JP2872034B2 - Manufacturing method of thin slab - 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 heat-treating continuously cast carbon steel thin slabs, and more particularly, to a thin cast steel having characteristics required for conventional hot-rolled steel sheets, such as strength-ductility balance and surface quality. The present invention relates to a method of manufacturing a piece to omit or simplify a hot rolling step.

[0002]

2. Description of the Related Art In recent years, due to the remarkable progress of continuous casting technology, cast slabs have been made thinner, and the omission and simplification of the conventional hot rolling process have been progressing. Further, such thinning is receiving attention from the viewpoint of cost reduction. As a device for continuously casting such a thin slab, for example, a twin drum type continuous casting device has been developed. This device, as shown in FIG.
A pair of cooling drums 1, 1 approaching each other and rotating in opposite directions, and a pair of side dams 2, 2 contacting both end surfaces of the cooling drums 1, form a pool 3, and the pool is formed. The molten metal injected into the part 3 is cooled and solidified on the peripheral surface of the cooling drums 1 and 1 to form solidified shells G and G.
A thin slab S of about 10 to 10 mm can be continuously cast.

[0003] However, since the thickness of this thin slab is close to that of the final product, a large reduction amount cannot be obtained unlike the conventional hot rolling process. For this reason, in thin cast slabs of carbon steel (hereinafter simply referred to as thin cast slabs), the metal structure becomes a coarse ferrite structure, so that not only cannot sufficient mechanical properties be obtained, but also rough surface during processing (orange peel) ) Has been pointed out. The reason for the coarsening of the ferrite structure is that the austenite structure when transforming from austenite to ferrite is coarse, so its grain boundary area is small, and there are few defects such as deformation bands, so the number of ferrite nucleation sites is small. .

[0004] In order to solve such a problem, the use of intragranular ferrite, which causes an austenite to transform from austenite grains positively, by using an oxide or the like as a nucleus of transformation from austenite to ferrite, has led to the use of thick steel sheets. It is mainly studied (for example, JP-A-61-213322). However, in this method, it is difficult to control the composition of the components and the melting conditions for uniformly dispersing the oxides and precipitates serving as the core of the transformation,
Further, since a ferrite side blade, bainite and the like generated from austenite grain boundaries compete during transformation, it is difficult to form a uniform structure. In addition, intragranular ferrite itself tends to become acicular Widmann-Stetten ferrite, and thus has problems such as low ductility and fatigue characteristics.

On the other hand, as another approach for preventing the ferrite structure from becoming coarse in a thin slab, for example, Japanese Unexamined Patent Publication No.
1-99630, JP-A-63-62822, JP-A-3-274231, JP-A-3-294419, and JP-A-4-21723. Transformation heat treatment is mentioned. This heat treatment method controls the cooling of the continuously cast and solidified thin slab from the Ar ₁ transformation point to generate intragranular ferrite, and then heats it again to the Ac ₁ transformation point or higher to form an austenitic single phase structure. Then, the cooling is controlled to form a polygonal ferrite structure.

However, the austenite single-phase structure obtained by this method strongly inherits the influence of the coarse prestructure, and therefore it is not possible to sufficiently obtain a uniform and fine polygonal ferrite structure. In addition, since this method is a method in which an in-line heat treatment apparatus is installed in a continuous casting apparatus, there are many operational problems. For example, in continuous casting, the frequency of maintenance of the equipment is high in order to handle molten steel at a high temperature, and the heat treatment apparatus must be stopped along with the stop of the continuous casting apparatus. Further, when restarting the operation of the heat treatment apparatus, it is necessary to raise the heat for a long time, thus lowering the productivity. Furthermore, in order to control and cool the slab in-line,
There is an economical problem because an expensive control cooling device such as that associated with the existing hot rolling mill is required. And furthermore, continuous casting of thin slabs has a slower threading speed than hot rolling,
There is a problem that the cooling rate becomes uneven due to the uneven flow of the cooling water on the thin slab upper surface.

[0007]

SUMMARY OF THE INVENTION The present invention solves the problems in in-line heat treatment of continuously cast thin slabs, and also requires the properties required for conventional hot-rolled steel sheets, such as strength-ductility balance and surface quality. To produce a thin slab having
It is an object to omit or simplify the hot rolling step.

[0008]

According to the present invention, there is provided a method of manufacturing a thin slab, comprising the steps of continuously casting carbon steel having a weight ratio of C ≦ 0.15% into a thin slab having a thickness of 10 mm or less. Casting and cooling the thin slab to below 600 ° C.
After heating to a temperature range of ~ 1100 ° C, it is cooled to 300 ° C at a cooling rate of 40 ° C / s or more, and then heated to a temperature range of 500 to 700 ° C, and then heat treated at this temperature range for 1 hour or more. It is characterized by doing.

[0009]

[Action] The characteristics of the crystal structure commonly found in hot-rolled steel sheets having excellent mechanical properties such as strength-ductility balance and fatigue properties are that the crystal grains are uniform and fine, and that each crystal grain has at least It has a polygonal ferrite structure. That is, even in a simplified process starting from a thin slab, if the microstructure transformed from austenite is finally uniformly fine polygonal ferrite, the required mechanical properties will be satisfied. The inventors of the present invention have conducted intensive studies in view of the above-mentioned circumstances, and have concluded that it is difficult to completely produce uniformly fine polygonal ferrite only by the intragranular ferrite transformation studied as a conventional technique. did.

The present inventors have found through various experiments a process for obtaining uniformly fine polygonal ferrite by one reverse transformation heat treatment and tempering heat treatment. The basic principle is that a fine bainite structure (partially martensite) is obtained by controlling the cooling rate when performing the reverse transformation heat treatment with the heat history as shown in FIG. The structure is a uniform and fine polygonal ferrite structure.

That is, the continuously cast thin slab cools and solidifies to a coarse ferrite structure or a coarse bainite structure. Next, when this thin slab is heated to 910 to 1100 ° C., a mixed grain austenite structure is obtained. Next, when the thin slab is cooled to 300 ° C. or less at a cooling rate of 40 ° C./s or more, large grains of the mixed austenite grains are transformed into bainite structure or martensite structure, and small austenite grains are converted to fine polygonal grains. Transforms to a null ferrite structure.

Next, when the thin slab is heated to a temperature range of 500 to 700 ° C. and tempered at 1 hour or more in this temperature range, the bainite structure or the martensite structure becomes fine polygonal ferrite. Transformed into a structure, the whole structure becomes a uniform and fine polygonal ferrite structure, and a thin cast piece excellent in mechanical properties and workability can be manufactured.

Next, the reasons for limiting each condition in the present invention will be described. C contained in carbon steel is the most important element that determines the structure morphology in the transformation from austenite to ferrite.In order to stably obtain a bainite structure or a martensite structure by the reverse transformation heat treatment in the present invention, The lower limit is preferably 0.01%. The upper limit is set to 0.15% so as not to deteriorate the weldability.

When the thickness of the thin slab exceeds 10 mm, the structure becomes coarse and the crystal grains are not sufficiently refined even when the reverse transformation heat treatment is performed. The temperature range of the reverse transformation heat treatment is Ac ₁ in carbon steel of C ≦ 0.15%.
Since the temperature at the point is 830 to 910 ° C., the lower limit is 910.
° C. If the temperature is 1100 ° C. or higher, austenite grains become coarse and the surface roughness of the product becomes large.
The temperature was set to 00 ° C or less. The cooling rate of the reverse transformation heat treatment is set to 40 ° C./s or more so that the coarse austenite grains have a bainite structure or a martensite structure. At a cooling rate lower than this, the coarse austenite grains have a coarse ferrite structure, and the processed surface of the product becomes rough. The temperature range of the tempering heat treatment is set at 500 to sufficiently diffuse C.
℃ or more, 700 ℃ in order to sufficiently precipitate solid solution C
It was as follows. The time of the tempering heat treatment was set to 1 hour or more in order to sufficiently precipitate solid solution C.

The composition of the components other than C is represented by Mn
Is an element added for the purpose of increasing the strength of steel, like C, and is an element that increases the hardenability. Therefore, the content is set to 0.05% or more in order to secure hardenability, and the upper limit is set to 0.6% in order to obtain necessary materials such as SPHC, SPHD, SS400, etc. which are JIS standards for general mass-produced hot-rolled steel strip. It is preferable to set the following. P is an element added to increase the strength of steel, but excessive addition deteriorates ductility and weldability.
The upper limit is preferably set to 0.050%, since S causes hot cracking. Al is necessary for deoxidation of steel, but excessive addition causes an increase in cost and also remains as inclusions in the steel and causes deterioration of workability of the product. % Is preferable.

Next, the manufacturing method of the present invention will be described with reference to FIG. In the present invention, carbon steel of C ≦ 0.15% is continuously cast into a slab having a plate thickness of 10 mm or less, wound up at 600 ° C.
Cool to below. During this time, hot rolling may be performed to adjust the surface roughness and shape of the thin slab. Next, this thin slab is heated to a temperature range of 910 to 1100 ° C. (Ac ₁ point or more) to perform reverse transformation, ie, transformation from ferrite to austenite, and then 300 ° C. or less at a cooling rate of 40 ° C./s or more. A reverse transformation heat treatment for transforming austenite into ferrite is performed by cooling to a temperature. It is first important that the structure be a mixed structure of fine polygonal ferrite, bainite, and martensite by controlling the cooling rate. This reverse transformation heat treatment makes the structure finer and prevents roughening during processing. Furthermore, this thin slab is heated at 500 to 700 ° C.
And a tempering heat treatment for heat treatment in this temperature range for 1 hour or more. By this tempering heat treatment, bainite,
The second important thing is to make the martensite structure into a polygonal ferrite structure and to precipitate solid solution C in the ferrite and to purify the matrix, thereby making the whole structure a uniformly fine polygonal ferrite structure. Thus, a thin cast piece having excellent mechanical properties and workability can be manufactured.

[0017]

EXAMPLE As an example of the present invention, carbon steel having the composition shown in Table 1 was melted, and a thin slab having a thickness of 3 to 10 mm was cast using the twin-drum continuous casting apparatus shown in FIG. Subsequently, two heat treatments as shown in FIG. 2 were performed under the conditions shown in Table 2.
The thin cast slab after the heat treatment was pickled and then subjected to the following mechanical tests. (1) Processed into JIS Z2201, No. 5 test piece,
241 was subjected to a tensile test in accordance with the test method described in 241 to measure tensile strength and total elongation. (2) Processed into JIS Z2201, No. 5 test piece and 15%
After stretching to elongation, the surface roughness was measured. As a comprehensive evaluation of the quality, a tensile strength of 270 MPa or more, a total elongation of 21% or more, and a surface roughness after 15% tensile of 2.5 μm or less were accepted. As a comparative example, a molten steel having a component composition shown in Table 1 was smelted, a thin slab having a thickness of 3 to 15 mm was cast, heat-treated under the conditions shown in Table 2, pickled, and then subjected to the above mechanical test. Was. Table 3
The results of mechanical tests of the method of the present invention and the comparative examples are shown in FIG.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

In the examples of the present invention, since the structure is a polygonal ferrite structure having a uniform and fine structure, the structure has an excellent balance between strength and ductility, and the surface does not become rough during processing. However, the thin slab produced in the comparative example is coarse. Since it has a ferrite structure or a bainite or martensitic structure, the strength-ductility balance is inferior to that of a hot-rolled steel sheet having the same composition, or the surface is roughened during processing, and the overall quality is rejected. Was.

[0022]

According to the present invention, it is possible to make the metal structure of a continuously cast thin slab a uniform and fine polygonal ferrite structure equivalent to that of a conventional hot-rolled steel strip. As a result, it becomes possible to manufacture thin steel sheets having the same strength-ductility balance as that of conventional hot-rolled steel strips and the quality of surface roughness after processing, etc., as well as dramatically improving productivity and reducing equipment costs. Reduction becomes possible.

[Brief description of the drawings]

FIG. 1 is a view showing a twin-drum continuous casting apparatus.

FIG. 2 is a view showing a heat history of a thin slab after casting in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Cooling drum 2 Side weir 3 Hot water pool M Melt G Solidified shell S Thin slab

Claims (1)

(57) [Claims] 1. A carbon steel having a weight ratio of C ≦ 0.15% is cast by continuous casting into a thin slab having a thickness of 10 mm or less, the thin slab is cooled to 600 ° C. or less, and then 910 to 1100.
After the temperature rises to a temperature range of
A method for producing a thin cast piece, comprising: cooling to a temperature of not higher than 00 ° C., and then raising the temperature to a temperature range of 500 to 700 ° C., and performing a heat treatment in this temperature range for 1 hour or more.