JPH11131186A - Hot rolled steel plate excellent in workability and scale adhesion, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably and inexpensively obtain a hot rolled steel plate having excellent scale adhesion and workability by providing a composition containing specific amounts of C, also providing a rolling texture having specific recrystallization ratio, and constituting this rolling texture of subgrains of specific fine grain size. SOLUTION: Hot rolling is applied to a steel slab containing <=0.2 wt.% C, and this hot rolling is finished in the ferrite region ranging from 700'C to the Ars point. By this procedure, a rolling texture of <=20% recrystallization ratio is formed and also this rolling texture is constituted of subgrains of <=10 μm grain size, and mechanical properties, particularly ductility, can be improved. Further, owing to the hot rolling at relatively low temp., the thickness of the scale forming at the surface of the resultant steel plate is regulated to <= 5μm and the adhesion of the scale can be improved, and the occurrence of inferior external appearance after working or after coating can be prevented. Then, this hot rolled steel plate is coiled at 650-500 deg.C. By this method, the progress of recrystallization of the steel-plate structure can be prevented, and also the formation of subgrains can be accelerated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet for processing or exterior use, which is used as a material for structural lightweight structural steel, round and square steel pipes, and a method for producing the same.

[0002]

BACKGROUND OF THE INVENTION hot-rolled steel sheet, the ends of the rolling at a temperature below Ar ₃ transformation point, because the structure after rolling is deteriorated mechanical properties become processed structure or coarse grain structure, the Ar ₃ transformation point It is common to roll in a high temperature range that exceeds this.

On the other hand, the rolling process in such a high-temperature region increases the thickness of the black scale at the same time as the rolling and immediately after the rolling. As a result, the scale is peeled off when the hot-rolled sheet is processed. It causes poor appearance after processing or painting. Further, in the production of a thin hot-rolled steel sheet, it is difficult to secure a temperature of _three or more points of Ar, so it is necessary to raise the heating temperature, which is economically disadvantageous.

[0004] From the above background, there has been a demand for an inexpensive hot-rolled steel sheet having a thin black scale and good workability. Here, the workability of the hot-rolled steel sheet is reduced by setting the end temperature of the hot rolling to Ar ₃ points or less, and performing rolling at a total draft of 60% or more within 2 seconds of the final stage of the hot rolling. An improvement has been proposed in JP-A-61-41970. Japanese Patent Application Laid-Open No. 1-249828 also proposes that rolling is performed at a total draft of 60% or more in a temperature range of 800 ° C. or less and 600 ° C. or more. The aim of these proposals is to obtain a microrecrystallized ferrite structure, but such a method requires an excessive rolling capacity and is not practical.

Japanese Patent Application Laid-Open Nos. 1-149924 and 6-306480 disclose techniques for obtaining a finely recrystallized ferrite structure by regulating the components and the cooling rate. The problem of rising is a problem.

[0006]

Therefore, an object of the present invention is to provide a hot-rolled steel sheet having excellent scale adhesion and workability as compared with a conventional hot-rolled steel sheet, and an inexpensive hot-rolled steel sheet. Another object of the present invention is to provide a stable manufacturing method.

[0007]

SUMMARY OF THE INVENTION In order to avoid the conventional problems, the present invention achieves improved scale adhesion and workability of a hot-rolled sheet even under low-cost and low-cost manufacturing conditions. The present invention has been completed by focusing on a technology that can utilize a fine region of sub-crystal grains.

[0008] That is, conventionally, it has been considered that a steel which has been rolled below the _Ar3 point causes surface defects such as a decrease in ductility or a rough surface due to coarsening of ferrite grains due to strain-induced grain growth. Further, the unrecrystallized structure was considered to have extremely low ductility. However, as a result of a detailed study of the relationship between the structure and the material of the steel rolled in the ferrite region, the inventors have found that although the microstructure is a non-recrystallized rolled structure, the microstructure has a fine substructure. It has been found that steel in which crystal grains (sub-grains) are formed exhibits good ductility.

[0009] The steel which has been rolled at _three points or less of Ar is
Because the rolling temperature is low, it is possible to reduce the scale thickness, to improve the adhesion of the black scale to the mother plate, and to improve the scale adhesion and processability at the same time. I also found something.

The present invention is based on the above-mentioned findings, and the gist of the present invention is to provide a hot-rolled steel sheet comprising a rolled structure containing C: 0.2 wt% or less and having a recrystallization ratio of 20% or less,
The rolled structure is a hot-rolled steel sheet having excellent workability and scale adhesion, characterized by having subcrystal grains having a grain size of 10 μm or less. In particular, setting the scale thickness of the steel sheet surface to 5 μm or less is advantageous for improving the scale adhesion.

When the hot-rolled steel sheet is subjected to hot rolling on a steel slab containing C: 0.2 wt%, the hot rolling is performed at 700 ° C.
Ar is completed in a temperature range of ₃ points, and then wound in a temperature range of 650 to 500 ° C.
Obtainable.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting each component of the present invention will be described below. First, C is an element having a large effect on workability. If its content exceeds 0.2 wt%, carbides increase and workability deteriorates.
% As the upper limit.

The other components are not particularly limited. For example, Si: 0.02 wt%, Mn: 0.40 wt%, P: 0.
020 wt%, S: 0.010 wt%, Al: 0.020 wt%, the composition consisting of the balance Fe and unavoidable impurities is recommended.

Next, the recrystallization rate and the subcrystal grain size are important requirements of the present invention. That is, the recrystallization rate is 20%
If the ratio exceeds the above range, recrystallized grains become coarse and ductility is reduced. Therefore, it is important that the recrystallization rate be 20% or less.

On the other hand, the subcrystal grains are formed as a substructure of an unrecrystallized structure having a recrystallization ratio of 20% or less, and are unique to the present invention. That is, the sub-crystals are formed by rearrangement of dislocations generated for releasing strain in the crystal grains subjected to the rolling process, and are generally formed by small-angle grain boundaries in the process of recovering the structure. Since the sub-crystal grains act similarly to one crystal grain with respect to deformation, each sub-crystal grain improves mechanical properties (particularly ductility) by the fineness of the grain size. However, if the grain size of the subcrystal exceeds 10 μm, the ductility decreases as well as the coarsening of the crystal grain. Therefore, the grain size of the subcrystal is set to 10 μm or less.

In the unrecrystallized structure, the sub-crystal grains are:
It is preferable to contain as much as possible, exceeding 80% by weight.

Further, in order to obtain a structure having subcrystal grains in an unrecrystallized structure having a recrystallization ratio of 20% or less, it is necessary to perform rolling in a ferrite region of Ar ₃ points or less. But 70
If the temperature is lower than 0 ° C., dislocation movement for strain release hardly occurs, and no subcrystal is formed. Therefore, in the present invention, the hot rolling end temperature is limited to 700 ° C. or more and ₃ points or less of Ar.

Further, when the winding temperature f exceeds 650 ° C., recrystallization proceeds, and the recrystallization rate exceeds 20%. In addition, coarsening of sub-crystal grains also occurs.
The following subcrystal grain size cannot be achieved. On the other hand, when the winding temperature is 500 ° C. or lower, no sub-crystal grains are formed. Therefore, the winding temperature is 650 ° C. or less and 500 ° C.
It was above.

In the hot rolling, the slab from the continuous casting line may be used as it is, or the cooled slab may be reheated, or direct rolling with auxiliary heating may be performed. The slab heating is not particularly limited, but from the viewpoint of ending the rolling at a low temperature, it is economically preferable to heat the slab at as low a temperature as possible.

Here, the scale thickness of the hot-rolled steel sheet surface is as follows:
5 μm or less. That is, in conventional temperature rolling at _three or more points of Ar, the scale thickness is about 8 to 15 μm, and when such a hot-rolled steel sheet is processed, its surface distortion is large, so that scale peeling was induced. However, these disadvantages are eliminated by suppressing the scale thickness to 5 μm or less. For that purpose, it is effective to limit the hot rolling end temperature to 700 ° C. or more and ₃ points or less,
Since the scale thickness can be suppressed to 5 μm or less, a dramatic improvement in scale adhesion can be achieved.

[0021]

EXAMPLES Hot rolled steel sheets were manufactured from steel slabs having the chemical components shown in Table 1 under various rolling and cooling conditions shown in Table 2.
The mechanical properties, scale thickness and scale adhesion of the hot rolled steel sheet thus obtained were investigated. The result is
Also shown in Table 2.

[0022]

[Table 1]

[0023]

[Table 2]

As shown in Table 2, it is clear that a steel sheet satisfying the requirements of the present invention has excellent workability due to good ductility, and also has a reduced scale thickness and excellent scale adhesion. I understand. In contrast, when the recrystallization ratio exceeds 20% and the subcrystal grain size exceeds 10 μm (No. 4), and when no subcrystal grains are formed (Nos. 7, 9, and 10), the ductility is low. The C content is 0.2 wt%
Also, the D steel (No. 13) having more than 30% has reduced ductility.

Here, FIG. 1 shows the result of comparing the bending workability of a steel sheet obtained by rolling in the conventional austenite region with a steel sheet according to the present invention by using a minimum bending radius. In addition, as shown in Fig. 2, the minimum bending radius is set to 0.25 mm in the V-block bending test method.
R and 0.5 mmR were obtained by pressing a steel plate having a thickness of 1.6 mm and measuring the inner radius of the steel plate after removing the load. FIG.
As is clear from the above, the steel sheet according to the present invention has a smaller minimum bending radius and a smaller bending radius than the gamma-rolled material (comparative example) in which the rolling temperature exceeds Ar ₃ points, although the ductility is the same. It can be seen that the properties, especially the mold adaptability, are remarkably excellent.

[0026]

As described above, according to the present invention,
Since both workability and scale adhesion, which have been problems in conventional hot-rolled steel sheets, are improved, a hot-rolled steel sheet suitable for processing or exterior use can be provided. Also,
ADVANTAGE OF THE INVENTION Since the manufacturing method of this invention promotes low temperature heating of a slab, the hot rolled steel sheet excellent in workability and scale adhesion can be provided at low cost.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of comparing the bending workability of a steel sheet obtained by finishing rolling in a conventional austenite region and a steel sheet according to the present invention.

FIG. 2 is a diagram showing a procedure for measuring a minimum bending radius.

Claims (3)

[Claims] 1. A hot-rolled steel sheet comprising a rolled structure containing C: 0.2% by weight or less and having a recrystallization ratio of 20% or less,
A hot-rolled steel sheet having excellent workability and scale adhesion, characterized in that the rolled structure is composed of sub-crystal grains having a grain size of 10 μm or less. 2. The hot-rolled steel sheet according to claim 1, wherein the scale thickness on the surface of the steel sheet is 5 μm or less. 3. When hot rolling is performed on a steel slab containing C: 0.2 wt%, the hot rolling is completed in a temperature range of 700 ° C. to Ar ₃ points, and then in a temperature range of 650 to 500 ° C. A method for producing a hot-rolled steel sheet having excellent workability and scale adhesion characterized by winding.