JPS5828329B2 - Manufacturing method of thick-walled high-toughness steel plate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a thick, high-toughness steel plate, which uses an existing coiler as it is to produce a hot-rolled steel plate with a thickness of 12 mvt or more, which has high strength and high toughness, using a hot strip mill. The object of the present invention is to provide a method by which a product having excellent low-temperature toughness can be obtained without embrittlement even after high-temperature rolling.

When hot-rolling a thick steel plate using a hot strip mill, the phenomenon of extreme deterioration of low-temperature toughness even when rolled at a high temperature of 650°C or higher has rarely been observed; In high-temperature rolling, it was even common to achieve the same level of toughness at the expense of some loss of strength.

However, even if the so-called controlled trawling technology was introduced to hot strip mills and it became possible to manufacture hot-rolled coils with relatively good toughness, the toughness level would be -40°C in vTs value (fracture transition temperature). This is due to the fact that it has a high toughness that is impossible to achieve.In recent years, hot strip mills have also been using comprehensive control trolling technology to produce high-tensile, high-toughness coils within the thickness range. It is becoming possible to manufacture
Under such circumstances, high-temperature winding embrittlement has become an important problem.

When manufacturing a coil with high tensile strength and high toughness within the thickness described above using existing equipment, it is often necessary to wind the coil at a high temperature due to the capacity limitations of the existing coiler. On the other hand, even if factors that cause embrittlement, such as those for example, come into play during high-temperature rolling, the actual degree of embrittlement changes depending on the level of toughness, and in the case of the conventional low toughness level as described above, The embrittlement, which could be virtually ignored due to the fact that the embrittlement is small and the toughness is generally good in the low-strength region, becomes greatly noticeable under these conditions of high strength and high toughness. Therefore, it is not possible to obtain a product with the desired performance.

The present invention was created after repeated studies in view of the above-mentioned circumstances, and aims to clarify the cause of embrittlement during high-temperature winding as described above, and to appropriately suppress the cause of embrittlement. We succeeded in manufacturing a thick steel plate with high strength and high toughness without high temperature rolling embrittlement.

That is, to further explain the present invention, a high-toughness steel plate as referred to in the present invention means that the fracture surface transition temperature vTs in the C direction (direction perpendicular to rolling) is 160°C or less when the plate thickness is 12 mm, and the plate thickness is 16 mm.
In mm, it also refers to a steel material with extremely excellent toughness, such as a vTs value of 150°C or less in the C direction, and as mentioned above, a steel material with a fracture surface transition temperature of approximately -40°C or higher in the C direction. It is clearly different from those with such toughness level.

As already mentioned, high-temperature roll-up embrittlement differs depending on the toughness level, and does not occur in steels with low toughness levels, but is a phenomenon that is first experienced in high-toughness steel sheets, which are the object of the present invention. When high-toughness steel sheets such as those mentioned above are obtained by hot strip milling, compared to the case where they are rolled at 500 to 630°C, which is usually referred to as low-temperature rolling,
This prevents the high toughness that would otherwise occur when rolled up at 0° C. to be significantly reduced.

However, when manufacturing steel plates using hot strip mills, the rolled steel plates are usually coiled using a coiler, and the steel plates are cooled very slowly in the temperature range below the coiling temperature. This is a unique thermal history that does not exist in steel plates manufactured in plate mills rather than hot strip mills.

Therefore, high-temperature winding embrittlement, which is a problem in the present invention, is a problem that is recognized only when manufacturing hot strip mill steel plates, and is different from the case of thick plate mills.

Furthermore, as described above, when manufacturing high-toughness steel by hot rolling in a hot strip mill, the existing coiler becomes insufficient in capacity due to thickening, widening, or increasing the strength of the steel plate, resulting in high-temperature winding. When trying to make up for this lack of ability with a steel plate, the addition of a specific amount of Mo prevents the resulting steel sheet from becoming significantly brittle due to the high-temperature winding embrittlement phenomenon.

Regarding control trawling technology for Nb-containing steel, many studies have been conducted, mainly regarding plate mills, such as heating temperature of billet or steel ingot, finishing temperature, reduction rate per pass, cumulative reduction rate, etc. It goes without saying that the high-toughness steel according to the present invention is manufactured specifically based on the results of these studies, and C: 0.
03 to 0.20%, Si: 0.7% or less, Mn: 0
．． 8-2.5%, Nb: 0.01-0.10%, s
ol, Al: 0.05-0.10% and as necessary ■: o, 2% or less, Cu: 0.5% or less, Ni
: Steel containing one or more of 0.5% or less, Cr: 0.5% or less, with the balance consisting of iron and unavoidable impurities, is heated to 1200°C or less, soaked, and then heated to a finishing temperature using a hot strip mill. In the production of controlled rolled high toughness steel plates with a thickness of 12 mW or more, which consists of hot rolling at Ar3 to 850°C and then coiling, the coiling temperature is set at 64°C.
The embrittlement caused by heating the steel to 0 to 700°C is caused by Mo: 0.
．． This is avoided by adding and containing 05-3.

To explain this more specifically, the attached drawing summarizes the aspects of high-temperature winding embrittlement, namely, first, a single Nb-based component, which is a conventional component, and then N
Table 1 below shows the compositions specifically used by the present inventors for some of the steels with B and V composite additions, as well as those with composite additions of Nb and Mo.

However, among these steels, 12
A and b in the attached drawings show the changes in vTs in the direction perpendicular to rolling (direction C) with the winding temperature using materials heated at 50°C and 1200°C. As a result, the strength decreases, and the toughness improves slightly accordingly, but if the overall toughness is high, such as a material heated to 1200℃, the toughness decreases extremely when rolled up at around 650 to 670℃. What to do is as shown in the diagram.

Steel B2 to which an appropriate amount of Mo was added according to the present invention
In the case of B3, such a decrease in toughness is not observed and high toughness can be maintained.

As a result of repeated studies aimed at developing thick-walled, high-tensile, and high-toughness hot-rolled steel sheets, the inventors of the present invention found that the embrittlement caused by high-temperature rolling as described above is due to the precipitation of film-like carbon at the grain boundaries. The authors confirmed that this type of grain boundary carbide precipitation can be most effectively and reliably suppressed by adding a small amount of Mo, and by utilizing these facts, high-temperature winding embrittlement This makes it possible to advantageously produce thick-walled, high-toughness steel plates that do not deteriorate.

The range of steel used in the present invention will be explained as follows.

In other words, it is effective for increasing the strength of CII'i, and 0.03% or more is necessary to ensure precipitation hardening of Nb, V, etc. On the other hand, if it exceeds 0.2%, weldability will be impaired. is the upper limit.

Si is effective in increasing tension through solid solution hardening, but 0
．． If it exceeds 7%, weldability deteriorates, so it should be added below this limit.

Furthermore, Mn is effective in increasing the tensile strength and toughness by making the grains of ferrite finer, and requires addition of 0, s% or more, but adding a large amount of Mn will result in the formation of upper bainite, which is harmful to the toughness. The upper limit is set at 2.5%.

However, Nb is an extremely effective and indispensable element for high-toughness non-temperature steel produced by controlled trolling because of its properties of increasing the tensile strength due to precipitation hardening and increasing the austenite recrystallization temperature. 01
% or more, and on the other hand, adding a large amount will only increase the undissolved content during heating, and precipitation hardening will become saturated, which may impair the cleanliness of the steel. The upper limit is 1%.

As mentioned above, Mo is a main and essential element in the present invention and is a basic element that suppresses high-temperature winding embrittlement, and in order to exhibit such basic characteristics, it is necessary to add 0.05% or more. , and is sufficiently effective with an upper limit of 0.3, and this Mo is also effective in increasing tension, but
In this case as well, as mentioned above, it is most effective in the range of 0.05 to 0.3%.

Note that AI is an element that plays the role of deoxidizing steel, adjusting grain size, and fixing N in steel, and is 0.0 as 5ol-Al.
If it is less than 0.05%, these effects cannot be properly detected, and if it exceeds 0.1%, the cleanliness of the steel will decrease, so it should be within the range of 0.005 to 0.1.

The effect of V, which is an optional element, in steel is basically the same as that of Nb, but its effect is smaller than that of Nb, and it can be added as necessary in an amount of 0.2 or less, and Cu.

Ni and Cr are both effective elements for increasing tensile strength, and Ni in particular is effective for increasing toughness, and is necessary within a range of 0.5% to maximize each characteristic. Depending on the situation, one or more of them may be added.

The hot rolling conditions of the present invention include a slab heating temperature of 1200°C or lower, and a finishing rolling temperature of 850°C or lower Ar3, but more preferably, in addition to these, the finishing entrance thickness and finishing thickness are The ratio is set to 2.5 or more.

Low-temperature toughness is basically controlled by rolling conditions, and among these rolling conditions, lowering the heating temperature of the billet (steel ingot) is the most effective way to increase the toughness of steel. If the heating temperature of the steel billet is lowered in this way, the cumulative reduction rate in the low temperature range during rolling will inevitably become small, making it possible to fully demonstrate the effect of low temperature reduction.

This is clear from the above-mentioned FIG. 1, and in the present invention, the heating temperature of the steel billet (steel ingot), which is a pre-rolling step, is set to 1200° C. or lower.

The finishing temperature needs to be at least 850°C or lower in order to fully utilize the effect of low-temperature reduction and obtain a steel plate with a high toughness level as described above in the present invention, but if it is lower than 3 Ar points, abnormal structure will occur. Ar
It must be more than that.

These rolling conditions, namely heating below 1200°C and Ar
Under the condition of finishing at 3 to 850℃, the conventional N-containing
The toughness level at which high-temperature winding embrittlement of material B becomes more likely to occur, that is, the vTs in the C direction at 12-thickness is -60°C or less, and the vTs in the C-direction at 16-thickness is -50°C or less, is high toughness. can be maintained appropriately.

In other words, the above-mentioned rolling conditions are essential conditions for obtaining such a level of toughness, and the effects of the present invention are maximized when this excellent low-temperature toughness is achieved. .

The coiling temperature is based on the premise that the capacity of the coiler (coiler) is weak and high temperature coiling is required to obtain the steel plate of the relevant grade, or when higher toughness is desired by reducing the strength to some extent. That is, it is rolled at a temperature of 640°C or higher, but if it is rolled at a temperature of 700°C or higher, a γ→α transformation occurs after rolling, and the crystal grains of the resulting steel sheet become large, making it impossible to obtain high-toughness steel. .

A preferred winding temperature is in the range of 640 to 680°C.

Thus, according to the present invention, a steel plate having excellent toughness without high-temperature winding embrittlement can be appropriately manufactured.

. Specific examples according to the present invention will be shown below, as well as comparative examples thereof.

Using comparative steels of AI and A2 and steels of the present invention of 81 to B3 as shown in Table 1 above, each of these steel slabs was hot rolled in a hot strip mill under various rolling conditions and winding conditions. The following Table 2 shows the mechanical properties in the rolling C direction of the steel plate obtained when the steel plate was rolled and the ratio of the finished inlet thickness to the finished thickness was both 2.6 or more.

That is, for comparative steels A1 and A2, compared to low-temperature rolling at 560 to 590'C, high-temperature rolling at 650°C and 670'C resulted in lower strength, higher vTs, lower impact strength, and significantly greater toughness. It is declining.

On the other hand, in the case of 81-B3 steel according to the composition of the present invention, the strength is slightly lower when rolled at 650-680°C under the same rolling conditions than when rolled at 550-600°C. However, it is clear that both vTs and vEo are at the same level or even slightly better.

However, steel B1 has a thickness of 16m1L and 191! In addition to the plate thickness change of i thickness, the values for different heating temperatures of 1200℃ and 1150℃ are also shown, but even if the general toughness level changes due to these factors, the above-mentioned It is also clear that the features of the invention are not affected in any way.

It should be noted that the steel according to the present invention does not lose its characteristics, not only when it is used directly from a hot-rolled coil for steel structures, but also when it is used after being subjected to a tempering treatment equivalent to winding. Of course there is no such thing.

According to the present invention as explained above, the hot strip mill produces 1211t11! A hot-rolled steel plate having a thick wall, high strength, and high toughness as described above is appropriately obtained by high-temperature rolling, that is, a hot-rolled steel plate with excellent low-temperature toughness without high-temperature rolling embrittlement is obtained, Therefore, it is possible to provide a product having the above-mentioned characteristics by using an existing coiler as is with conventional equipment, and this invention is industrially very effective.

[Brief explanation of the drawing]

The drawings show an embodiment of the present invention and a conventional one, in which a is a single Nb steel and b is a Nb steel. ■ Composite addition steel, c and d are charts showing the relationship between the winding temperature and fracture surface transition temperature vTs for Nb, Mo composite addition steel and Nb, Mo, V composite addition steel according to the present invention; 125 for Nb, Mo, V composite addition steel
The case of heating at 0°C is also shown.

Claims (1)

[Claims] IC: 0.03-0.20 ratio, Si: 0.7 ratio or less, Mn: 0.8-2.5%, Nb: 0.01-
0.10%, sol, Al: 0.005-0.1%
V: 0.2% or less, Cu: 0.5% as necessary
Hereinafter, a steel containing one or more of Ni: 0.5% or less and Cr: 0.5% or less, with the balance consisting of iron and unavoidable impurities, is heated to a temperature of 1200°C or less to finish at a temperature of A. r In the production of controlled-rolled high-toughness steel plates with a thickness of 12 mm or more that involve hot rolling at 3 to 850°C and then coiling, embrittlement can be prevented by setting the coiling temperature to 640 to 700°C. Mo in the steel: 0.05-0.
A method for manufacturing a thick, high-toughness steel plate, characterized in that the prevention is achieved by adding 3%.