JPH05295428A - High toughness plate manufacturing method - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a method for economically manufacturing a thick steel plate having excellent internal quality and toughness. [Structure] In the rolling in which a heated slab is finished at Ar ₃ points or more, the slab thickness t (mm) and the cooling rate V (° C / sec) are measured from the start of rolling of the slab to the end of recrystallization. Is V>
Cooling that satisfies the relationship of (18 / t) ^0.5 ,
Hydrogen H (ppm) in steel at the center of plate thickness is H ≦ 1.09-0.0
A method of manufacturing a high-toughness thick plate, characterized by performing a dehydrogenation treatment to obtain 007σy. Where σy: Yield stress of steel plate at room temperature (MPa)

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a thick steel plate, and more particularly, to provide a method for manufacturing a thick steel plate having high toughness by a combination of cooling during rolling and dehydrogenation.

[0002]

2. Description of the Related Art In recent years, due to the increase in size of structures and the stricter conditions of use, the demands on the steel materials to be used have become higher than ever before in terms of weldability and low temperature toughness. As a method of simultaneously satisfying such requirements, C
There has been proposed a method of reducing the carbon equivalent by adding an alloying element such as u, Ni or the like, and carrying out controlled rolling centering on two-phase rolling or fine graining followed by accelerated cooling. Furthermore, the present inventors have already proposed in Japanese Patent Application No. 3-73386 a method for obtaining a high toughness steel sheet by refining austenite by cooling before rolling during rolling to achieve fine grain structure after transformation. is suggesting. However, alloy addition and controlled rolling have an unbearable drawback in terms of economy, while cooling during rolling has a weak point that impairs the internal quality of steel.

[0003]

DISCLOSURE OF THE INVENTION The present invention provides a method for economically producing a thick steel sheet having excellent internal quality and toughness without the above-mentioned problems of the prior art. ..

[0004]

Means for Solving the Problems The gist of the present invention is that in a rolling process in which a heated slab is finished at an Ar ₃ point or more, the slab thickness t from the start of rolling of the slab to the end of recrystallization. (m
m) and the cooling rate V (° C / sec) satisfy V> (18 / t) ^0.5 , and the hydrogen H (ppm) in the steel at the center of the plate thickness is H ≤ 1.09-0. It is characterized in that a dehydrogenation treatment is performed to obtain a value of 0.707σy. Further, in the present invention, after the rolling, the cooling rate is 5 ° C./sec or more, the cooling stop temperature is 650 ° C.
Also in the case of performing the following accelerated cooling, also after the completion of rolling,
It is also possible to use together with the method of performing quenching and tempering.

The reasons for limiting the above conditions will be described below.
The heating condition is Ac ₃ or higher for solution treatment. The present invention performs cooling during rolling in order to increase the toughness of the thick steel plate,
The cooling rate V (° C / sec) at that time was V> (18 / t) ^0.5.
If the above condition is not satisfied, a coarse microstructure is generated and toughness is deteriorated, so this cooling condition is essential.

Further, accelerated cooling after rolling improves toughness by refining the transformed structure, so that it can be used according to required characteristics such as strength and toughness. At that time, the cooling rate is 5 ° C /
If the cooling stop temperature is less than 650 ° C. and the cooling stop temperature exceeds 650 ° C., the structure cannot be refined. Further, it can be applied depending on the strength and toughness of the steel sheet that requires quenching and tempering after rolling.

In the present invention, although the range of chemical components is not particularly specified, all the elements generally used for thick plates can be used. Therefore, the strength level can be widely applied from mild steel to high strength steel depending on the applied chemical composition.

[0008]

According to the present invention, in the final ultrasonic flaw detection test (UST) of a product manufactured by cooling during rolling, there are rare cases where the product does not pass the test. About
The present invention has been completed by finding that UST defects can be sorted out in relation to the amount of hydrogen in steel at the center of plate thickness.

That is, the relationship is shown in FIG. 1. In the relationship with the yield strength σy of the steel sheet at room temperature, hydrogen at the center of the sheet thickness is H ≦ 1.09-0.0007 as shown in FIG.
It was found that by satisfying the condition of σy, generation of minute internal defects due to hydrogen in steel can be prevented, and as a result, ultrasonic flaws can be prevented.

When cooling the steel sheet during rolling as in the present invention, hydrogen in the steel is less likely to be released. Therefore, when hydrogen in the steel is high in the hot rolling stage, the internal quality of the steel sheet is impaired due to defects caused by hydrogen, and it is detected as ultrasonic defects. Therefore, regulation of hydrogen concentration in steel is essential. Is.

The reason why the degree of occurrence of internal defects due to hydrogen depends on the strength is not always clear. However, the higher the strength, the more hydrogen trap sites such as dislocations are generally absorbed, the more hydrogen is absorbed, and the higher the susceptibility to cracking by hydrogen. It is thought to be because.

When accelerated cooling is applied after rolling, it is expected that the sensitivity due to hydrogen will further increase and the amount of hydrogen to be controlled will become stricter. However, from the experiments, it was confirmed that air cooling was performed regardless of the cooling pattern after rolling. Good results were obtained within the limited range of the present invention by either accelerated cooling or quenching and tempering. Therefore, the formula for limiting the amount of hydrogen of the present invention can be applied in any case.

For removing hydrogen in the above-mentioned steel, there are conventionally known methods such as (a) vacuum degassing of molten steel, (b) gradual cooling of steel pieces, (c) soaking diffusion of steel pieces, and (d) steel. Piece thickness reduction (breakdown, sizing) and subsequent dehydrogenation slow cooling,
One or more of (e) hydrogen release during thick plate rolling and (f) light reduction of the final solidified portion of the continuously cast slab may be performed in combination.

[0014]

EXAMPLE A continuous cast slab having the components shown in Table 1 was subjected to controlled rolling shown in Table 2, cooling during rolling, and cooling after rolling to obtain a thickness of 3
A 5 mm thick steel plate was manufactured. The mechanical properties and UST results are shown together in Table 2.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

[0018]

INDUSTRIAL APPLICABILITY The present invention makes it possible to economically produce a steel sheet having excellent toughness by combining a cooling technique during rolling and a dehydrogenation technique for a steel sheet. The effect on is enormous.

[Brief description of drawings]

FIG. 1 is a chart showing the tendency of hydrogen defects to occur in the relationship between the room temperature strength of a steel sheet and the hydrogen concentration at the center of the steel sheet.

Claims (3)

[Claims] 1. In a rolling process in which a heated slab is finished at an Ar ₃ point or more, a slab thickness t (mm) and a cooling rate V (° C./° C.) are measured from the start of rolling of the slab to the end of recrystallization. Second) is V>
Cooling that satisfies the relationship of (18 / t) ^0.5 , and
Hydrogen H (ppm) in steel at the center of plate thickness is H ≦ 1.09-0.0
A method of manufacturing a high-toughness thick plate, characterized by performing a dehydrogenation treatment to obtain 007σy. However, σy: Yield stress (MPa) of the steel sheet at room temperature 2. The method for producing a high toughness thick plate according to claim 1, wherein after the completion of rolling, accelerated cooling with a cooling rate of 5 ° C./sec or more and a cooling stop temperature of 650 ° C. or less is performed. 3. The method of manufacturing a high toughness thick plate according to claim 1, wherein quenching and tempering treatment is performed after the rolling is completed.