JPS6082616A - Production of extra low carbon cold rolled steel plate for deep drawing - Google Patents

Abstract

PURPOSE:To produce cold rolled steel plate having high deep drawability from an extra low carbon steel as blank material and can be subjected to continuous annealing in a process for hot rolling and then cold rolling an extra low carbon steel having a specified compsn. for breadth rolling of a rough rolling stage in hot rolling. CONSTITUTION:A billet consisting by wt%, <=0.004 C, further if necessary, <=0.05 in total of at least two kinds among W, Ti, Nb, B, Zr, Al, and rare earth element, and residual wt% Fe is hot-rolled. In the rough rolling stage of this hot rolling, broad side rolling or width-reducing rolling of >=20% total breadth draft is performed. By performing such rolling, a cold rolled steel plate of extra low carbon content for deep drawing having superior deep drawability and surface property is produced with high productivity.

Description

[Detailed Description of the Invention] The present invention relates to a method for producing a cold rolled steel sheet for ultra-low carbon deep drawing,
In particular, it relates to a method for improving deep drawability by width rolling in the rough rolling process of hot rolling.

Cold-rolled steel sheets used for automobile fender panels, gasoline tanks, etc. require excellent deep drawability. In order to obtain excellent deep drawability, the mechanical properties of the steel sheet require low yield strength, high ductility such as high elongation, and a high Lankford value. Conventionally, low-carbon aluminum-killed steel sheets annealed by box annealing have been used for this type of application, but the biggest drawback of this steel sheet is that it is difficult to manufacture by continuous annealing. Compared to conventional box annealing, the continuous annealing method has extremely short processing time and high productivity, and the material is uniform, with Mn,
It has advantages such as less concentration of alloying elements such as Si on the surface and excellent chemical conversion treatment properties in pre-painting treatment. However, from a metallurgical point of view, continuous annealing is a rapid heating and cooling process.
The growth of crystal grains is insufficient and the grains tend to become fine, which is preferable for the r value (1111 recrystallization coarse weave is difficult to develop, so it is not necessarily suitable for producing steel sheets for deep drawing.

Under these circumstances, there have been numerous reports of methods for producing cold-rolled steel sheets for deep drawing by continuous annealing, but the most promising method is to reduce the carbon content in the steel to 0.005% or less. An example of this is JP-A-55-5.8333. However, if the material is simply made of ultra-low carbon steel, the quality of the material will not improve that much. The reason for this is obvious and difficult, but it is probably due to the large in-plane anisotropy of the material. A large in-plane anisotropy means that when a test is performed in each direction of the surface of a steel plate, the material quality changes greatly depending on the direction.

With ultra-low carbon steel, the quality of the material in one direction improves markedly, but the quality of the material in other directions does not change at all, but it is thought that it may even deteriorate. In such a case, not only will the increase in the average material level in each direction be small, but in actual press forming, large selvages will occur and the yield of the material will decrease.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method for manufacturing a cold-rolled steel sheet made of ultra-low carbon steel and capable of continuous annealing and having excellent deep drawability.

The gist of the present invention is as follows.

That is, it contains C: 0.004% or less by weight ratio,
Furthermore, W, Ti, Nb, B, as necessary.
A steel plate containing 0.05% or less of one or more selected from Zr, At, and rare earth elements, either alone or in total, with the balance consisting of Fe and unavoidable impurities, is hot-rolled and then cold-rolled. In the method for producing a cold rolled steel sheet for deep drawing with a low internal diameter, the full width reduction ratio is 2 in the rough rolling step of the hot rolling.
This is a method for producing a cold-rolled steel sheet for ultra-low carbon deep drawing, characterized by comprising a step of width rolling or width rounding rolling of 0% or more.

The reasons for limiting the components in the present invention will be explained. For high ductility, it is desirable to have a small amount of C, but in particular 0.004
If it exceeds 0.004%, not only will the effect of the width reduction of the present invention disappear, but also the adverse effect on the material will become significant, so it is limited to 0.004% or less. The cold rolled steel sheet for deep drawing of the present invention has C: 0.004%
The following limitations are the basic ingredients, but if necessary, W,
Ti, Nb, B.

If one or more of Zr, AL and rare earth elements are contained individually or in total at 0.05% or less, the material will be stable and the object of the present invention can be achieved even more effectively. However, the addition of these elements alone or in total is 0.05%.
If the content exceeds 0, the effect will not only reach saturation, but also cause a decrease in ductility, deterioration of surface quality, etc., so these contents should be reduced to 0.
．． It was limited to 0.05% or less. There is no need to particularly regulate the amounts of component elements other than those listed above, but in order to manufacture high-strength steel sheets with an excellent balance of strength and ductility, Mn and Si must be present at 1.5% or less.
The content of P is preferably 0.15% or less.

Next, a method for manufacturing a cold-rolled steel sheet having the above-mentioned limiting components will be explained. A combination of a converter and a degassing device is suitable for the melting process, and either blooming or continuous casting may be used for the next process of producing steel billets.

The hot rolling process of the steel billet is the most important point of the present invention,
Regarding the heating of the steel billets, the present invention is characterized by adding width rolling or width-rolling with a full width reduction rate of 20% or more in the rough rolling process of the steel billets in terms of temperature and method. . This full width reduction may be performed using one bus or multiple passes, and in the case of multiple passes, the total width may be 20% or more. It is preferable that this full width reduction ratio is 30% or more. Here, the full width reduction ratio refers to the percentage of the widened width in the case of width rolling, and the percentage of the narrowed width in the case of width rounding rolling, to the width before rolling. Further, a combination of width rolling or width rolling (hereinafter both are referred to as width rolling) and normal rolling is also possible, and in this case, the total reduction ratio of normal rolling is preferably 70% or more. The rolling speed for width rolling is preferably 1Qm/min or more. The type of rolling mill for width rolling does not matter as long as width rolling is possible.

Although the details of the reason for the effect of width rolling are unknown, the coarse structure of ultra-low carbon steel slabs or unique textures such as textures in specific directions are improved and broken by the addition of strain in the width direction, resulting in deep drawing. It can be inferred that the gender is the same as above. However, if the full width reduction ratio in width rolling is less than 20%, the above-mentioned structure improvement is not sufficient.
limited to the above.

Alternatively, a width roughing mill may be installed on the exit side of the continuous casting machine to perform width rolling, and then heating or heat retention treatment may be performed to provide hot rolling.

Finishing hot rolling of the steel material that has undergone the above rough rolling has a finishing temperature of 50
0 to 900°C, and the winding temperature is preferably 200 to 700°C.

Next, as for the recrystallization annealing after pickling and cold rolling of the hot rolled steel strip, continuous annealing is preferable from the viewpoint of productivity, uniform material quality, etc. as described above.
It is also possible to manufacture using the box annealing method.

Annealed materials may be subjected to temper rolling of 10% or less for the purpose of shape correction.

As mentioned above, the cold-rolled steel sheet manufactured by the method of the present invention, which is manufactured by rough rolling (/''1), width rolling, and continuous annealing after cold rolling, has good surface quality as well as good deep drawability. Met.

Example: Steel having the composition shown in Table 1 was melted using a bottom-blowing converter and an RH degassing process, and cast into a continuous casting machine to form a sheet with a thickness of 220 to 280 mm.
The slab had a plate width of 900 to 1400 taps. Among these slabs, A10 was directly hot-rolled, and the remaining slabs were once cooled to near room temperature, then soaked at the heating temperature shown in Table 2, and rolled into width rolls installed before the rough rolling mill. Width rolling was performed at the full width reduction ratio shown.

The sample materials 5 and 7 are width rolling, and all others are width rolling, and the number of passes is 1 to 5. After width rolling, hot rolling was performed using a 5-row rough rolling mill and a 7-row finishing mill at the hot rolling finishing temperature and coiling temperature shown in Table 2.

The obtained hot rolled steel strip was pickled and cold rolled at a cold rolling reduction of 75 to 79% to form a copper strip having a thickness of 0.8 mm.

The cold-rolled steel strip is soaked at 820℃ in a continuous annealing line.
Cooling treatment was performed at 20°C/SeC. After this annealed steel strip was subjected to 0.5 to 12% temper rolling, its mechanical properties were measured, and the results are also shown in Table 2. The tensile test was performed using TIS No. 5 in three directions: 0 degrees, 45 degrees, and 95 degrees with respect to the rolling direction, and the average values of each were yield stress YS, tensile strength ITS, and elongation Et. , Lankford value lower, and in-plane anisotropy ΔEt,
I got △r.

Sample material A2 does not satisfy the limiting condition of the present invention in terms of C content, and sample material 3 has a full width reduction ratio lower than the limiting condition of 20%, and both are comparative examples.

It can be seen from Table 2 that all of the inventive examples have superior ductility and deep drawability compared to the comparative examples. In addition, the sample material! 9, 11.12 are Mn, P + S, respectively
This is a high-strength steel sheet for deep drawing with an increased amount of .

As is clear from the above-mentioned embodiments of the present invention, the present invention is to perform width rolling or width rounding with a full width YL reduction ratio of 20% or more in the rough rolling process of hot rolling of ultra-low carbon steel having limited components. As a result, even if continuous annealing was performed after cold rolling, an ultra-low carbon cold-rolled m-plate for deep drawing with excellent deep drawability and surface texture could be produced with high productivity.

Agent: Patent Attorney Takeo Nakaji

Claims (1)

[Claims] (1) Contains C: 0.004% or less in weight ratio, and further contains W, Ti, Nb + B, Zr, as necessary.
Ultra-low carbon steel that contains 0.05% or less of one or more selected from AA and rare earth elements, either alone or in total, with the balance consisting of Fe and unavoidable impurities, is hot-rolled and then cold-rolled. A method for producing a cold rolled steel sheet for deep drawing, characterized in that the rough rolling step of the hot rolling includes a step of performing width rolling or width rolling with a full width reduction of 20% or more. A method for producing cold-rolled steel sheets for deep drawing.