. As a result, the amount of scale is reduced and the shape is improved. Conversely, 65
If wound at 0 ° C. or higher, the hot-rolled grain size grows abnormally and the r value deteriorates.

[Example] The composition steels shown in the table were melted.

A1 to A3, B1, B3, and C1 to C3 have the same composition. C1 to C3 are comparative examples in which the amount of C is higher than the range of the present invention.

All of A1, A2, B1, C1 and C2 were kept above 550 ℃ after solidification. On the other hand, A3, B3, and C3 are comparative examples that were once cooled to room temperature.

A1, A2, B1, C1, C2 were below 1050 ℃, so 1050 to 1180 ℃
Was reheated to the range. The heating rate for reheating is 8 ° C./min or more in all cases.

The winding temperature is 650 ° C or less in all cases except C2. C2 is 700
Performed at ° C.

These hot rolling conditions are also shown in the table.

After hot rolling and cold rolling at 75%, 720 ℃ × 1.5min〜4
Continuous annealing was performed at 00 ° C for 3 min. In addition, the mechanical properties were investigated after a skin pass of 0.6%.

From the table, all the cold-rolled steel sheets (A1, A2, B1) manufactured under the conditions within the scope of the present invention have deep drawability, that is, r
It is clear that the value, E1 has improved dramatically, and that the unit consumption of the heating furnace has been reduced. Therefore, according to the present invention, a cold rolled steel sheet having excellent deep drawability can be manufactured at low cost.

EFFECTS OF THE INVENTION According to the present invention, it is possible to solve the above-mentioned problems of the prior art and provide a method for producing a cold-rolled base material for continuous annealing that can provide a cold-rolled steel sheet product having excellent deep drawability. it can.

Claims (1)

[Claims] 1. By weight%, C; 0.01-0.06% Mn; 0.02-0.3% S; <0.01% sol Al; 0.01-0.06% O; <0.01% N; <0.006%, with the balance iron and In a method for producing a base material of a cold-rolled steel sheet for continuous annealing consisting of unavoidable impurities, after casting a molten metal, a slab is obtained by solidifying the molten metal, and the slab after solidification does not fall below 550 ° C. Temperature is maintained, heating is performed at 105-1180 ° C at a heating rate of 8 ° C / min or more, and then hot rolling is performed to precipitate AlN, and the hot rolling temperature ends at the Ar ₃ transformation point or higher. Then 650
A method for producing a cold-rolled steel sheet for continuous annealing, which comprises winding at a temperature of ℃ or less.