JPH02267229A - Production of cold rolled steel sheet excellent in workability from thin cast slab - Google Patents

Abstract

PURPOSE:To improve workability by a process in which hot rolling stage is simplified by forming a low-C steel in which respective contents of Mn, S, O, and B are specified into a thin cast slab and subjecting the above to hot rolling, descaling, cold rolling, and annealing after a specific time from solidification. CONSTITUTION:A molten steel having a composition containing, by weight, <=0.005% C, <=0.5% Mn,<=0.02% S, 0.06-0.012% O, and 0.6-1.5N B is continuously cast into a thin cast slab of <=60mm thickness. Hot rolling is applied to the above after >=3min from the conclusion of solidification, and the resulting plate is descaled, cold-rolled, and successively subjected to continuous annealing together with overageing treatment, followed by temper rolling, if necessary. By this method, the cold rolled steel sheet having superior workability and reduced in age deterioration can be produced at a low cost while obviating the necessity of using dead-soft C steel.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing cold-rolled steel sheets with excellent workability at low cost from thin continuously cast strips.

(Prior technology) Conventionally, cold-rolled steel sheets with excellent workability have been made by continuously casting steel with limited ingredients into a 200-250 mm thick slab, heating it, hot rolling it to a thickness of 2-6 mm, and pickling it. It is manufactured through , cold rolling and annealing processes. In this case, hot rolling is 200~25k
In order to carry out rolling from m to 2 to 6 yen, a powerful rolling equipment is required, and its consumption and energy are also large.

Recently, attempts have been made to make the slab thickness as close to the product thickness as possible as a way to overcome the above-mentioned drawbacks. Specific methods include one in which a cold-rolled material is directly produced by continuous casting without hot rolling, and one in which the hot-rolled material is produced as a slab with an intermediate thickness to simplify the hot-rolling process.

The former is superior in that it eliminates a process, but the manufactured steel sheet is hard and has poor workability, and even though the product has fine grains, the surface becomes rough during processing, making it unusable for processing.

In order to obtain a steel plate with excellent workability using the latter method, Japanese Patent Application Laid-Open No. 8
It is necessary to use ultra-low C steel as disclosed in Japanese Patent No. 3-14820. Furthermore, as disclosed in Japanese Patent Application Laid-Open No. 63-14819, ultra-low C steel and Si
, ooe% or less will result in good deep drawability, and cracking during winding into a coil before hot rolling cannot be avoided. Therefore, the steel manufacturing cost increases and the merit of simplifying the hot rolling process is reduced.

(Problems to be Solved by the Invention) The present invention solves the problems in the prior art described above, uses cold-rolled steel sheets with excellent workability from thin cast slabs, and can produce steel using a simplified hot-rolling process. The purpose of this work was to provide a manufacturing method using low C steel that does not increase costs, that is, a method for manufacturing cold rolled steel sheets with excellent workability at low cost.

(Means for solving the problem) As a result of various studies on steel composition, slab thickness, and thermal history after solidification, the present inventor found that if the steel composition and the time from solidification to the start of hot rolling are controlled, cracking will occur during hot rolling. It was discovered that a steel plate with excellent workability and no occurrence of oxidation can be produced even by continuous annealing.

The gist of the present invention is C, 0.05% or less, M
n; 0.5% or less, S; 0.02% or less, o; o, oo
eo~0.012%, B;0. [Processing characterized by forming molten steel containing iN to 1.5N into a thin slab of 60 mm or less, hot rolling after 3 minutes or more has elapsed from solidification, followed by descaling, cold rolling, and annealing. The method of manufacturing cold-rolled steel sheets with excellent properties.

First, the steel composition, which is an important component of the present invention, will be described.

It has been known that the lower the carbon content, the better the workability. However, since the purpose of the present invention is to reduce costs, it is preferable that the content be 0.012% or more, which can be melted in a converter. Good workability can be obtained even with a lower L1CHk than this. Since workability deteriorates as the amount of C increases, the upper limit was determined to be 0.05% in order to obtain good workability.

It is known that, like C, when the amount of Mn added increases, the workability of the steel sheet deteriorates. For this reason, the upper limit of Mn was specified as 0.5%. From the viewpoint of workability, the lower the Ml, the better, but in the present invention, since the thin slab is directly hot-rolled, Ml of 0°05% is necessary to ensure the soundness of the slab surface. If the surface of the slab is poor, surface defects will occur frequently on the steel plate after cold rolling annealing, and the yield of the product will decrease. A preferable range is 0.1 to 0.20% in order to obtain a steel plate with good workability and a good surface.

S is well known as an element that causes hot embrittlement, and in the present invention as well, if the amount of S exceeds 0.02%, cracks will occur in the hot rolled sheet due to hot embrittlement. Furthermore, the lower the S content, the better in order to improve the surface condition of the slab, and for this reason as well, the upper limit of the S content was specified as 0.02%. Even if S becomes low, the characteristics of the present invention are not impaired, so there is no particular limitation.

O is an important component of the present invention. We will discuss the experimental facts that yielded the knowledge to identify on. Various types of molten steel are melted in the laboratory and made into thin slabs of lθ~60m+* thickness.
．． It was directly hot rolled to a thickness of 0m. After pickling the hot rolled sheet, 0
．． It was cold rolled to 8-1, and successive annealing including recrystallization annealing at 775° C. for 1 minute and overaging treatment at 350° C. for 3 minutes during cooling was performed, and 1.5% temper rolling was performed.

The composition range of the steel is C; 0.015 to 0.015%, Sl;
0.01%, Mn; 0.10-0.30%, P: 0
．． 003-0.015%, N, 0.0012-0.
The range is 0.0020%, 0.0001% to 0.035%.

First, FIG. 1 shows the relationship between the degree of edge cracking in a hot rolled sheet and Oll. The edge cracking of hot rolled sheets is graded into 4 grades: ◎ for those with no edge cracks at all, 0 for those with almost no cracks or slight cracks, Δ for those with a few cracks, and Δ for those with a lot of cracks. Those that occurred were evaluated as ×.

As can be seen from the figure, when the O content is close to zero, edge cracks occur frequently in the hot-rolled sheets. O amount is o, ooeo%
When it is contained, the occurrence of edge cracking is drastically reduced, and at 0.0080%, there is no edge cracking at all. On the other hand, when the O content exceeds 0.012%, edge cracking of the hot rolled sheet increases, which means that an optimum Oi exists.

Figure 2 shows the relationship between the material properties after cold rolling annealing and the amount of O added.

The elongation and j value become better as the addition amount increases up to 0 addition zo, oeo%, and as the amount of O increases beyond this point, the elongation and i value become slightly better. The aging index (AI), which is an index of aging property, also becomes 2 kg/yI when the amount of O becomes 0.0080%, which is a value where there is no need to worry about aging property in practical terms.

However, when Oji increases, the aging index increases, and the properties as a non-aging steel sheet are lost. Based on this experimental fact, the O content is set to 0.0060 to 0 as a condition for obtaining a steel plate that does not cause cracking in the hot rolled sheet and has both good workability and aging resistance.
．． It was identified as 0.012%.

B is an essential element for the present invention. In general, constant C cold rolled steel sheets have AI added, so N in the steel combines with A and Q and precipitates as AfIN, suppressing the adverse effects of solid solution N on strain aging and recrystallization texture. . However, since the present invention is characterized by leaving O in the steel, it is not possible to add enough AfI to fix N. To fix N, set B to 0.8
It is necessary to add a small amount to N1. The upper limit of B was specified as 1.5N because as Bll increases, the amount of solid solution B increases, increasing the recrystallization temperature and deteriorating deep drawability.

The basic composition has been explained above, and although the characteristics of the present invention are not impaired by the presence of impurities such as Sl, P, and N, the workability of cold-rolled steel sheets generally decreases when the content of these components increases. Since these elements deteriorate, it is preferable to have less of these elements.

The above-mentioned composition is melted in a normal converter or electric furnace and, if necessary, subjected to vacuum degassing treatment. The molten steel is cast into thin slabs, but the solidification method,
That is, the features of the present invention are not impaired by the CC method. The thinner the slab is, the better, in order to simplify the hot rolling process, but if it becomes too thin, the productivity of continuous casting will deteriorate, so in the present invention, it is set to the ioam thickness or more.

If the thickness of the slab exceeds 60 degrees, a lot of energy is required for hot rolling, which is inconsistent with the purpose of simplifying the hot rolling process of the present invention. From this point of view, the slab thickness was specified to be BO+om or less.

The slab after solidification is hot rolled, and the time from solidification to the start of hot rolling is important.

Figure 3 shows the relationship between the time from the end of solidification to the start of hot rolling and the material properties after cold rolling annealing.

The solid lines in Figure 3 are C: 0.020%, Mn: 0.14%,
S: 001O%, N: 0.0019%, B: o,
otl1%, O; 0.095%, dashed line is C: 0.02
0%, Mn; 0.14%, S, 0.010%, N,
0.0019%, An); 0.038%, B, 0
．． These are the results for steel containing almost no O at 0.016%, 0.0020%, and 0.0020%.

Other manufacturing conditions are the same as in FIG.

Steels with components other than the present invention, in which almost no 0 remains, improve as time passes from solidification to the start of hot rolling, but the El
In order to obtain l-45%, i″-1.6 or more, 20 minutes or more is required. Also, it takes 20 minutes or more from solidification to the start of hot rolling to eliminate edge cracks in the hot-rolled sheet. However, , O in the steel of the present invention, the workability rapidly improves within 3 minutes after solidification, and the level of workability becomes high.

At the same time, edge cracks in hot-rolled sheets are also eliminated. The time between solidification and the start of hot rolling was determined based on this experimental fact.

The form of existence of the slab from solidification to the start of hot rolling may be in the form of a coil or in the form of a plate.

As for the hot rolling conditions, it is preferable that the hot rolling is completed at the desired temperature, similar to the conventional method. Since the winding temperature is based on continuous annealing, the workability will be slightly impaired when the winding temperature is 600° C. or less, but there is less dependence on the winding temperature compared to conventional methods, and there is no need to specify anything in particular. The preferable range of the winding temperature is 650 to 750°C in order to obtain better workability.

The hot-rolled sheet produced in this way is cold-rolled after descaling, and the cold-rolling reduction ratio is preferably 60% or more, as in the conventional method.

Annealing is performed by continuous annealing with overaging treatment.

The annealing temperature needs to be higher than the recrystallization temperature, and if it is AC3 or lower, the higher the annealing temperature, the better the workability, but in the present invention, it is mainly carried out at 700°C to 850°C. Overaging treatment is mainly carried out at 250°C to 400°C.

Although the main purpose of the present invention is continuous annealing, a cold rolled steel sheet having good workability can be manufactured even if box annealing is performed without impairing the features of the present invention. The annealed plate is subjected to skin pass rolling as required and then used as a product.

The present invention is mainly carried out on cold-rolled steel sheets, but Zn, Z
n-Fe, Zn-AII ablation plating, electroplating,
Even when applied to a surface-treated steel sheet such as Sn plating, the characteristics of the present invention are exhibited, and a steel sheet having good workability can be obtained.

(Example) The steel shown in Table 1 was melted in a converter, and the slab with the thickness shown in Table 1 was cast with a twin-belt continuous casting machine, and the thickness was 3.7 mm under the manufacturing conditions shown in Table 1. It was hot rolled to thickness.

After pickling, this hot-rolled sheet was cold-rolled to a thickness of 775 mm.
Continuous annealing including recrystallization annealing at 300° C. x 1 ml and overaging treatment at 300° C.

The results are shown in Table 1.

The edge cracking of hot-rolled sheets is evaluated on a four-grade scale: ◎ indicates that there is no edge cracking at all; Those that occurred frequently were given a score of ×.

A JIS No. 5 test piece was used for the bow tension test.

The coils kA-1 and G are made of steel whose composition falls within the range of the present invention.

Coils B, C, D, E, F, H, and I are steels that are compositionally outside the scope of the present invention. Are all coils whose components are outside the scope of the present invention an indicator of workability? The value and growth are low.

A whose components are within the scope of the present invention and whose manufacturing conditions are also within the scope of the present invention.
-1.G indicates that there is no edge cracking at all, the i value and elongation are high, the aging index is low, and the workability is good. Coil A-2, whose components were within the scope of the present invention but whose manufacturing conditions were outside the scope of the present invention, had edge cracks in the hot-rolled sheet and was poor in workability.

It should be noted that the material properties of the coil NαH could not be investigated because the hot-rolled sheet had significant edge cracking and could not be tested in the process after cold rolling.

It can be seen that by specifying the components and manufacturing conditions in this way, excellent workability can be obtained even in the process of directly hot rolling a thin slab.

(Effects of the invention) As described above, the present invention does not require extremely low C.
, Even without adding a large amount of carbonitride-forming elements such as TI to fix N, just leaving O in the steel can produce cold-rolled steel sheets with excellent workability and less aging deterioration, resulting in low steel manufacturing costs and low C.
It can be manufactured using a process of directly hot-rolling thin slabs of steel and continuous annealing.

In addition, hot rolling cracks do not occur even if the thin slab is directly hot rolled.

Therefore, both the steel composition and manufacturing process cost are significantly lower than those of conventional methods, making this invention industrially useful.

[Brief explanation of drawings]

Figure 1 is a chart showing the relationship between Offfi and edge cracking in hot rolled sheets.
Figure 2 is a diagram showing the relationship between material properties after cold rolling annealing and Offi, and Figure 3 is a diagram showing the relationship between the time from solidification to the start of hot rolling, edge cracking in hot rolled sheets, and material properties after cold rolling annealing. This is a chart showing the following. Figure 2 o C/, ) 0 (Z)

Claims (1)

[Claims] In weight%, C: 0.005% or less, Mn: 0.5% or less, S: 0.02% or less, 0: 0.060 to 0.012%, B: 0.6 -Excellent workability characterized by making molten steel containing ~1.5N into a thin slab of 60mm or less, hot rolling after 3 minutes or more after solidification, followed by descaling, cold rolling, and annealing. A method for producing cold-rolled steel sheets.