JPH11181520A - Production of thick steel plate having high toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently producing a thick steel plate excellent in the strength and toughness of the steel, in actual thick plate rolling, by promoting recrystallization in the austenitic recrystallization temp. region and highly securing the cumulative rolling reduction in the anstenitic unrecrystallization temp. region. SOLUTION: At the time of subjecting a thick steel plate to tandem rolling using two rolling mills arranged on the same rolling line, the interpass time in the rolling by the two rolling mills in the austenitic recrystallization region is regulated to <=5 sec, and then, >=70% cumulative rolling reduction is applied by the two rolling mills in the temp. region from not more than the unrecrystallization temp. to not less than the Ar3 transformation temp.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a rolling method for obtaining a thick steel plate having excellent toughness.

[0002]

2. Description of the Related Art Thick steel plates are used for various purposes such as construction, shipbuilding, marine structures, line pipes and the like. In recent years, the requirements for materials for thick steel plates used in these materials have been increasingly sophisticated, and further higher strength and higher toughness are desired. Conventionally, controlled rolling has been widely used to meet such demands.

[0003] Controlled rolling is basically based on the chemical composition of steel,
The purpose is to obtain a fine structure by optimizing the heating conditions and the rolling conditions during hot rolling, and to achieve high strength and high toughness simultaneously by making the structure fine. The main ideas are (1) to increase the reduction rate per pass in the high-temperature recrystallization region of austenite and repeat the recrystallization of austenite to reduce the grain size of austenite; By increasing the cumulative draft in rolling in the low-temperature unrecrystallized region, increasing the degree of austenite grain elongation, and introducing a number of deformation zones, the number of ferrite nucleation sites during ferrite transformation is increased in the subsequent ferrite transformation. Of fine particles.

[0004] The promotion of recrystallization of austenite in a high temperature range plays a major role in that it serves as a basis for the subsequent refinement of the structure and in obtaining a uniform structure. Recrystallization is more easily performed as the austenite grains become finer, and thus it is desirable to employ a lower heating temperature. However, in actual rolling, even in a high temperature range, a large rolling reduction that causes recrystallization in one pass, for example, about 20
It is not easy to give a% reduction.

[0005] It is also known that in one pass, as a result of superimposition of a reduction to such an extent that recrystallization does not occur, rolling distortion is accumulated and recrystallization occurs. However, in actual plate rolling on the premise of a reverse mill, it is impossible to continuously apply rolling reduction to a rolled material in a short time. Therefore, during the unavoidable waiting time between rolling passes, the rolling strain introduced in the previous pass is recovered, and the progress of recrystallization due to the accumulation of the strain cannot be sufficiently exhibited in many cases.

On the other hand, the cumulative rolling reduction of austenite in the low-temperature unrecrystallized region has the effect of refining ferrite based on the mechanism (2) described above and significantly improving the material. For example, FIG. 1 shows a steel A which is a C-Mn steel described later.
FIG. 4 shows changes in strength and toughness of steel when the rolling reduction was variously changed in a temperature range of 850 ° C. (unrecrystallization temperature) or lower after heating to 1100 ° C. It is clear that both the strength and the toughness continuously increase with the increase of the rolling reduction in the unrecrystallized region of austenite.

[0007] Therefore, in controlled rolling, it is desirable to ensure as large an accumulated rolling reduction as possible in the non-recrystallization temperature range of austenite. However, on the other hand, since the rolling end temperature is desirably equal to or higher than the Ar3 transformation point temperature,
The fact is that there is no choice but to limit the cumulative rolling reduction.

That is, in actual rolling, the material to be rolled undergoes a large temperature drop during rolling due to the heat removal by the rolling rolls of the plate rolling mill and the use of roll cooling water and descaling. For this reason, the deformation resistance of the material increases, and the rolling reduction per pass cannot be increased, and the rolling temperature decreases in a superimposed manner. Further, although the reduction below the Ar3 transformation point improves the strength toughness, the development of the rolling texture becomes remarkable and there is a problem that the anisotropy of the steel material is promoted. Not preferred.

Therefore, when the finish sheet thickness (product sheet thickness) of the steel sheet and the rolling end temperature are determined, the temperature drop behavior during the rolling is univocal, and the most important factor in the controlled rolling is the unrecrystallized area. The cumulative rolling reduction will be limited. As a result, for example, it is difficult to secure a cumulative rolling reduction of 70% or more in a non-recrystallized region, and the effect of improving strength and toughness as shown in FIG. 1 cannot be fully utilized.

There are several methods for increasing the cumulative rolling reduction of austenite in the non-recrystallized region.
The first method is a method of increasing the temperature at which non-recrystallization starts due to the inclusion of a microalloy element such as Nb and Ti, and expanding the non-recrystallization region. 0.10% for this method
By including the following Nb or Ti, the unrecrystallized region is expanded by about 50 ° C. to the high temperature side. However, Nb, Ti
The effect is saturated even if the content is further increased, and the increase in the content of these elements generally impairs the weldability significantly. There is.

A second method is a method for preventing heat loss in a hot rolling line. Regarding this, for example,
No. -9644, Japanese Patent Publication No. 53-142955,
Japanese Patent Publication No. 56-51518 discloses a technique for suppressing heat loss during the transportation of billets. However, these are all technologies for preventing heat loss during the transportation of the billet between the pre-rolling process and the rolling process, and suppressing the temperature drop of the billet during rolling to reduce the cumulative rolling reduction in the unrecrystallized region. It does not increase.

Further, JP-A-5-43934 and JP-A-5-295432 disclose that a temperature reduction during rolling is prevented by using a heat retaining system, and a cumulative rolling reduction in an unrecrystallized region is ensured. A method is disclosed. However, these methods require the installation of dedicated equipment for keeping the temperature warm, and are not necessarily advantageous in terms of cost effectiveness.

On the other hand, a third method is disclosed in
No. 8703 discloses that two rolling mills are arranged on the same rolling line and used as a tandem reverse mill, so that an extremely low heating temperature is adopted in hot rolling of a thick steel plate, A tandem rolling method that raises the temperature is disclosed.

According to the present invention, the slab heated at a low temperature is subjected to tandem rolling in order to finish the slab in the recrystallization temperature range. The lower limit of the heating temperature is substantially 900 ° C. and the lower limit of the finishing temperature is substantially 800 ° C. However, cryogenic heating not only has many problems in terms of slab heating efficiency and uniformity of the heating temperature, but also does not refer to rolling and cooling conditions in the present invention. It is not considered that the toughness is improved.

[0015]

Although the present invention has been theoretically well established as described above, the austenitic method is used in a controlled rolling method in which the effect cannot be sufficiently exerted in actual plate rolling. The purpose of the present invention is to provide a method for efficiently producing a thick steel plate having excellent toughness by repeatedly promoting recrystallization in a recrystallization temperature region and securing a large cumulative draft in an austenite non-recrystallization temperature region. It is assumed that.

[0016]

The inventors of the present invention have conducted intensive studies on the effects of rolling and cooling conditions on the toughness of a steel material. As a result, in tandem rolling, a detailed study has not been conducted so far. By regulating the time between rolling passes and providing a sufficient cumulative draft in the non-recrystallization temperature range, we succeeded in significantly improving the properties of thick steel plates.

That is, according to the present invention, when a steel plate is tandem-rolled using two rolling mills arranged on the same rolling line, the time between passes of rolling by the two rolling mills in the austenite recrystallization region is 5 seconds. Manufacturing of a high-strength steel plate having high toughness, wherein a cumulative rolling reduction of 70% or more is applied by two rolling mills in a temperature range not higher than the austenite non-recrystallization temperature and not lower than the Ar3 transformation point. Is the way.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the respective manufacturing conditions in the present invention will be described below. First, the configuration of the rolling line is such that two rolling mills are arranged on the same line. This is because by performing rolling in two passes continuously by two rolling mills, it is possible to substantially impart a large rolling reduction to the steel per pass, and the following two effects are obtained.

That is, the first effect is to promote the recrystallization of austenite, and the second is to improve the rolling efficiency in the temperature range not higher than the non-recrystallization temperature and higher than the Ar3 transformation point, and to obtain a sufficient cumulative rolling reduction in this temperature range. The effect can be.
Even if three or more rolling mills are arranged on the same line,
Similar effects can be expected.

Incidentally, a hot strip mill is well known as a method of manufacturing a steel sheet by arranging a plurality of rolling mills on the same rolling line. However, the reason why the hot strip mill has multiple stands is that the dimensions and shape of a relatively thin steel plate and its productivity are taken into consideration. Also,
Since the length of the rolled material is extremely long, it is impossible to perform reverse rolling, and in order to roll a thin steel sheet, it is necessary to perform rolling without lowering the temperature of the material after rough rolling, and The rate is in the range of 20 to 50% per stand (pass), which is impossible with thick plate rolling.

On the other hand, in terms of material, even when used as a structural material, the upper limit of the plate thickness that can be manufactured is limited, and toughness is rarely required. Therefore, the concept of introducing a tandem mill into thick plate rolling in which a reduction rate cannot be increased as in the present invention is fundamentally different.

Next, in the austenite recrystallization temperature range, the time between passes of rolling by two rolling mills is set to 5 seconds or less. FIG. 2 shows a process in which the time between passes when applying rolling at 950 ° C. is changed in a process of rolling steel A by heating at 1100 ° C. and rolling by two rolling mills in an austenite recrystallization temperature range. It shows the austenite grain size later. If the time between rolling passes is 5 seconds or less,
Before the rolling distortion given by the first rolling mill was not fully recovered, the rolling distortion was given by the second rolling mill, so that the rolling strain was accumulated and a large rolling reduction was given in one pass. The same effect as described above can be obtained.

For this reason, recrystallization proceeds promptly thereafter, and austenite is refined. However, when the time between rolling passes exceeds 5 seconds, rolling strain is recovered between passes, so that accumulation of strain is not sufficient and austenite recrystallization does not sufficiently occur. Further, depending on the conditions of the temperature and the rolling reduction, recrystallization occurs partially, resulting in mixed grains, which may deteriorate toughness.

There is no restriction on the inter-pass time when the tandem rolling is performed again after the rolling reduction by the two rolling mills, that is, when the reverse rolling is performed. This is because when the time between passes before starting reverse rolling is 5 seconds or less, the effect of accumulating strain is obtained in the same manner as in the pass applied before the reverse rolling, and recrystallization is performed by one of the rolling passes. Because it wakes up. Further, when the length of the rolling material is long and the inter-pass time is 5 seconds or more, recrystallization may occur during the standby time until the start of the reverse rolling due to the stored distortion.

Austenite below recrystallization temperature Ar3
In the temperature range above the transformation point, a cumulative rolling reduction of 70% or more is applied by two rolling mills. In this temperature range, the degree of elongation of the austenite grains and the density of the deformation zone in the grains increase with an increase in the rolling reduction. Accordingly, the area of the austenite grain boundary and the area of the deformation zone per unit volume which can be a nucleation site of ferrite at the time of γ / α transformation are increased, and the structure after transformation is refined, so that the strength and toughness are improved. I do.

At this time, if the cumulative rolling reduction is less than 70%,
Since the degree of extension of austenite grains and the density of the deformation zone in the grains are not sufficiently high, a sufficiently fine-grained structure after transformation cannot be obtained, and sufficient toughness for thick steel plates, for example, for low-temperature steel containing Ni, No comparable toughness can be obtained. Therefore, the cumulative draft in the temperature range from the non-recrystallization temperature to the Ar3 transformation point or more is set to 70% or more.

The cooling rate after the completion of the rolling is not particularly limited, but may be air-cooled or cooled in a temperature range of 650 ° C. to 400 ° C. at a cooling rate of 2 to 20 ° C./sec.
It is possible to adjust the strength of the steel sheet.

In the present invention, the chemical composition of the steel plate is not particularly limited, but C: 0.
02 to 0.20%, Si: 0.01 to 1.0%, Mn:
0.5-2.0%, Al: 0.01-0.08%, and if necessary, Cu: 0.05-1.0%,
Ni: 0.05 to 1.0%, Cr: 0.05 to 1.0
%, Mo: 0.05 to 1.0%, Nb: 0.005 to
0.10%, V: 0.005 to 0.10%, Ti: 0.
It is desirable to contain one or more of 005 to 0.10%. The reason why steel having such a composition range is desirable is as follows.

C: 0.02 to 0.20% C is an element necessary for securing the strength of the steel material.
If the content is less than 0.02%, the strength is insufficient, and if the content exceeds 0.20%, the weldability is impaired.
~ 0.20% is preferred.

Si: 0.01 to 1.0% Si is necessary not only to increase the strength of the steel material but also as a deoxidizing agent in the steel making process, but if the content is less than 0.01%, the effect is insufficient. %, The briquette property of the welded portion is deteriorated.
% Is preferred.

Mn: 0.5 to 2.0% Mn is contained in order to increase the strength of the steel material.
If the content is less than 2.0%, the strength is insufficient, and if the content exceeds 2.0%, the brittleness of the base material and the welded portion is deteriorated and the welded product is deteriorated. Therefore, the content is preferably 0.5 to 2.0%. .

Al: 0.01 to 0.08 Al is mainly contained as a deoxidizing agent. Content is 0.
If the content is less than 01%, the effect is not stabilized. On the other hand, if the content exceeds 0.08%, the cleanliness of the steel is reduced. Therefore, the content is preferably 0.01 to 0.08%.

Cu: 0.05-1.0%, Ni: 0.0
5 to 1.0%, Cr: 0.05 to 1.0%, Mo: 0.
0.05 to 1.0% Cu, Ni, Cr and Mo are effective in increasing the strength,
If each is less than 0.05%, the effect is not exhibited,
If the content exceeds 1.0%, the weldability deteriorates, so the content is preferably 0.05 to 1.0%.

Nb: 0.005 to 0.10%, V:
0.005 to 0.10%, Ti: 0.005 to 0.10
% Nb and Ti have the effect of raising the non-recrystallization temperature as described above, and Nb, V and Ti also have the effect of forming carbonitrides or nitrides to refine ferrite grains and strengthen precipitation. It is an element effective for improving the toughness and strength due to the above. If the content is less than 0.005%, the effect cannot be exhibited effectively, and if it exceeds 0.1%, the brittleness of the welded portion is deteriorated, so that the content is 0.005 to 0.10. % Is desirable.

In addition to these elements, P, S, O and N are inevitably contained in the steel as impurity elements, but this does not impair the properties of the steel of the present invention.

[0036]

Embodiments of the present invention will be described below.
Steel having the chemical composition shown in Table 1 shown in FIG.
It heated to 00 degreeC, and rolled from the initial slab thickness of 250 mm to the steel plate of 20 mm in finish plate thickness. Here, steel A, B, C, D
Does not contain Nb and Ti, its non-recrystallization temperature is 850 ° C. The steels E, F, G, and H are steels containing either Nb or Ti, and have an unrecrystallization temperature of 900 ° C. Tables 2-1 and 2 shown in FIGS. 4 and 5
-2 shows the production conditions and mechanical properties of each steel sheet.

FIG. 1 shows that after heating steel A to 1100 ° C.,
It shows the change in strength and toughness of steel sheets (sheet numbers A-1 to A-5) rolled at a finishing temperature of 810 to 800 ° C. with various reductions in a temperature range of 0 ° C. or lower (unrecrystallization temperature). . As described above, it is apparent that both the strength and the toughness are continuously improved with the increase in the cumulative rolling reduction in the austenite unrecrystallized region.

In particular, plate numbers A-1 and A-2 in which the cumulative rolling reduction in the non-recrystallization temperature region was secured by 70% or more by using tandem rolling.
-2 and A-3 have a ductile-brittle fracture transition temperature of -80 ° C or less in the Charpy impact test, and show toughness comparable to low-temperature steel. On the other hand, in the plate numbers A-4 and A-5 rolled by a normal one-stand rolling mill, since the finishing temperature was equal to or higher than the Ar3 transformation point, the cumulative rolling reduction in the non-recrystallization temperature range was 60%.
Only about% can be secured. Therefore, the toughness of the sheet number A-
1, inferior to A-2 and the like.

Further, plate numbers B-1, B-2, C-1, D-
1, D-2, E-1, E-2, F-1, G-1, H-
1 and H-2 were all produced under the conditions of the present invention as shown in Table 2-1. As apparent from the mechanical properties of the steel sheet shown in Table 2-2, good results were obtained. A tough steel plate has been manufactured. In addition, although the plate numbers D-2 and G-1 are subjected to controlled cooling after rolling, higher strength can be achieved as compared to air-cooled steel plates after rolling.

On the other hand, plate numbers B-3, C-2, E-3, E-
4, H-3 and H-4 are comparative steels. Here, plate number B-
3, E-3, E-4, and H-4 used conventional one-stand rolling, so that a sufficient rolling reduction at the non-recrystallization temperature or lower and at the Ar3 transformation point or higher could not be ensured or the non-recrystallization temperature or lower. This is an example in which the finishing temperature was lower than the Ar3 transformation point when the cumulative draft in the above was secured. Sheet numbers C-2 and H-3 are examples in which tandem rolling is performed but the inter-pass time exceeds 5 seconds. As apparent from the mechanical properties of the steel sheet shown in Table 2-2, these comparative steels are not sufficiently tough or strong as compared with the examples of the present invention.

[0041]

As described above, in a thick steel plate rolling line, tandem rolling is performed using two rolling mills arranged on the same line, so that the rolling pass is performed in the high temperature recrystallization region of austenite. By controlling the time interval, recrystallization is promoted, and a sufficiently high cumulative rolling reduction can be imparted in the non-recrystallized region of austenite. Therefore, it is possible to efficiently produce a high-strength steel plate having extremely high toughness as compared with the conventional method. In particular, in the conventional method, in order to compensate for the insufficient rolling conditions, there was a case where the content of Ni having a large toughness-improving effect was required. However, in the present invention, the expensive Ni was not required. The economic advantages are also great.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of the cumulative draft in steel A on a temperature range of 850 ° C. or lower and an Ar3 transformation point or higher on the strength and toughness of a steel sheet.

FIG. 2 is a diagram showing the austenite grain size after processing when the time between passes when applying a reduction at 950 ° C. is changed in steel A.

FIG. 3 is a diagram showing, as Table 1, chemical components of a test steel used in Examples.

FIG. 4 is a table 2-1 showing a relationship between rolling conditions, cooling conditions, and mechanical properties of a steel sheet used in Examples.

FIG. 5 is a diagram showing, as Table 2-2, a relationship between rolling conditions, cooling conditions, and mechanical properties of a steel sheet used in Examples.

Claims (1)

[Claims] 1. When tandem rolling a thick steel plate using two rolling mills arranged on the same rolling line, the time between passes of rolling by the two rolling mills in the austenite recrystallization region is set to 5 seconds or less; A method for producing a steel plate having high toughness, wherein a cumulative rolling reduction of 70% or more is applied by two rolling mills in a temperature range from an austenite non-recrystallization temperature to an Ar3 transformation point or more.