JP2583165B2 - Manufacturing method of cold rolled steel sheet with excellent press formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel sheet which does not cause roughening during press forming and has extremely excellent workability.

[0002]

2. Description of the Related Art Cold rolled steel sheets having excellent press formability are manufactured by using Ti.
It is mass-produced by continuously annealing ultra-low carbon steel to which carbonitride forming elements such as Nb and Nb are added. recent years,
The application of high-workability cold-rolled steel sheet, for example, taking automotive parts as an example, does not stop at small parts that have become more complex in shape than before. From time to time, it has spread to large parts that are molded as an integral part. Furthermore, from the viewpoint of reducing the weight of automobile bodies,
There is a demand for a thinner gauge than ever before.

[0003] When such a thin and wide cold-rolled steel sheet is manufactured by continuous annealing, since the steel sheet is easily broken in an annealing furnace, measures such as reducing the line speed and lowering the annealing temperature are taken. This is necessary, and the advantages of continuous annealing cannot be fully utilized in terms of operation and material.

[0004]

On the other hand, in the case of box annealing, which has been conventionally used, there is hardly any size restriction of the steel sheet as in continuous annealing. High-temperature annealing such as annealing is not always necessary.

A method for producing ultra-low carbon steel to which Ti or Nb is added by box annealing is described in, for example, Japanese Patent Application Laid-Open No. 63-210.
No. 243 and Japanese Patent Publication No. 29728/1990 propose a Ti-added ultra low carbon steel sheet.

However, when a very low carbon cold rolled steel sheet is actually manufactured by box annealing, there are the following problems.
This has been clarified by the study of the present inventors.

[0007] Due to the high purity of the steel, the grain growth is good,
If the annealing temperature is too high, the crystal grains are easily coarsened, and there is a concern about roughening during pressing. In particular, if the ambient gas temperature is too high, coarse particles are likely to be generated on the coil surface or end even if the inside of the annealing coil is fine and fine. In addition, when exposed to a high temperature range for a long time, abnormally coarse grains due to secondary grain growth (particle number 2 to 4)
Is easy to occur.

Normally, when box annealing is performed, a gas mainly composed of N _{2, which} is called HNX gas, is used as an atmosphere gas. As a result, the workability is significantly deteriorated.

Usually, in box annealing, to improve productivity,
In many cases, a plurality of coils of two or more coils are simultaneously annealed in the same furnace. In this case, when the cold-rolled coil inserted at the same time is a low-carbon steel, the C content in the extremely low-carbon steel is annealed. It rises later, and deterioration of workability due to carburization is observed.

[0010] The present invention solves the above-mentioned problems of the prior art, and increases the crystal grain coarseness accompanying the production by box annealing of an ultra-low carbon cold rolled steel sheet to which a carbonitride forming element such as Ti or Nb is added.
It is an object of the present invention to provide a method capable of effectively solving quality problems such as nitriding and carburizing.

[0011]

Means for Solving the Problems In order to solve the above problems, the present inventors have conducted intensive studies on the conditions of conventional box annealing together with the composition of steel, and as a result, have completed the present invention. It is.

That is, according to the present invention, C ≦ 0.005%;
N ≦ 0.005%, Mn ≦ 0.30%, P ≦ 0.020
%, Al: 0.010 to 0.100%, and 4C
+ 3.42N ≦ Ti ≦ 0.100%, and if necessary, further contain Nb ≦ 0.030%, with the balance consisting of Fe and inevitable impurities hot rolled and cold rolled,
At a plurality of coils in a box annealing to process simultaneously in the same furnace
H ₂ + N ₂ containing H ₂ or 30% or less of N ₂ is used as the atmosphere gas, and the temperature of the atmosphere gas is 800 ° C. or less and the coldest point of the coil is equal to or higher than the recrystallization temperature for 5 hours or less. The gist of the present invention is a method for producing a cold-rolled steel sheet having excellent press formability, characterized by heating.

Hereinafter, the present invention will be described in more detail.

[0014]

[Action]

First, the reasons for limiting the chemical components in the present invention will be described.

C: Since C deteriorates ductility, the smaller the amount, the more preferable. In the present invention, C is Ti
However, from the viewpoint of cost reduction by reducing the Ti content, the C content is preferably as small as possible, and the upper limit is made 0.005%.

N: Since N also deteriorates ductility similarly to C, the smaller the amount, the more preferable. Also, in the present invention, N is precipitated as a nitride by Ti. The smaller the amount, the better. The upper limit is 0.005%.

Mn: Since Mn deteriorates deep drawability and hardens steel, the smaller the better, the better the upper limit is 0.30.
%.

P: In the case of exposing extremely low carbon steel containing a large amount of Ti to a high temperature by box annealing for a long time, P: P
Is preferred because it forms a phosphorus compound, deteriorates ductility, and significantly impairs processability. The upper limit is 0.020.
%. In addition, from the viewpoint of further improving ductility, the smaller the better, the better. Particularly when securing high ductility, 0.010 is preferred.
% Or less.

Al: Al is at least 0.0 for deoxidation.
It is necessary to add 10%. However, 0.10
Even if added in excess of 0%, the steel will harden, so the Al content should be less than 0.1%.
The range is 010 to 0.100%.

Ti: Ti fixes at least C and N in order to fix C and N in the steel as precipitates and to improve deep drawability.
Requires the addition of at least the equivalent of (48/12) × C and (48/14) × N. Therefore, the lower limit of the Ti amount is 4C +
3.42 N (%). On the other hand, even if it is added excessively, the effect is only saturated and the cost increases, so the upper limit is set to 0.1.
100%.

Nb: Nb is also a carbide-forming element and, like Ti, improves deep drawability, and may be added as necessary. In particular, when the purpose is to improve the anisotropy or when the grain size is to be further reduced in order to prevent roughening at the time of pressing, it is better to add positively. However, if the Nb content is too large, the recrystallization temperature rises. Therefore, the upper limit of the Nb content is 0.030%.

The smaller the impurity, the better. For example, Si,
It is desirable to reduce S and the like to Si ≦ 0.10% and S ≦ 0.020%, respectively.

Next, the manufacturing method of the present invention will be described.

The steel having the above-mentioned chemical components is formed into a slab by continuous casting or ingot making in a conventional manner, and the slab is subjected to hot rolling after being uniformly heated in a heating furnace. On this occasion,
The slab may be either cooled once to normal temperature or charged into a heating furnace at a high temperature. Further, hot rolling may be performed directly at a high temperature without being charged into the heating furnace. The slab heating temperature is not particularly limited.
For example, heating may be performed at 1000 to 1300 ° C., but it is preferable to pay attention to the fact that the lower the temperature, the larger the precipitates are and the better the material is.

Rough rolling and finish rolling may be performed in the usual manner, but in order to minimize the temperature difference between the end and the center, it is not possible to locally heat the edge using induction heating or gas. It is desirable from the viewpoint of improving the thickness accuracy and uniforming the material. As shown in FIG. 1, the crystal grain size of the annealed sheet corresponds well to the crystal grain size of the hot-rolled sheet. When the crystal grain size of the hot-rolled sheet is small, the crystal grain size of the annealed sheet tends to be small. In the present invention, as described above, hot rolling is basically good according to a conventional method, but in particular, in order to prevent steel from having high purity and coarse grains, the hot finishing temperature and It is effective to perform cooling after hot rolling under the following conditions. Hot rolling finishing temperature: 880-930 ° C Cooling: 800 ° C at 40-100 ° C / sec immediately after rolling
Cool down to below.

That is, the finishing temperature is usually made random at the stage of hot-rolled sheet, so that the texture after annealing becomes more preferable due to the deep drawability, so that the Ar ₃ transformation point is approximately 890 to 900 ° C. Therefore, when the finishing temperature is 880 to 930 ° C., the hot-rolled sheet crystal grains are the finest grains, and therefore, the annealed sheet crystal grains are also fine grains (see FIG. 2), and even when the r value is equal to or less than the Ar ₃ transformation point, There is no rapid deterioration, but rather lower when the temperature greatly exceeds the transformation point (see FIG. 3).
This is considered to be related to the r-value of the texture of the hot-rolled sheet and the magnitude of the effect of the fine grains of the hot-rolled sheet.
Above, it is considered that the r value is degraded because the effect of coarsening the hot-rolled sheet fine grains is remarkable, and that the r value is degraded at 880 ° C. or lower because the adverse effects of the fine grains and the texture are both effective.

For the cooling after rolling, it is effective to carry out strong cooling with cooling water in a time as short as possible after rolling, in order to reduce the grain size. When the cooling rate is 40 ° C./sec or less, the effect of grain refinement is weak. At 100 ° C./sec or more, there is a problem of equipment cost, and a large amount of cooling water is sprayed on the coil.
The strong cooling end temperature is preferably set to 800 ° C. or less, because crystal growth can be suppressed due to a longer residence time in a high temperature region with slow cooling. The winding temperature is not particularly limited, but 630 to 730 ° C.
High temperature winding is preferred.

After hot rolling, cold rolling is performed. The cold rolling rate is not particularly limited, and a conventional method may be used. A rolling reduction of 65 to 85% is desirable from the viewpoint of deep drawability.

The conditions of box annealing performed after cold rolling are the most important in the present invention as described below. In the present invention, an open coil is applicable as long as box annealing is performed.

Heating rate: The heating rate is not particularly specified.
In order to more positively reduce the grain size of the box annealing, as shown in FIG. 4, the average heating rate, that is, (the coldest soaking temperature-the temperature before heating) / (the coldest soaking temperature). (Time required to reach the temperature) is preferably 50 ° C./hr or more. The upper limit of the heating rate is 300 ° C./hr due to facility restrictions.

Soaking temperature, time: In soaking, it is necessary that the coldest point of the coil is heated above the recrystallization temperature. Lion, the soaking of more than 5 hours so easily lead to abnormal coarsening due to secondary grain growth of crystal grains, soaking time is not more than 5 hours.

Composition and temperature of atmosphere gas: The atmosphere gas is substantially H ₂ , but may contain N ₂ . That is, if N ₂ is large, nitridation is likely to occur, so N ₂ can be contained if it is 30% or less. The higher the amount of H _{2, the} better, but it is 70% H ₂ + 30% N _{2. In} practice, the material degradation due to nitridation can be reduced to a level that does not cause a problem. Therefore, the upper limit of N ₂ is set to 30%. In addition, instead of H ₂ , A
Even if an inert gas such as r is used, nitriding can be prevented. If the temperature of the atmosphere gas is not set to 800 ° C. or lower, the crystal grains are likely to be coarsened particularly at a portion directly in contact with the atmosphere gas, such as the surface and the end of the annealing coil.

Simultaneous charging coil: In the box annealing according to the present invention, a plurality of coils are simultaneously processed in the same furnace. In an embodiment, the plurality of coils have a chemical component within the scope of the present invention. In some cases, among the plurality of coils, any one of the coils has a chemical component within the scope of the present invention, while the other coil (coil outside the scope of the present invention) has a component content within the scope of the present invention ( (Except C)
This is the case for the coil whose C content is within the scope of the present invention even if the above condition is not satisfied. As in the above embodiment, a coil having a chemical component outside the scope of the present invention can be inserted simultaneously with a coil having a chemical component within the scope of the present invention, but the latter has a C content of 0.005% or less. There is. This is because when the C content of the latter coil exceeds 0.005%, the amount of carburizing of the coil with the chemical components within the scope of the present invention increases, and the deterioration of the material reaches a level that cannot be ignored. . Although the details of the mechanism of carburization are not necessarily clear, it is important to note that the chemical reaction with atmospheric gas in a high temperature range and that the steel having a chemical composition within the scope of the present invention contains Ti which is easily linked to C. We think that it causes charcoal.

After annealing, temper rolling may be performed as necessary to adjust the grain size. However, from the viewpoint of ensuring ductility, a skin pass under light pressure of about 0.5% or less is preferable. Further, even if various platings such as zinc plating and chromium plating are performed according to the application as a post-process, the effect of the present invention is not impaired at all.

Next, examples of the present invention will be described.

[0037]

【Example】

[Table 1]

[Table 2] Steel slabs having the scientific composition shown in Table 1
After heating to 00 ° C. to form a hot-rolled steel sheet having a thickness of 4.0 mm at a finishing temperature of 915 ° C., it is cooled to 750 ° C. at a rate of 70 ° C./sec, then cooled at 30 ° C./sec, and wound at 700 ° C. I took it. After pickling, cold rolling to a thickness of 0.8 mm, Table 2
Each was subjected to box annealing under the following conditions. After performing skin pass rolling of 0.3%, the results of examining the material properties are shown in Table 2.
Shown in In Table 2, the C amount of the simultaneously charged coil indicates the maximum C amount of the simultaneously charged coil, and the grain size number of the annealed sheet was measured at the center of the sheet thickness in the width and length directions. The carburizing amount and the nitriding amount indicate the amounts of change in the amounts of C and N before and after annealing, respectively.

As is clear from Table 2, in the present invention example, 5
High elongation of 0% or more, high r-value of 2.0 or more, fine-grained granules with a level that does not cause roughening during pressing (No. 7 or more)
Is shown. On the other hand, in the comparative example, even if the chemical component is within the range of the present invention, the annealing conditions are out of the range of the present invention (No. 2,
In the case of 3, 5, 7), deterioration of the material due to carburization or nitriding or coarsening of crystal grains are recognized. Further, when the chemical component is out of the range of the present invention (No. 8, 9, 10, 11, 12),
The r value, elongation, or level of YP are worse than those of the present invention.

[0039]

As described in detail above, according to the present invention,
In box annealing of ultra-low carbon cold rolled steel sheets to which carbonitride forming elements such as Ti and Nb are added, there is no quality problem such as grain coarsening, nitriding, carburizing, etc. It is possible to manufacture a cold-rolled steel sheet which is extremely excellent in workability and free of any problem.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the grain size of a hot-rolled sheet and the grain size of an annealed sheet.

FIG. 2 is a diagram showing a relationship between a hot-rolling finishing temperature and a hot-rolled sheet crystal grain size.

FIG. 3 is a graph showing the relationship between the hot-rolling finishing temperature and the grain size and r-value of an annealed plate.

FIG. 4 is a diagram showing the relationship between the annealing heating rate and the grain size of an annealed plate.

Claims (5)

(57) [Claims] 1. In terms of% by weight (the same applies hereinafter), C ≦ 0.005.
%, N ≦ 0.005%, Mn ≦ 0.30%, P ≦ 0.020
%, Al: 0.010 to 0.100%, and Ti as 4C +
In the case of box annealing in which 3.42N ≦ Ti ≦ 0.100% is included and the balance is Fe and unavoidable impurities, steel is hot-rolled and cold-rolled, and a plurality of coils are simultaneously processed in the same furnace. H ₂ or 30% or less N ₂ as gas
Using H ₂ + N ₂ containing at least an atmosphere gas temperature of 800 ° C. or lower and a coil coldest point at a temperature higher than the recrystallization temperature for 5 hours or less. Manufacturing method of rolled steel sheet. 2. The method of claim 1 wherein said steel further comprises Nb ≦ 0.030%. 3. The method according to claim 1, wherein an average heating rate of the box annealing is 50 ° C./hr or more. 4. A hot-rolling machine in which a rolling finish temperature is 880.
-930 ° C, immediately after rolling at 40-100 ° C / sec
The method according to claim 1, 2 or 3, wherein the method is cooled to not more than 00 ° C. 5. A coil other than at least one of a plurality of coils to be charged in the same furnace.
The coil which does not satisfy the component content (except for C) described in (1), but satisfies the C content.
The method described in.