JPS5989723A - Manufacture of steel sheet for working by continuous casting and direct hot rolling - Google Patents

Abstract

PURPOSE:To obtain a steel sheet for working with superior intrasurface anisotropy and surface properties by continuously casting a dead soft steel contg. a very small amount of a rare earth element, Ca, Ti or Mg, and hot rolling it in the red-hot state. CONSTITUTION:A molten steel consisting of, by weight, <=0.015% C, <=0.030% S, <=0.08% O, 0.002-0.040% in total of >=1 kind among a rare earth element, Ca, Ti and Mg, and the balance Fe with inevitable impurities is continuously cast to form a billet. The billet is hot rolled in the red-hot state. A steel obtd. by this method has superior surface properties, low yield strength, high elongation and a high Lankford value as well as superior workability.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing thin steel sheets for processing by continuous casting and direct hot rolling.

Traditionally, hot-rolled steel sheets and cold-rolled steel sheets are manufactured by using a blooming method or continuous casting method, then cooling the steel billets to near room temperature and then subjecting them to surface treatments. After going through the above steps, it was reheated in a heating furnace, soaked, and then hot rolled. However, the reheating method requires (a) a large amount of energy to heat;
There were serious drawbacks such as (b) a decrease in yield due to the formation of oxides and (c) an adverse effect on quality due to an increase in internal oxidation products. On the other hand, recent advances in continuous casting technology have made it possible to stably produce slabs by KIJ without requiring surface care. Based on the above background, the method of continuous hot rolling after continuous casting (hereinafter referred to as C
The C-DR method (referred to as C-DR method) has been adopted for manufacturing thin steel sheets.

On the other hand, hot-rolled steel sheets and cold-rolled steel sheets used for purposes such as drawing and bending are required to have low yield strength and high elongation, and furthermore, cold-rolled steel sheets are required to have a high Lankford value (r value). Hot-rolled steel plate for this narrowing process.

In the production of cold-rolled steel sheets, a carbon content of 0.015% or less or an extremely low carbon density is often used. However, it has become clear that when this ultra-low carbon steel is used for manufacturing by the CC-D It method described above, there are the following serious drawbacks compared to the conventional reheating method.

A) Large in-plane anisotropy of mechanical properties. If the in-plane anisotropy is large, ears will occur during drawing processing, and cracks and wrinkles will occur due to non-uniform deformation during other processing as well.

B) A mixed grain structure is generated. Coarse crystal grains and fine grains become mixed, resulting in rough surface during press processing.

This causes problems such as wall cracks.

C) Surface quality deteriorates due to hot rolling cracks.

During hot rolling, cracks are likely to occur on the rolled surface and side surfaces, and oxides may be included, significantly deteriorating the surface quality of the steel sheet. In particular, since cold-rolled steel sheets are used for the outer panels of automatic vehicles, an unsightly surface is a fatal flaw.

An object of the present invention is to provide a method for manufacturing thin steel sheets for processing by continuous casting and direct hot rolling, which can solve the problems of the prior art described above.

The gist of the present invention is as follows.

That is, in terms of weight ratio, c + o, ots% or less, S: 0
．． 030% or less, o+o, os% or less, and 1 selected from among rare earth elements, Ca, TI, and Mg.
The method comprises the steps of: continuously casting a steel of 0.002 to 0.040 in total of one or more kinds into a slab; and successively hot rolling the slab in a red-hot state. This is a method for manufacturing thin steel sheets for processing by continuous casting and direct hot rolling.

The present invention is based on the addition of trace amounts of rare earth elements (hereinafter referred to as REM), Ca, T1Mg, etc. to ultra-low carbon steel, and CC-DR.
This method was chosen based on the knowledge that steel sheets with small in-plane anisotropy, a well-defined grain structure, and excellent surface quality can be obtained by this method.

The reason why the components of the thin steel sheet for processing of the present invention are limited will be explained.

C+ C has a great influence on the workability of steel sheets.

If it exceeds 0.015%, it causes deterioration of ductility and drawability, so the upper limit was set at 0.015%.

If S, O+ S exceeds 0.030%, and 0 exceeds 0.08%, the increase in inclusions will cause significant deterioration of the material, especially ductility, and adverse effects on the surface texture, so SIO, 0.030% or less,
It was limited to o+o, os% or less.

Flea LEM, Ca, Ti, M
The important point in the present invention is the addition of trace amounts of g sREM, Ca, T'i, and Mg, and one or more of these elements should be added in a total amount of O, On2 to 0.040%. However, it reduces the in-plane anisotropy of the steel sheet material, leading to improvements in surface texture and grain structure. Although the reason for this is not clear, the following may be considered.

That is, REV, Ca, Ti, and Mg are all elements that form oxides and sulfides at high temperatures. On the other hand, C: o, o
Although little is known about the characteristics of the grain structure and texture of ultra-low carbon steel with i5% or less during casting or before the γ→α transformation, it is clear that the formation of compounds such as I (EM) described above occurs during continuous hot rolling. It is fully expected that changes will be made to the previous tissue.According to basic experiments, REM, Ca, Ti
, Mg exhibits its effect when added alone or in combination.

0, (l O less than 2% has no effect (,0,04
If it exceeds 0%, the material quality and surface properties will deteriorate significantly due to an increase in inclusions, so the total amount is 0.002 to 0.040%.
limited to the range of REM, Ca, Ti, M
Among g, REM and Ca greatly contribute to improving the surface properties of the material 1. Therefore, it is preferable to add Ti1Mg in combination with REM and Ca rather than adding it alone.

Although the present invention does not particularly limit other components, N is 0.01% in order to ensure ductility and prevent aging deterioration.
The following are preferred. Also, C in a solid solution state.

Nitride- and carbide-forming elements such as At, B, Nb, and Zr are added in order to further improve the aging properties by fixing NY precipitation, and in particular to obtain a higher r value in cold-rolled steel sheets. That is beneficial.

Next, the manufacturing process of a thin steel plate for processing having the above-mentioned limiting components will be explained. The cooling rate of the slab produced by continuous casting until it is subsequently hot rolled in a red-hot state, the presence or absence of holding treatment, and the presence or absence of side heating are not essential issues in the present invention. Slab slabs also include thin pieces with a thickness of 10 to 100 fins manufactured with sheet bar casters.

The type of hot rolling mill does not matter. For example, a high-reduction rolling mill consisting of several rows or a hot strip mill consisting of a conventional roughing mill and a finishing mill may be used.

Controlling the hot rolling temperature, especially finishing temperature, was extremely important in conventional manufacturing methods, but it is not so important in the method of the present invention (if hot rolling is finished in the range of 600 to 1000°C, The desired steel plate can be manufactured.Cooling conditions after hot rolling The coiling temperature can be set as desired, but coiling at %650°C or below is beneficial for improving pickling properties.Hot rolled steel plate products For this, skin pass rolling with a rolling reduction of 5.0% or less and pickling treatment are carried out as necessary.

In order to produce a cold-rolled steel sheet, a hot-rolled steel sheet may be pickled, cold rolled, and recrystallized annealed by a conventional method. It is advantageous for the cold rolling reduction rate to be 50% or more in improving the r value.

The annealing method may be either box type or continuous type.650~85
Soaking temperatures in the range of 0°C are preferred. In continuous annealing, after soaking, rapid cooling at a cooling rate of 30 to 150°C/sec and overaging treatment at 300 to 500°C may be effective in improving the material quality. The overaging treatment does not need to be carried out in a continuous annealing furnace; a sufficient effect can be obtained by winding the coil into a coil and annealing it in a box, or leaving it to stand for slow cooling treatment. On the other hand, it is also possible to perform plating treatment with molten metal continuously after continuous annealing, and in this case as well, it is advantageous to carry out overaging treatment using the various methods described above.

According to the present invention, a hot-rolled or cold-rolled thin sheet having excellent in-plane anisotropy and surface texture can be obtained by the above manufacturing method. Furthermore, even when a steel plate containing inclusions such as tEM oxides in the steel is enameled, it has excellent enameling resistance and can be used as a enamel steel plate for processing.

Examples Hot-rolled and cold-rolled steel sheets were manufactured from steel having the chemical composition shown in Table 1 under the manufacturing conditions shown in Table 2. Test material J%1~SFi
This is an example of the present invention, and a comparative example of sample materials A6 to A7. Sample material A6 has more C than the limiting conditions of the present invention, and A7t'i (
REM+Ti) is more than the limited condition, ya 8ke REM,
Ca, Ti, Mg? Contains no. The temper rolling shown in Table 2 was performed on cold rolled annealed sheets.

The tensile test value, n value, Lankford value r% aging index AI, etc. using JIS No. 5 tensile test pieces of the produced hot rolled steel sheets and cold rolled steel sheets were measured, and the results are shown in Table 3. Note that the average value and anisotropy are at angles of 0 degrees, 45 degrees, and 9 degrees with respect to the rolling direction.
X is the characteristic value of 0 degrees. -1X4. , Lee, and was calculated using the following formula.

The aging index AI is the difference between the deformation stress at a pre-strain of 7.5% in the rolling direction and the descending stress when the stress is unloaded and then stretched after artificial aging at 100℃ for 30 minutes. This is what is shown.

Table 3 shows that the inventive examples have lower yield strength [YS
, a high n value and elongation Et, and a high T value in cold rolled steel sheets, indicating that it has excellent workability. In addition, the aging index AI of the present invention example is low at 4.0 positive/- or less,
The in-plane anisotropy of the above characteristics is also small. As a result of surface inspection, Example 1 of the present invention had no problematic surface defects, and the hot-dip galvanized cold-rolled steel sheet of sample material A5 had excellent plating properties.

As is clear from the above examples, the present invention adds a limited amount of REM%Ca%Ti%Mg to extremely low carbon and produces a thin steel plate by the so-called CC-DR method. It is possible to obtain hot-rolled thin sheets and cold-rolled steel sheets with excellent workability such as bending, a regular grain structure, and good surface properties.

Agent Takeo Nakaji

Claims (1)

[Claims] (1) Weight specific pressure, CIo, 015% or less, 810.0
30% or less, 0IO108% or less, ¥ contains one or more selected from rare earth elements, Ca, TI, Mg in a total of 0.002 to 0.040%, and the balance is Fe and unavoidable elements. Continuous casting - by direct hot rolling, characterized by comprising the steps of continuous casting of molten steel containing impurities into a slab, and subsequent hot rolling of the slab in a red-hot state. A method for manufacturing thin steel sheets for processing.