JP3467767B2 - Steel with excellent brittle crack arrestability and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material excellent in brittle crack propagation stopping property used for large-scale structures such as ships, offshore structures, low temperature storage tanks, line pipes, and construction / civil engineering structures, especially collisions. TECHNICAL FIELD The present invention relates to a steel material having excellent brittle crack propagation arresting characteristics even after being subjected to an unexpected large deformation due to an earthquake, a large earthquake, or the like, and a manufacturing method thereof.

[0002]

2. Description of the Related Art In large-scale structures such as ships, offshore structures, low temperature storage tanks, line pipes, and construction / civil engineering structures, accidents due to brittle fracture have a large impact on the economy and the environment, and high safety Is required. Therefore, low temperature toughness is often required for steel materials used for these structures. In recent years, in particular, even if a structure cracks due to an accident or the like, a brittle crack propagation stopping property at a low temperature, a so-called arrest property, is being demanded from the viewpoint of preventing damage. Further, since structures are often subjected to large plastic deformation due to an accident, steel materials are required to have a predetermined low temperature toughness even after being damaged by plastic deformation.

That is, in the case of ships, the demand for brittle crack propagation stopping properties for materials has been further enhanced from the viewpoint of ensuring safety when the ships are reused after a collision accident. For example, in a steel material for ships, a high brittle crack propagation arresting property even after being subjected to plastic deformation, for example, after being subjected to 10% plastic deformation, Kca (0 ° C.) ≧ 600 k
In some cases, a fracture toughness value of gf / mm3 / 2 is required.

Further, from the experience of the brittle and destructive damage accident of a large building structure due to the Great Hanshin-Awaji Earthquake, aftershock stress of repeated earthquakes or even after the structure receives large force and large plastic deformation, From the viewpoint of safety in the subsequent continued use of the structure, steel materials for building structures are also required to have sufficient low temperature toughness after undergoing plastic deformation.

Such a high brittle crack propagation arresting property is
Even in the as-rolled state that is not subject to plastic deformation, it cannot be easily realized. In addition, it is generally difficult to realize excellent brittle crack propagation arresting properties in the state after plastic deformation because steel materials generally undergo toughness when plastically deformed and work hardened.

[0006] As a means for improving the low temperature toughness of steel materials, especially the brittle crack propagation arresting property, a method of increasing the Ni content has been conventionally known, and 9% Ni is used in a liquefied natural gas (LNG) storage tank. Steel is used on a commercial scale. However, an increase in the amount of Ni necessitates a significant increase in cost, so it is difficult to apply it to applications other than the LNG storage tank.

On the other hand, in the case of so-called cold region specifications or winter specifications in Japan, which do not reach extremely low temperatures such as LNG, a thermomechanical treatment method conventionally combined with controlled rolling and controlled cooling, a so-called TMCP method (Thermo) is used.
-Mechanical Control Proce
ss) has been widely used. In this method, (1) the recrystallization of austenite is repeated to make the austenite finer, and (2) the rolling reduction in the low temperature unrecrystallized region of austenite is increased, and the austenite grains are expanded. By increasing the number of deformation zones to increase the nucleation site of ferrite in the subsequent ferrite transformation and to reduce the grain size of ferrite, and (3) γ / α in controlled cooling after rolling. It is characterized in that ferrite is made finer and fine bainite structure is introduced by adjusting the conversion ratio.

[0008] The TMCP method can impart extremely excellent brittle crack propagation stopping characteristics to a relatively thin steel material having a plate thickness of 40 mm or less used for ships and line pipes. However, when the plate thickness of the steel material increases, for example, a steel material having a plate thickness of more than 40 mm, there is room for improvement in terms of brittle crack propagation stopping properties.

Further, in controlled rolling, by rolling the transformed ferrite to develop a texture,
The TMCP method for improving the brittle crack propagation stopping property is also known. This is a method of increasing the resistance to brittle fracture by causing separation on the fracture surface of the steel material in a direction parallel to the plate thickness direction to relax the stress at the tip of the brittle crack. However, as the steel plate becomes thicker, it becomes difficult to sufficiently bring out the effect of TMCP. Further, if excessive working is performed below the transformation point, there is a problem that the toughness of the steel material in the plate thickness direction deteriorates.

On the other hand, in recent years, a technique of making the microstructure of the surface layer portion of the steel material ultrafine without increasing the alloy cost has been proposed as a means for improving the brittle crack propagation stopping property. For example, in Japanese Unexamined Patent Publication No. 4-141517, attention is paid to the fact that a shear lip (plastic deformation region) generated in a steel material surface layer portion when a brittle crack propagates is effective in improving the brittle crack propagation stopping property, and a shear lip portion A method of refining the crystal grains of (1) to absorb the propagation energy of the propagating brittle crack is disclosed.

As a method for embodying this, a step of cooling the surface layer portion to an Ar3 transformation point or lower by controlled cooling after hot rolling, and then stopping the controlled cooling to reheat the surface layer portion to a transformation point or higher is carried out. This is repeated more than once, and during this period, a reduction is applied to the steel material to repeatedly transform or process and recrystallize, thereby forming an ultrafine ferrite structure or bainite structure in the surface layer portion.

[0012] Further, in JP-A-5-271863, the structure of the surface layer portion of the steel material for improving the brittle crack propagation stopping property is required to have a fine ferrite crystal grain or bainite structure having an average circle equivalent diameter of 3 µm or less. It is disclosed that it is effective that the fine crystal grains form a texture and the crystal grains are flat.

However, in these disclosed inventions, only the surface layer of the steel material is once cooled, reheated, and processed during reheating to obtain a structure effective in the brittle crack propagation arresting property. Yes, this is a process that is considered to be difficult to control on an actual production scale. Although a fine structure is obtained by utilizing the fact that ferrite is processed and recrystallized, processed recrystallized ferrite tends to grow and lacks structural stability. There is a problem that uniformity is likely to occur.

Therefore, in Japanese Unexamined Patent Publication No. 8-225836, a descaling device attached to a rolling mill is improved to
An invention for stabilizing the above unstable structure by a method of controlling the timing of cooling and rolling is disclosed. However, since the present invention uses the descaling device arranged in a limited space for a purpose different from the original purpose, it requires a modification for the purpose of greatly increasing the capacity. There are challenges. Moreover, it is considered that the method for fundamentally solving the instability of the ferrite structure, which is easily affected by the thermal history, has not been reached.

Further, even if the ultrafine structure excellent in the brittle crack propagation arresting property described in the prior art is stably obtained, it is shown in Welding Society Papers, Vol. 15, No. 1, pp. 148-154 (1997). As described above, the ultrafine structure inevitably increases the hardness of the surface layer portion. Therefore, the hardness of the surface layer portion is significantly higher than the hardness of 160 to 170 at the center portion of the plate thickness such that the Vickers hardness exceeds about 200, and the ultrafine structure on the surface layer side can obtain brittle crack propagation stoppage. There is a fear that the characteristics may be offset.

In view of this, according to Japanese Patent Laid-Open No. 8-253812, for the purpose of making the hardness distribution uniform in the plate thickness direction, a steel containing V is used to directly cool a heated slab to impart a temperature difference, thereby providing ferrite. And then start rolling,
There is disclosed a method of reheating in the temperature range near the transformation point during or after rolling. This is a technical idea in which the precipitation hardening of V is applied only to the central part of the plate thickness, the hardness distribution in the plate thickness direction can be made uniform, and the brittle crack propagation stopping property can be improved. However, in order to properly precipitate the V compound, it is necessary to make the process more complicated, and it is considered that it is not always linked to the solution of the above-mentioned instability of the structure.

As described above, these prior arts, which are intended to form an ultrafine structure on the surface of a steel material to improve the brittle crack propagation arresting property, are industrial scale,
It is considered difficult to obtain a stable organization. Further, in the prior art, although it is clear that the steel material itself produced using the disclosed technology has excellent brittle crack propagation arresting characteristics, the fracture toughness value after undergoing plastic deformation is mentioned. Therefore, it is not clear whether or not the brittle crack propagation arresting property is sufficient.

[0018]

DISCLOSURE OF THE INVENTION The present invention clarifies the structure required to have excellent brittle crack propagation arresting properties even after undergoing plastic deformation, and it is possible to obtain such a structure stably. An object of the present invention is to provide a method of manufacturing a steel material that can be performed without using an expensive alloy element.

[0019]

Means for Solving the Problems As a result of intensive studies aimed at achieving the above-mentioned problems, the present inventors have found that, as will be described below, excellent brittle crack propagation arresting properties are obtained even after undergoing plastic deformation. The invention of the steel material and the manufacturing method for stably obtaining the steel material has been completed. That is, in order to improve the brittle crack propagation arresting property, in the steel material having ferrite as the main structure, not only the grain refinement of the ferrite crystal grains but also the subgrains formed in the ferrite crystal play a large role. It was possible to clarify the relationship between the ferrite and subgrain structures and the TMCP condition, which is a manufacturing condition.

A first invention is a steel material having a main structure of ferrite, and contains subgrains having a maximum diameter of 5 μm or less in ferrite crystal grains having an oblateness of 2 or more and a minor axis diameter of 5 μm or less. The structure (hereinafter referred to as "sub-grain structure") occupies 5% or more of the steel plate thickness, and is a steel material excellent in brittle crack propagation arresting characteristics. A steel material having such a subgrain structure can obtain excellent brittle crack propagation stopping properties even after undergoing a large plastic deformation.

The second to fifth inventions are methods for stably obtaining the fine grain ferrite structure and the subgrain structure in actual production. That is, the second invention is a method of manufacturing a steel material having excellent brittle crack propagation stopping characteristics, which is characterized by the following steps when manufacturing a heated steel slab into a steel material having a predetermined shape and thickness. (A) A step of temporarily stopping the rolling of the steel slab at a temperature of 3 points or higher of Ar, and (b) 6 points in a temperature range of -20 ° C. or lower of 3 points of Ae.
Step of staying for 0 seconds or more, (c) Ar 3 points or less Ar 3
A step of rolling at a cumulative rolling reduction of 50% or more in a temperature range of -100 ° C or higher, and (d) immediately after that, cooling rate of 2 ° C /
Step of controlled cooling to a temperature range of 650 ° C. or higher at s or higher.
According to the present invention, it is possible to stably obtain a fine grain ferrite structure and a subgrain structure that contribute to the improvement of the brittle crack propagation arresting property after undergoing plastic deformation.

The third invention is, before the step (a),
It is a method for producing a steel material having excellent brittle crack propagation stopping properties, which is characterized by adding a rolling step of applying a reduction of cumulative reduction of 50% or more in an austenite non-recrystallization temperature range. According to the present invention, the ferrite structure can be more stably made to have an oblateness of 2 or more and a minor axis diameter of 5 μm or less.

In a fourth aspect of the invention, the step (b) is performed by Ae
Excellent brittle crack propagation arresting property, characterized by having a step of staying in a temperature range of 3 points −100 ° C. or lower for 5 seconds or more instead of a step of staying in a temperature range of 3 points −20 ° C. or lower for 60 seconds or longer. It is a method of manufacturing a steel material. According to the present invention, a subgrain structure can be obtained more stably.

A fifth aspect of the present invention is, before the step (a),
A rolling step for applying a rolling reduction of cumulative reduction of 50% or more in the austenite non-recrystallization temperature range, and a step of allowing the step (b) to stay in a temperature range of Ae3 point−20 ° C. or lower for 60 seconds or longer. Instead, it is a method for producing a steel material having excellent brittle crack propagation arresting characteristics, which comprises a step of allowing the material to stay in a temperature range of Ar 3 points −100 ° C. or lower for 5 seconds or more.
According to the present invention, it is possible to more stably obtain a subgrain structure and a fine-grain ferrite crystal grain that are effective in improving the brittle crack propagation arresting property.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention aim to develop a steel material having excellent brittle crack propagation arresting characteristics not only in the state in which the steel material is manufactured but also after undergoing plastic deformation,
The relationship between organization and characteristics was investigated. In particular, the ultrafine structure generated in the steel surface layer portion presented in the prior art lacks stability due to changes in thermal history, and inevitably increases the hardness of the surface layer portion, thus improving the brittle crack propagation arresting property. May be disturbed. Therefore, a stable structure effective for the brittle crack propagation arresting property was found, and the relationship between it and the TMCP conditions was examined.

In the examination, general structural steel having a tensile strength of 400 MPa class or 490 MPa class, which is often used for large-scale structures, was used. Its chemical composition is C: 0.0
2 to 0.2%, Si: 0.05 to 0.5%, Mn: 0.
5 to 2.0%, P: 0.005 to 0.02%, S: 0.
001 to 0.005%, Sol. Al: 0.01-0.
08%, containing an appropriate amount of Nb, V, Ti, Cu, Ni as necessary, and its Ceq (WES formula):
It is 0.40% or less and Pcm: 0.20% or less. After the test steel was formed into a steel slab by continuous casting, a steel material having a plate thickness of 16 to 30 mm was manufactured under various TMCP conditions. Based on these experimental results, the reason why the structure of steel and the manufacturing conditions are limited in the present invention will be described below.

(1) Structure of Steel Material The present invention is intended for a steel material having ferrite as a main structure.
That is, as a structure of steel, ferrite is approximately 80%.
Occupying the above, the balance consists of pearlite and / or bainite. The target was limited in this way because the tensile strength frequently used for large-scale structures is 400 MPa class or 4
This is because it is a typical structure of general structural steel of 90 MPa class.

The most common method for improving the brittle crack propagation arresting property of steel materials is to reduce the grain size of the ferrite structure and to cause separation. This is because the latter has the role of relaxing the stress at the crack tip when the brittle crack propagates, as described above. It is known that separation can promote its development by developing a texture. Therefore, first, the effects of grain refinement of the texture and the ferrite structure on the brittle crack propagation arrest characteristics before and after the plastic deformation was investigated using various steel materials with different rolling conditions. As the plastic deformation, a tensile prestrain of 10% was applied to the steel material.

The development of the texture was evaluated simply by the elongation of the ferrite crystal grains, that is, the flatness of the crystal grains observed using an optical microscope. Here, the flatness of the ferrite crystal grains means the ratio of the major axis diameter to the minor axis diameter of the ferrite crystal grains as shown in FIG. Further, the evaluation of the brittle crack propagation stopping property was carried out by collecting NRL drop weight test pieces from the rolling direction and
This was done by measuring the DTT. The target value of NDTT was −60 ° C. or lower. The NRL drop weight test was performed in accordance with the ASTM E208 standard, and the test piece shape was a P-3 test piece (thickness 15.9 mm, width 51 mm, length 127 mm).

The relationship between the flatness of the ferrite crystal grains and the minor axis diameter of the flat ferrite crystal grains and the NDTT is shown in FIGS. The steel material that has undergone plastic deformation naturally has a lower NDTT than the steel material that has not undergone plastic deformation, but with or without plastic deformation, the flatness of ferrite crystal grains increases (texture development). ) And reduction of the minor axis diameter of the flat ferrite crystal grains (fine graining), it is clear that NDTT is improved. In particular, in the case where the ferrite crystal grains have a flatness of 2 or more and a minor axis diameter of 5 μm or less, an NDTT of −60 ° C. or less is obtained even after undergoing a large plastic deformation of 10%. You can

The steel material having this good NDTT contains N
Clear separation was observed on the fracture surface of the test piece that broke at a temperature near DTT, but on the other hand, despite the fact that the same structure was observed and the occurrence of separation was observed, ND
For some steel materials, the TT does not reach -60 ° C. Therefore, the finer graining of ferrite and the occurrence of separation (increase of the oblateness of ferrite crystal grains) are conditions necessary for improving the brittle crack propagation arresting property after being subjected to plastic deformation. It turns out that the purpose of can not be achieved.

Therefore, it is considered that the subgrain, which is a substructure of ferrite, may also influence the brittle crack propagation arresting property, and a scanning electron microscope (SEM) or a transmission electron microscope (TEM) is used. Detailed observations were made.
As a result, in the steel material having an NDTT after plastic deformation of −60 ° C. or less, many subgrain boundaries (sub-grain boundaries) are observed in the elongated ferrite crystal grains,
It can be seen that the substructure of crystals such as subgrains also plays an important role.

FIG. 3 shows the relationship between the subgrain size of the steel material before plastic deformation and the NDTT after plastic deformation. It is clear that the NDTT improves with the refinement of the subgrains, but the NDTT after undergoing plastic deformation is stratified by the size of the minor axis diameter of the ferrite crystal grains as the matrix phase. That is, when the minor axis diameter of the ferrite crystal grains exceeds 5 μm, the NDTT is not improved so much even if the subgrain size is 5 μm or less. On the other hand, when the minor axis diameter of the ferrite crystal grains is 5 μm or less and the size of the subgrain is 5 μm or less,
Many meet the target of NDTT-60 ° C or lower.

Further, the larger the proportion of the subgrain structure, the more stable and excellent the brittle crack propagation arresting property after undergoing plastic deformation. FIG. 5 is a diagram showing the relationship between the ratio of the subgrain structure in the plate thickness direction of the steel material and NDTT. By this structure occupying 5% or more of the plate thickness, NDTT-60 ° C. or less is obtained. It turned out that it can be obtained stably. In addition, the subgrain structure is preferentially generated from the front and back surfaces of the steel material from the heat history at the time of manufacturing the steel material.

As described above, in order for the steel material to have excellent brittle crack propagation arresting properties even after undergoing a large plastic deformation, in addition to the refinement of the ferrite structure and the development of the texture structure of the steel material, As a result, the subgrain structure generated within the ferrite crystal grains plays an important role. Therefore, controlling the size of the subgrain structure and the amount of generation thereof can achieve the object of the present invention.

The term "subgrain" as used herein generally means that the walls (cells) formed by the dislocations introduced into the metal by plastic deformation and the dislocations inside the cells are rearranged by recovery by annealing. The sub-crystal grains that are almost the same size as the first cell and have a very small misorientation to each other (Metal Material Technical Term Dictionary, edited by Metal Material Technical Research Institute, Nikkan Kogyo Shimbun). Since the subgrain structure is different from the ferrite grain boundary, even if a corrosive liquid that develops the structure of steel materials (for example, nital and picral) is used,
It is difficult to see clearly under the magnification of an optical microscope, but S
It is clearly observed when using EM. Therefore, in the present invention, the size of the subgrain is measured by measuring the maximum diameter of SEM images or SEM photographs at several locations using an image processing apparatus. Also, with the maximum diameter of the subgrain,
The diameter of the smallest circle that can include an elliptical or polygonal subgrain inside, and the concept is shown in FIG.

Although the mechanism of the effect of such a subgrain structure on the brittle crack propagation arresting property after plastic deformation is not clear, the subgrain boundary plays a role similar to that of a ferrite grain boundary, and the size of the subgrain is large. Is believed to substantially control the low temperature toughness of the steel. Further, it is presumed that this subgrain structure plays a role of alleviating the adverse effect of work hardening (dislocation structure introduced) accompanying plastic deformation.

Furthermore, this subgrain structure is relatively easy to control and can be stably obtained, as described later. That is, the ultrafine structure of the surface layer portion of a conventional steel material is obtained by applying a reheating process to the steel material and utilizing work recrystallization of ferrite. On the other hand, in the present invention, the ferrite grains which are transformed from austenite and have no anisotropy are subjected to a reduction so that the ferrite grains are anisotropic (texture).
And at the same time form a subgrain structure,
It does not utilize work recrystallization of ferrite. Therefore, it is considered that the subgrain structure itself also has anisotropy (texture), and has the effect of improving the brittle crack propagation stop property of the steel material itself, but especially the brittle crack propagation stop property in the rolling direction It is considered that this is a factor that has significantly improved.

The subgrain structure has a hardness of about 180 or less as measured by a micro hardness meter (load 98N). Therefore, even if this fine structure is generated in the front and back surfaces of the steel,
It is considered to have the effect of suppressing the decrease in ductility of the front and back surfaces, and this is also considered to contribute to the improvement of the brittle crack propagation arresting property of the steel of the present invention.

From the above, it is a steel material having a main structure of ferrite, and (a) the ferrite crystal grains have an oblateness of 2
As described above, the minor axis diameter is 5 μm or less, (b) the subgrains having a maximum diameter of 5 μm or less are formed in the ferrite crystal, and (c) the fine ferrite structure containing the subgrains is a steel plate. Occupying a portion of 5% or more of the thickness is a necessary condition for a steel material having excellent brittle crack propagation arresting properties even after undergoing plastic deformation.

(2) TMCP Conditions for Steel Material Next, in order to stably obtain this subgrain structure, when the heated steel slab is manufactured into a steel material having a predetermined shape and thickness, the following rolling and cooling are performed. Conditions are required.
That is, (a) rolling conditions for ensuring fine ferrite crystal grains, (b) rolling conditions for generating a fine ferrite structure in a portion of 5% or more of the steel plate thickness, and (c) developing a texture in the fine ferrite. Under the rolling conditions in which the dislocations introduced by processing (rolling) are rearranged by thermal energy to form subgrains, (d) the cooling conditions for suppressing coarsening of the formed fine ferrite crystal grains and fine subgrain grains. is there.

The step corresponding to (a) above is to apply a cumulative reduction of 50% or more in the unrecrystallized region of austenite of 3 or more points of Ar. As a result, the austenite structure can be expanded, a large number of deformation zones can be introduced into the austenite grains, and the minor axis diameter of the final ferrite crystal grains can be 5 μm or less. On the other hand, if the cumulative rolling reduction in the austenite unrecrystallized region is less than 50%, the desired fine ferrite structure cannot be obtained. Prior to this rolling, it is desirable to optimize the heating temperature of the steel slab to prevent coarsening of the heated austenite crystal grains and to reduce the size of the austenite grains by recrystallization.

The step corresponding to the above (b) is a step in which the rolling after the step (a) is temporarily interrupted and the steel material is allowed to stay in the temperature range of Ae3 point−20 ° C. or lower for 60 seconds or longer. The step of interrupting the rolling releases the strain in the crystal structure introduced in the previous rolling, so that a uniform structure can be obtained thereafter. Further, by the step of allowing the steel material to stay in the temperature range of Ae3 point−20 ° C. or lower for 60 seconds or more, ferrite transformation is promoted and a predetermined ferrite phase fraction can be secured. Temperature range to stay is Ae3 transformation point -20 ℃
If the temperature is lower than this, the supercooling during transformation is small and the time required for ferrite transformation becomes long, which is not preferable because the rolling efficiency is reduced in industrial scale production.
Further, if the staying time is less than 60 seconds, the ferrite transformation may not proceed sufficiently depending on the staying temperature. This step includes both the case where the temperature is kept isothermally and the case where the temperature range is cooled for 60 seconds or longer.

The Ae3 point is the boundary temperature between the γ single-phase region and the γ + α two-phase region in the equilibrium state of steel. Here, the reason why the Ae3 transformation point is used as the control factor is to take into consideration the case where the temperature of the steel material drops at a slower rate than air cooling in this step.

The Ae3 point is simply the Ac3 point which is the transformation point when the steel is heated and the Ar point which is the transformation point when the steel is cooled.
It can be calculated as an average temperature of three points. In this case, the difference between the Ae3 point and the Ar3 point is empirically 20 to 30 ° C. The Ac3 point and the Ar3 point can be obtained by the following empirical simple formula. Ac3 = 854-179.4xC + 44.4xSi-1.
3.9xMn Ar3 = 310xC-80xMn

On the other hand, in this step, from the viewpoint of suppressing the growth of transformed ferrite crystal grains, a low temperature and a short time are preferable. That is, after the step (a), even in the step of cooling to a temperature of Ar 3 point −100 ° C. or lower by water cooling or the like and staying in that temperature range for 5 seconds or more, ferrite cooling is promoted due to large overcooling, Moreover, coarsening of ferrite crystal grains can be suppressed, which is preferable. If the staying temperature is too low, the driving force for transformation becomes insufficient. Therefore, for example, Ar3 point −200 ° C. or higher is desirable.

The steps corresponding to (c) above are (a),
For the steel material that has undergone the process of (b), Ar 3 points or less and Ar 3 points-1
The rolling is finished after applying a rolling reduction of 50% or more in a temperature range of 00 ° C. or more. This is a step for elongating the transformed ferrite grains and at the same time introducing dislocations necessary for developing subgrains in the ferrite grains. The reduction rate in the temperature range of 50% or more is because the flatness of the ferrite crystal grains is 2
The reason is to ensure the strain required for the development of subgrains. Further, the reason why the rolling temperature is set to the Ar3 point or lower is that when the rolling temperature is high, the transformed ferrite may cause grain growth, and the ferrite may be recrystallized. The reason for setting Ar3 point to −100 ° C. or higher is that a certain amount of thermal energy is required to rearrange the introduced dislocations.

The last step (d) is as follows:
650 at a cooling rate of 2 ° C./s or more after the step (c) is completed.
This is a step of controlled cooling to below ℃. When cooling at a slower cooling rate than this, or when the temperature at which controlled cooling ends is 65
This is because when the temperature exceeds 0 ° C., the generated fine ferrite crystal grains and fine subgrain grains are coarsened.

As a method of controlling the temperature of the steel material in the actual manufacturing, there are a method of directly measuring the surface temperature of the steel material and a method of obtaining it by computer simulation from a cooling curve of the surface temperature. Further, when the process (a) is shifted to the process (b), as a method of lowering the temperature to a target temperature range after the rolling is interrupted at the Ar 3 point or more, forced cooling by using not only water cooling but also wind blast cooling is used. It is also possible to perform cooling.

In the relationship between the temperature range in which the steel material stays in the step (b) and the rolling temperature range in the step (c), if the former is high, the rolling in the step (c) can be carried out as it is. When the former is low temperature, it is necessary to heat to a predetermined rolling temperature range. In this case, heat retention or heating may be performed by a gas heating device, an induction heating device, a direct current heating device, or the like. Further, as the method of controlled cooling in the step (d), a water cooling device, an air cooling device or the like can be used.

As the material for rolling in the present invention, slabs, blooms, billets and ingots which have been refined by a converter method or an electric furnace method, which are generally used in the art, and cast by a continuous casting method or a soul-making method. Etc. can be used.

[0052]

EXAMPLES Examples of steels produced according to the invention having good brittle crack propagation arresting properties even after undergoing plastic deformation are given below. Table 1 shown as FIG. 7 shows the chemical composition of the sample steel. The table also shows the temperatures of Ar3 point and Ae3 point. From a steel slab having these chemical components, a steel plate having a plate thickness of 30 mm was rolled to evaluate the characteristics. Regarding the manufacturing conditions of the test steel sheet, Table 2 shown in FIG. 8 shows an example of the present invention, and Table 3 shown in FIG. 9 shows a comparative example. Further, various characteristics of the test steel sheet are shown in Table 4 shown in FIG.
Table 5 shown as FIG. 11 shows a comparative example.

Here, the maximum diameter of the subgrain was obtained by the above method. Further, the brittle crack propagation stopping property was evaluated by NDTT by an NRL drop weight test and Kca (0 ° C.) by a double tensile test for a steel material which was given 10% prestrain as it was rolled. The double tensile test is based on the Japan Welding Association (WE
The test was carried out in accordance with the brittle fracture propagation stop test stipulated in the steel type certification test method of S). For reference, the Vickers hardness of the relevant structure is also shown.

Steel Nos. 1 to 21, which are examples of the present invention, have an NDTT of −60 ° C. even after being subjected to plastic deformation of 10%.
Hereinafter, the excellent brittle crack propagation stopping property of Kca (0 ° C.) of 600 kgf / mm3 / 2 or more was shown.

On the other hand, in the steel No. 22 of the comparative example, since the temperature at which rolling is interrupted is in the two-phase region at the Ar3 point or less, the ferrite crystal grains are elongated, but the subgrain structure cannot be obtained, and brittle cracking occurs. The propagation stopping property was not improved. Steel Nos. 23, 24, 29, 30, 33, 35 and 36 are Ae
Since the stay time at -20 ° C or lower at 3 points was short, ferrite transformation in this temperature range did not sufficiently occur,
As a result, the subgrain structure was not sufficiently obtained. Therefore, the brittle crack propagation arresting property after being subjected to plastic deformation is insufficient.

Steel Nos. 25, 26, 27, 31 and 32 are
The residence time at Ae 3 points −20 ° C. or lower or Ar 3 points −100 ° C. or lower is within the range of the present invention, and the ferrite transformation is sufficiently secured. However, in Comparative Example 25, the rolling temperature after the ferrite transformation (rolling restart temperature in Table 2) was Ar.
Since the rolling temperature was below 3-100 ° C., the rolling temperatures of Steel Nos. 26 and 31 exceeded the Ar3 point, and the rolling ratios of Steel Nos. 27 and 32 did not reach the range of the present invention. Was not obtained. Further, in steel Nos. 28 and 34, the target subgrain structure was obtained immediately after the final rolling, but the appropriate subgrain structure was lost because the controlled cooling was not performed after rolling. Is. These comparative steels, as shown in Table 5,
The brittle crack propagation arresting property may be excellent until rolling, but the brittle crack propagation arresting property after undergoing plastic deformation is not sufficient.

EFFECTS OF THE INVENTION The steel material having a subgrain structure of the present invention has excellent brittle crack propagation arresting properties even after undergoing plastic deformation. Further, when the production method of the present invention is used, the stability of the generated subgrain structure is high.
Therefore, when used in a large structure, even if the steel material is largely deformed due to an accident or the like, it is possible to prevent a large-scale collapse of the structure. Further, in the embodiments of the present invention, the thick steel plate manufactured by the thick plate rolling is illustrated, but it is needless to say that the idea of the present invention can be applied to other steel material manufacturing processes, for example, a shaped steel rolling process.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an oblateness of ferrite crystal grains and an NDTT before and after applying plastic deformation.

FIG. 2 is a diagram showing a relationship between a minor axis diameter of a flattened ferrite crystal grain and an NDTT before and after applying plastic deformation.

FIG. 3 is a diagram showing a relationship between a maximum subgrain diameter and a minor axis diameter of a matrix ferrite crystal grain, and NDTT after undergoing plastic deformation.

FIG. 4 is a diagram showing a method for evaluating the oblateness of ferrite crystal grains.

FIG. 5 is a diagram showing a relationship between a ratio of a subgrain structure in a plate thickness direction of a steel material and NDTT after undergoing plastic deformation.

FIG. 6 is a diagram showing a method of evaluating the maximum diameter of subgrains.

FIG. 7 is a diagram showing Table 1 showing the chemical composition of the sample steel shown as an example.

FIG. 8 is a table showing Table 2 as to the manufacturing conditions of the test steel sheet of the invention example.

FIG. 9 is a diagram showing Table 3 as to the manufacturing conditions of the test steel sheet of the comparative example.

FIG. 10 is a table showing various characteristics of the test steel sheet of the invention example.

FIG. 11 is a table showing various characteristics of a test steel sheet of a comparative example.

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-11-256229 (JP, A) JP-A-5-148544 (JP, A) JP-A-8-41536 (JP, A) JP-A-5- 9651 (JP, A) JP-A-8-295982 (JP, A) JP-A-56-25926 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 6 / 00,8 / 00-8/10 C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. In mass%, C: 0.02-0.2%, S
i: 0.05 to 0.5%, Mn: 0.5 to 2.0%,
P: 0.005-0.02%, S: 0.001-0.0
05%, sol. Al: 0.01 to 0.08%, the balance
A thick steel plate having a composition of Fe and unavoidable impurities and having ferrite as a main structure, having a maximum diameter of 5 μm or less in ferrite crystal grains having an oblateness of 2 or more and a minor axis diameter of 5 μm or less. The plastic deformation characterized by the structure containing subgrains occupying 5% or more of the steel plate thickness
Thick steel with excellent brittle crack propagation arresting properties after receiving
Board . 2. Further, Nb: 0.048% or less in mass%.
Below, V: 0.055% or less, Ti: 0.011% or less,
Cu: 0.18% or less, Ni: 0.09% or less
Or 2 or more kinds are contained, The statement in Claim 1 characterized by the above-mentioned.
Excellent brittle crack propagation arrest characteristics even after undergoing plastic deformation
Thick steel plate with properties . 3. When manufacturing a heated steel slab into a steel material having a predetermined shape and thickness, the thickness having excellent brittle crack propagation stopping properties even after undergoing plastic deformation characterized by the following steps: Steel plate manufacturing method. (A) a step of temporarily stopping the rolling of the steel slab at a temperature of 3 points or higher of Ar, and (b) air cooling to a temperature range of -20 ° C. or lower of 3 points of Ae , and
A step of staying in the temperature range for 60 seconds or more, and (c) a step of rolling at a cumulative rolling reduction of 50% or more in a temperature range of Ar3 point or less and Ar3 point-100 ° C or more, and (d) immediately after that, cooling rate 2 ° C / 650 ° C above s
Controlled cooling to the following temperature range. 4. The rolling step of applying a rolling reduction of a cumulative rolling reduction of 50% or more in the austenite non-recrystallization temperature region before the step (a).
Excellent brittle crack propagation stop characteristics after receiving plastic deformation
The manufacturing method of the thick steel plate which has .