JPH06128639A - High strength and high toughness structural steel plate manufacturing method - Google Patents

Abstract

(57) [Summary] [Purpose] The purpose of the present invention is to economically manufacture a Mo-added structural steel sheet in which both the base metal and the heat affected zone (HAZ) have excellent low temperature toughness with good productivity. [Structure] In a rolling process in which a slab of Mo-added structural steel heated to 1000 ° C. or higher and less than 1200 ° C. after completion of solidification is finished at an Ar ₃ point temperature or higher, a cooling rate V> (18 / t) Perform cooling to satisfy ^0.5 . [Effect] It is possible to manufacture a structural steel sheet having excellent base metal toughness and HAZ toughness without using alloy addition, waiting for rolling temperature, low temperature heating rolling, reheating rolling, etc., which have been inevitably used during rolling. This makes it possible to increase the productivity of this type of steel sheet and significantly reduce the manufacturing cost.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for economically producing high strength and high toughness structural steel plate with high productivity.

[0002]

2. Description of the Related Art In recent years, there has been proposed a technique for reducing the amount of alloying elements added to steel for improving weldability and producing a thick steel sheet having high strength and high toughness by an accelerated cooling method. The steel sheet manufactured by the company has been attempted to be applied to structures mainly in fields such as shipbuilding and marine structures.

For example, as disclosed in Japanese Patent Publication No. 62-130216, "Ti-added steel is hot-rolled in a low-temperature austenite unrecrystallized region, and immediately thereafter, accelerated cooling is performed at a cooling rate of 1 ° C./sec or more to obtain high strength and high strength. A method of manufacturing a tough steel plate ",
There is a proposal such as "a method for obtaining an extremely thick steel plate by immediately accelerating and cooling a steel slab at a predetermined water amount density in consideration of carbon equivalent immediately after hot rolling, which is found in Japanese Patent Publication No. 62-164820.

Even when the "accelerated cooling method" is applied, precipitation type alloying elements such as Nb, V, and Ti are usually added in order to enhance the strengthening effect by the rolling-accelerated cooling although the amount is small. It is said that the strengthening action of these elements is almost proportional to the amount of re-solid solution during heating, and the addition amount of alloying elements is adjusted according to the target strength after considering the adverse effect on weldability. .

As a method for further improving this accelerated cooling method, there is a method for controlling high-strength and high-toughness of a thick steel sheet by controlling precipitation during rolling which precipitates in austenite and controlling precipitation during accelerated cooling. See, for example, -275719.
This method is used during finishing rolling and finishing rolling.
At least 7 between 000 ° C and 850 ° C
Precipitation control in a temperature range of 850 ° C to 1000 ° C, which is a precipitation nose due to rolling strain of Nb carbonitride, V carbonitride, etc. by cooling in a temperature range of 0 ° C or more at a cooling rate of 0.5 ° C / sec or more. The strength is increased by about 9 kgf / mm ² .

The inventors of the present invention previously disclosed in Japanese Patent Application No. 3-75368 that the temperature is 1000 ° C. or higher 120 after completion of solidification.
In rolling in which a slab of structural steel heated to less than 0 ° C. is finished at an Ar ₃ point temperature or higher, a slab thickness t (mm) and a cooling rate are measured from the start of rolling of the slab to the end of recrystallization. V (° C
/ Sec) was cooled such that the relationship of V> (18 / t) ^0.5 was satisfied, and a method of manufacturing a thick steel plate for high strength and high toughness structural use was proposed.

[0007]

The present invention was obtained by the prior art without adding a large amount of alloying elements, staying / waiting for temperature adjustment, and reheating rolling in a low temperature range. It is an object of the present invention to provide a method for economically and efficiently manufacturing a structural steel sheet having a high strength and a high toughness equivalent to or higher than that.

[0008]

According to the present invention, after the solidification of Mo-added structural steel is completed, the slab heated to a temperature of Ac ₃ point or higher is rolled at a temperature of Ar ₃ point or higher during rolling, t (m
m) and the cooling rate V (° C./sec) satisfy the relationship of V> (18 / t) ^0.5 .

The structural steel to which the present invention is applied is based on the relationship of action and effect which has been conventionally confirmed in the use in the field of the art in order to obtain the required material for the ordinary welded structural steel. It is a structural steel containing 1% or less of Mo in addition to the specified additive elements. The reason and amount of addition of each of these component elements are shown below.

C is added as an effective component for improving the strength of the steel, but if the content exceeds 0.20%, island-like martensite precipitates in the HAZ and the HAZ
Since the toughness is significantly deteriorated, the content is limited to 0.20% or less.

Si is added as a deoxidizing element and a strength increasing element of molten steel, but if it is less than 0.01%, the deoxidizing effect is insufficient, and if it exceeds 1.0%, the workability of the steel deteriorates. However, since the toughness of HAZ decreases, the addition amount is 0.01-
Regulate to 1.0%.

Mn is also necessary as a deoxidizing component element,
If it is less than 0.3%, the cleanliness of the steel is lowered and the workability is impaired. Further, it is necessary to add 0.3% or more as a component for improving the strength of the steel material. However, since Mn remarkably deteriorates the weldability due to excessive addition, the upper limit is 2.0%.

Al and N are necessary because the grain size of steel can be refined by Al nitride. However, if the addition amount is small, the effect is not exerted, and if it is excessive, the toughness of the steel deteriorates. Therefore, the addition amount of Al is regulated to 0.001 to 0.20%, and the unavoidably contained N is 0. It is limited to 0.020% or less.

Mo is an element capable of maintaining a solid solution by cooling during cooling, and fine precipitation occurs during tempering during accelerated cooling, and dislocations accumulated in large amounts by cooling during cooling become precipitation sites during precipitation, so 1% or less is added. It

The basic components of the structural steel targeted by the present invention are as described above. On the basis of this, when an alloying element is added according to required properties for the purpose of increasing the strength of the base material or improving the toughness of the joint, it becomes difficult to secure the weldability if too much is added. Therefore, the alloy addition amounts are Ni, Cr,
Cu, W, P, Co, V, Nb, Ti, Zr, Ta, H
f, rare earth element, Y, Ca, Mg, Te, Se, B 1
The total number of types is regulated within 4.5%.

As for the rolling end temperature, when the surface of the steel sheet, which is the lowest temperature among the steel sheets, becomes less than the Ar ₃ point temperature, the austenite grains become coarse and the surface toughness deteriorates. Surface temperature is included above ₃ points.

[0017]

In order to solve the problems of the prior art and to achieve the objects of the present invention, the inventors of the present invention selected the steel grades shown in Table 1 of the examples as test steels representing typical structural steels. Various experimental studies were repeated using 2 and 9.

It is known that the austenite grain size of this structural steel sheet changes depending on the heating temperature. As a result of investigating the relationship between the heating temperature and the grain size of the crystal grains in the structural steel by changing the heating temperature of the structural steel cast piece, the present inventors have found that the core temperature of the cast piece is 1000 ° C. or more and less than 1200 ° C. Then, it was found that the added Nb was sufficiently solid-dissolved and the crystal grains became 100 μm or less as shown in FIG. 1 to prevent coarsening.

Next, the inventors of the present invention have recognized that the cooling rate during rolling is 0.4 to 0.5 ° C./second in order to search for a method of improving the strength and the toughness of the base material economically with good productivity. The relationship between the slab thickness and the cooling rate during conventional rolling was investigated. As a result, the actual state of the cooling rate corresponding to the thickness of the material to be rolled, which has never been utilized in the conventional rolling technology, has been found. The actual condition is shown by curve A in FIG.

The inventors of the present invention enjoy the effects of increasing the strength and toughness of the precipitates described above while adding a large amount of alloying elements, staying / waiting for temperature adjustment, and further reheating rolling in a low temperature range. Without carrying out, the experiments and studies were repeated in order to establish a method for manufacturing a structural steel sheet having high strength and high toughness equal to or higher than that obtained by the conventional technique.

In this experimental study, the strength was 47 kgf / mm ² , and the vTrs value in the Charpy impact test showing the base material toughness was −9.
The vTrs value showing the base metal toughness at 9 ° C and the same strength is -95.
VTrs value which shows the base metal toughness at ℃ and the same strength is -10
Fig. 2 shows the cooling rates by thickness for each 8 ° C curve B.
Indicate. This curve is (18 / t) ^0.5 based on the slab thickness t.
It turns out that can be approximated by. By utilizing this, even if the thickness of the material to be rolled changes during rolling, the cooling rate V
It was found that the object of the present invention can be achieved when (° C / sec) is maintained at (18 / t) ^0.5 or more.

FIG. 3 shows the cooling condition t × V ² for steel type 1 (Table 1).
[Mm · (° C / sec) ² ] and strength of rolled steel sheet, 1 / 2t
The relationship of base material toughness is shown. According to Fig. 3, strength 49kgf / m
The structural steel sheet of Steel Grade 9 obtained by the conventional rolling method showing m ² and toughness (vTrs value) −50 ° C. was significantly improved, and 5
It was found that it exhibited a strength of 4 kgf / mm ² and a toughness of -70 ° C.

Further, in order to improve the strength of the structural steel sheet obtained as described above by accelerated cooling, any cooling agent such as water, steam or a steam mixture is used after the rolling is finished and the cooling rate is 5 ° C. It has been found that rapid cooling at a cooling stop temperature of 650 ° C. or less may be performed for 1 / second or more. Similarly, it was found that even if the structural steel sheet is rolled and then quenched and tempered, the strength and toughness can be improved without impairing the effects of the present invention. The present invention is based on the above findings.

[0024]

[Examples] The composition of the test steel of the present invention may be any combination as long as it is an element and an addition amount of the above-mentioned general structural steel, but the present invention is used as a typical structural steel having different strength levels. The chemical composition of the steel used in the examples is shown in Table 1, the production conditions are shown in Table 2,
The rolling pass schedules used at that time and the cooling conditions during rolling are shown in Tables 3 and 4, and the obtained materials are shown in Table 5 together with the conventional example.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[Table 4]

[0029]

[Table 5]

Steel Nos. 1 to 3 shown in Table 1 are 40 kg class steels, and steel Nos. 4 to 7 are 50 kg class steels. An alloy element is added to each of them, if necessary. As shown in Table 2,
No. The invention examples A1 to A7 are capable of producing structural steel sheets having strength or toughness equal to or higher than that of the base material obtained by the prior art in both designated pass cooling and zone cooling with good productivity and economy. did it.

On the other hand, according to the conventional example No. 1 in which the temperature was waited for rolling and cooling was not performed during rolling. B1 to B7
Had their respective problems, and a structural steel plate satisfying the above requirements could not be obtained.

In addition, B having a heating temperature of 1200 ° C. or higher
1 and B4 are -44, respectively, as compared with invention examples A1 and A4.
The base metal toughness was low at ℃ and -57 ℃. B6, which has a heating temperature of less than 1000 ° C, has a strength of 57 kg even though it has a 60 kgf / mm ² component and Nb is not sufficiently dissolved in solid solution.
It was f / mm ² . Further, B7 was quenched and tempered after rolling but had a high finishing temperature, and had lower toughness than A6 and A7 of the present invention examples which were not accelerated cooled.

[0033]

INDUSTRIAL APPLICABILITY The present invention is equivalent to or more than that obtained by the prior art without adding a large amount of alloying elements, staying / waiting for temperature adjustment, and reheating rolling in a low temperature range. It is possible to smoothly and stably produce a structural steel sheet having high strength and high toughness based on high productivity, and to bring about the effect brought to the application field of the present invention and the related fields. The ripple effect is extremely large.

[Brief description of drawings]

FIG. 1 is a chart showing a relationship between a heating temperature of a structural steel cast product and an austenite grain size.

FIG. 2 is a chart showing the relationship between the thickness of a cast piece during rolling and the average cooling rate in the plate thickness direction according to the thickness.

FIG. 3 is a table showing the relationship between the cooling conditions of the present invention, the strength of 1/2 thickness, and the toughness of the base material.

Claims (3)

[Claims] 1. A slab thickness t (mm) and an average cooling rate V (° C./second) during rolling in which a slab heated to a temperature of Ac ₃ point or higher after solidification of a Mo-added structural steel is finished at a temperature of Ar ₃ point or higher. The method for producing a high-strength, high-toughness structural thick steel sheet, characterized in that rolling is carried out while cooling such that the relationship between V and (18 / t) ^0.5 is satisfied. 2. After the rolling is finished, the rolling is continued at 5 ° C. /
The method for producing a high-strength, high-toughness structural thick steel sheet according to claim 1, wherein accelerated cooling is performed to a temperature of 650 ° C or lower at a cooling rate of 2 seconds or more. 3. The method for producing a thick steel plate for high strength and high toughness structure according to claim 1, wherein quenching and tempering are performed after the rolling is completed.