JPH0765113B2 - 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 has the same good deep drawability as in the case of performing high temperature winding in the hot rolling step, and can continuously produce a cold rolled steel sheet product having good surface texture. The present invention relates to a method for manufacturing a base material of a cold rolled steel sheet for annealing.

[Prior Art] It is already known that the deep drawability of cold-rolled steel sheets is dramatically improved by adding carbonitride forming elements such as Ti and Nb to ultra-low carbon steel. 50-31531
Japanese Patent Publication No.

The ultra-low carbon cold-rolled steel sheet containing Ti and Nb exhibits extremely excellent deep drawability because the most harmful C and N for improving the deep drawability is Ti.
This is because they can be fixed as C, TiN, Nb (C, N).

However, the effect of solid solution C and N removal cannot be fully exerted depending on the precipitated form. For example, fine and densely deposited
TiC and the like prevent grain growth during annealing and prevent improvement of r value.

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, the slab made by the continuous casting method or the slab obtained by slab forging after casting is once cooled to room temperature at a cooling rate of 3 to 30 ° C./min, and then heated in a heating furnace for a long time at a high temperature. Is heated (1200 to 1300 ° C. × 1 hour), hot rolled, and wound at a winding temperature of 650 ° C. or higher.

In the above technique, the coiling at a high temperature of 650 ° C. or higher is to obtain deep drawability, that is, r value, in the cold-rolled steel sheet that is continuously annealed and is comparable to that of the conventional cold-rolled steel sheet by box annealing. That is, in the above method, during cooling of the slab
TiC or the like precipitates, and the precipitated TiC or the like re-dissolves during heating and soaking during hot rolling. Re-dissolved Ti
C and the like precipitate after hot rolling, but the distribution state of this precipitate differs depending on the crystal grains. That is, there are crystal grains with a large amount of precipitates and crystal grains with a small amount. Such a phenomenon is remarkable when the cooling rate during solidification is gradually cooled at 50 ° C./min or less and TiC or the like precipitates during slab cooling and reheats or re-dissolves. 650 ℃ there
The high temperature winding causes coarse precipitation of TiC and the like. This is because the higher the winding temperature is, the coarser precipitation of TiC or the like occurs, which is desirable for obtaining 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, TiC and the like precipitated during cooling are decomposed and re-dissolved in solid solution and finely re-precipitated during subsequent hot rolling. This finely re-precipitated TiC, etc. 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 as high as 650 ° C or higher. When the winding temperature is increased in this way, surface scale occurs,
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 kept below 650 ° C. in order to prevent this, continuous annealing cannot produce r-values comparable to those obtained by box annealing.

[Means for Solving Problems] The above problems are in terms of weight%; C; 0.02 to 0.015% Mn; 0.05 to 0.4% S; <0.01% sol Al; 0.01 to 0.07% O; <0.01% N; <0.006% and Ti and / or Nb sol Ti; 0.02-0.15% sol Nb; 0.02-0.15% sol Ti / (C + N) atomic concentration> 1.0 sol Nb / (C + N) atomic concentration> 1.0, balance iron And in the method of producing a base material of continuous annealing cold-rolled steel sheet consisting of unavoidable impurities, after casting the molten metal, to obtain a slab by solidifying the molten metal, so that the slab after solidification does not fall below 550 ° C. The temperature of the slab 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 TiC, and the hot rolling temperature is set to an Ar ₃ transformation point or higher. Finished and 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 Component Limitation) When the amount of Cc increases, the amount of TiC precipitation increases and the recrystallization temperature increases, so 0.015 in order to enable formability by low temperature and short time continuous annealing. % Is the upper limit. However, if the amount of C is too small, it becomes difficult or impossible to perform melting in a steelmaking furnace on a general commercial scale, and the formability is deteriorated due to a rapid increase in the oxygen content in the steel. Therefore
The lower limit is 0.002%.

Mn Mn has an effect of preventing hot embrittlement due to S, and therefore it is preferable that Mn is present in an amount of 0.05% or more. 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 40%.

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 or the like. 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 0.07%.

Ti, Nb Ti, Nb enhances the deep drawability as the amount of addition increases, and in particular, by completely fixing C and N in steel as carbonitrides (Ti (C, N)), the deep drawability is improved. It is known to be extremely good. However, it also causes a large increase in manufacturing cost. Therefore, in the present invention, the Ti and Nb addition amounts
The atomic concentration ratio (sol (Ti, Nb)) of the total amount of solTi and / or solNb excluding titanium oxide in the added Ti and the total amount of C and N is set to 0.2% or more while the upper limit is 0.15%. /
By defining (C, N) to be 1.0 or more, it is possible to maximize the effect of adding Ti and Nb and sufficiently enhance the deep drawability.

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.

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 intends to precipitate TiC 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 below ℃, these precipitates occur in the slab cooling process, decomposition and re-solid solution occur during reheating soaking, and fine re-precipitation occurs after hot rolling. On the other hand, when the lower limit of the cooling temperature is set to 550 ° C. or higher, TiC and the like do not precipitate during slab cooling, resulting in a supersaturated state.

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

Precipitation of TiC, etc. during soaking is promoted rather by cooling the slab to less than 550 ° C and reheating it to 1050-1180 ° C at a heating rate of 8 ° C / min and hot rolling. The present inventor has found out. The mechanism is not always clear, but TiC is cooled during slab cooling.
C and N are in solid solution in supersaturation without progressing precipitation, and C and N in solid solution in supersaturation become the driving force for precipitation in the soaking process by heating at the above heating rate. Etc. are considered to be deposited.

Therefore, there is no fine precipitation that precipitates in the ferrite region and hinders the grain growth during cold rolling-annealing, and a continuously annealed cold rolled steel sheet having a high r value can be obtained. Further, although steel having a carbonitride forming element added thereto has a drawback that the recrystallization temperature becomes high, the recrystallization temperature can be significantly lowered in the present invention.

The hot rolling start temperature is set to 1050-1180 ° C in order to promote the precipitation of TiC and the like and save heat. If the temperature is lower than 1050 ° C, it will be difficult to secure the temperature during finish rolling.

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

As described above, the continuous annealed cold rolled steel sheet must be wound at 650 ° C or higher in order to obtain excellent deep drawability in the conventional process. As described above, this is because coarse precipitation of TiC and the like occurs in the high temperature winding process. 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 is possible to wind at a low temperature of 650 ° C or lower.
As a result, the amount of scale is reduced and the shape is improved. On the contrary, if rolled at 650 ° C or higher, the hot rolled grain size grows abnormally and
It causes deterioration of the value.

[Examples of the Invention] The composition steels shown in the table were melted.

A1 to A3, B1, B3, C1 to C3, and D1 to D3 have the same composition. D1 to D3 are comparative examples containing no Ti and Nb.

All of A1-A2, B1, C1-C2, D1-D2 were kept above 550 ℃ after solidification. On the other hand, A3, B3, C3, and D3 are comparative examples that were once cooled to room temperature.

A1-A2, B1-B2, C1-C2, D1-D2 were all below 1050 ℃, so they were reheated to the range of 1050-1180 ℃. The heating rate for reheating is 8 ° C./min or more in all cases.

The coiling temperature is 650 ℃ or less in all cases except B3 and D3. B3, D
3 was performed at 690 ° C.

These hot rolling conditions are also shown in the table.

After hot rolling and cold rolling at 75%, 800 ℃ × 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, C1, C2) manufactured under the conditions within the scope of the present invention have dramatically improved deep drawability, that is, r value and elongation (E1). In addition, it is clear that the unit consumption of the heating furnace is being 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: 0.02 to 0.015% Mn; 0.05 to 0.4% S; <0.01% sol Al; 0.01 to 0.07% O; <0.01% N; <0.006% and Ti and / or Nb. sol Ti; 0.02 to 0.15% sol Nb; 0.02 to 0.15% sol Ti / (C + N) atomic concentration> 1.0 sol Nb / (C + N) atomic concentration> 1.0 with the balance iron and unavoidable impurities In a method for producing a base material of a rolled steel sheet, after casting a molten metal, a slab is obtained by solidifying the molten metal, and the temperature of the slab is maintained so that the slab after solidification does not fall below 550 ° C, 8 ° C / Precipitate TiC by performing hot rolling after heating to 1050-1180 ° C. at a heating rate of min or more, and finish the hot rolling temperature at Ar ₃ transformation point or higher, 650
A method for producing a cold-rolled steel sheet for continuous annealing, which comprises winding at a temperature of ℃ or less.