JPH05271860A - Structural steel excellent in brittle fracture resistance and its production - Google Patents

Abstract

PURPOSE:To economically produce a structural steel plate excellent in brittle fracture propagation characteristic with high productivity. CONSTITUTION:This plate is a steel plate which has ferrite structure of <=3mum average circle equivalent diameter or bainite structure in the surface layer part and where the aspect ratio (the ratio of major axis to minor axis) of texture colony having the same crystal orientation in the surface layer part structure is regulated to >=4. A steel slab or a steel plate is cooled rapidly down to a temp. not higher than the temp where ferrite fraction becomes >=50% through the region of >=0.02Xt0(mm) at least in a plate thickness direction from the surface layer when t0 represents the plate thickness at the time of water cooling in the course of rolling. Subsequently, rolling is started or resumed at the point of time when the surface layer part reaches a temp. not lower than the Ar1 point and rolling is done in the range between (Ac3-100) deg.C and Ac3 deg.C. Then, cooling is done at least down to the Ar1 point at a rate of >=1 deg.C/ sec without recuperation up to a temp. not lower than the Ac3 point.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a brittle fracture propagation arrest (arrest) performance, which is one of the important performances of a steel sheet for ensuring the safety of a structure, added with an expensive alloy element such as Ni element. The present invention relates to a steel sheet and a method for manufacturing the same, which dramatically improves the production efficiency without depending on the above.

[0002]

2. Description of the Related Art As a means for improving the arresting performance of brittle fracture propagation (arrest), as described in JP-A-59-47323, a production method of sufficiently reducing in a non-recrystallized region, or positively The second phase particles, which tend to cause brittle fracture, are dispersed in the cracks to cause many microcracks at the tips of the brittle cracks to relax the stress state at the crack tips, and to stop the cracks by ductile fracture that occurs during the coalescence between the microcracks and the main cracks. A method of making it easier has been proposed.

However, those proposals are for modifying the structure of the central part of the plate thickness and improving the brittle crack propagation stopping performance, and the NDT characteristic in the drop weight test mainly determined by the structure of the surface part of the plate thickness. Does not necessarily improve. In addition, when the plate thickness of the steel plate increases, the above-described grain refinement of the central portion of the plate thickness may not be achieved, and in particular, development of a technique for improving the arrest performance of the steel plate having a plate thickness of 25 mm or more is desired.

On the other hand, a method for producing a steel sheet having a fine grain structure on the surface layer of the steel sheet is described in JP-A-61-235534, and the surface layer is defined as having a structure of 5 μm or less. Materials and Processes, 6 (1990), p.
As described in 1796, even a ferrite grain of 3 μm or less easily causes brittle fracture at −120 ° C. or less, and there is a limit to the method of improving the arrest performance for forming a fine grain structure in the surface layer portion.

Further, Japanese Patent Application No. 02-24509 discloses a technique for cooling and reheating the surface layer portion up to ⅓ of the plate thickness and improving the structure by improving the structure of the surface layer portion. ing. However, with this method, it is necessary to realize cooling recuperation over a wide range up to 1/3 of the plate thickness, and without an external heat source, processed ferrite is generated in the center part of the plate thickness and toughness may deteriorate. The nature is great. Further, although the arrest performance can be improved by such a manufacturing method, the structure necessary for improving the arrest performance is not clear, and the surface structure necessary for efficiently improving the arrest performance and its necessary thickness are unknown. ..

[0006]

DISCLOSURE OF THE INVENTION The present invention clarifies the required structure and required thickness for improving the Kca property and NDT property which are the arrest performance by modifying the structure of the surface layer portion, and greatly increases the manufacturing cost. An object of the present invention is to provide a steel sheet having good arrestability and a method for manufacturing the steel sheet without adding expensive Ni element or the like.

[0007]

In order to achieve the above object, the present invention provides (1) a ferrite structure having a mean equivalent circle diameter of 3 μm or less over a range of at least 2% of the plate thickness from the surface layer, or A steel sheet having a bainite structure and having an aspect ratio (ratio of major axis / minor axis) of a texture colony having the same crystal orientation of its surface layer structure is 4 or more is a basic means.

Further, according to the present invention, (2) a steel slab or a steel plate having a temperature of Ac ₃ or more is 0.02 × t from the surface layer at least in the plate thickness direction when the plate thickness during water cooling during rolling is t _{0. 0}
The ferrite fraction is 50 at 2 ° C / sec in the area above (mm).
% Or more, and then the surface layer portion starts or restarts rolling from a temperature of Ar ₁ point or more,
Rolling is completed within the range of (Ac ₃ points −100) ° C. to Ac ₃ points, and then 1 ° C. without reheating to the Ac ₃ points.
The second means is characterized in that cooling is performed to at least the Ar ₃ point for at least ₃ sec / sec, and (3) a steel slab or a steel plate having a temperature of the Ac ₃ point or more is set to have a thickness during water cooling during rolling. When t ₀ , it is 0.0 from the surface layer at least in the plate thickness direction.
A region of 2 × t ₀ (mm) or more is rapidly cooled to a temperature at which the ferrite fraction is 50% or more at 2 ° C./sec, and then the surface layer portion starts or restarts rolling at a temperature of Ar ₁ point or more. Then, the rolling is completed in the range of (Ac ₃ -100) ° C to Ac ₃ ° C, and thereafter, accelerated cooling is performed at a cooling rate of 5 ° C / sec or more without reheating to the Ac ₃ point. And (4) applying a tempering heat treatment at 550 ° C. or lower is the fourth means.

In the present invention, the target structural steel is
For example, as described in JP-B-58-14849 mentioned above, as will be described below, in order to obtain the required material for the ordinary structural steel, the actions and effects that have been conventionally confirmed in the utilization in the field of the art. The same action and effect can be obtained by similarly using the types and amounts of the additional elements defined based on the relationship of. Therefore, the present invention is intended for steels containing these elements.

The respective constituent elements, the reasons for their addition and the amounts thereof are as follows. C is an effective component for improving the strength of steel and is added in an amount of 0.02% or more.
If the content exceeds 0.1%, not only does the deformation resistance during the two-phase region rolling increase and rolling becomes difficult, but island martensite precipitates in the welded portion, significantly degrading the toughness of the steel. It is regulated to 02% to 0.20%.

Si is necessary as a deoxidizing element for molten steel,
Although it is useful as a strength increasing element, if it exceeds 1.0%, the workability of the steel is deteriorated and the toughness of the welded portion is deteriorated.
%, The deoxidizing effect is insufficient, so the addition amount is 0.01
Regulate to ~ 1.0%.

Although Mn must be added in an amount of 0.3% or more as a component for improving the strength of the steel material, the addition of Mn lowers the transformation temperature. Therefore, the upper limit is 2.0%.

Al and N are added not only for refining steel by Al nitride but also for solid solution during rolling process, matching of crystal orientation of steel due to precipitation and recrystallization, but when the addition amount is small, it is effective. However, since the addition of an excessive amount deteriorates the toughness of steel, Al is 0.001 to 0.20% and N is 0.020.
% Or less.

P and S are 0.01% or less and 0.01% or less, respectively, in order to secure the toughness of the base material. The above is the basic composition of the steel to be the subject of the present invention, for the purpose of increasing the base metal strength or improving the joint toughness, depending on the properties required, when adding an alloying element, transformation If the temperature is lowered too much, the deformation resistance in the two-phase region increases and rolling becomes difficult, so the alloys to be added are Ni, Cr, M.
o, Cu, W, P, Co, V, Nb, Ti, Zr, T
a, Hf, rare earth element, Y, Ca, Mg, Te, Se,
One or more types of B can be used, but the total addition amount is restricted to 4.5% or less. Note that the average circle-equivalent grain diameter is obtained by focusing on individual grains of a corresponding tissue, obtaining the diameters of circles assumed to have equal areas, and averaging the diameters.

[0015]

The present inventors focused on the fact that even if the ferrite grain size of the ferrite-pearlite steel sheet containing no Ni element was reduced to 5 μm or less, the base material toughness vTrs was hardly improved. Through the elucidation of the mechanism, consideration about the resistance to brittle fracture necessary for improving the toughness of the steel sheet and the experiment were conducted.

It is already known that brittle fracture occurs when the local stress at the tip of the crack or notch exceeds the critical microscopic fracture stress determined by the structure of the steel sheet. That is, in order to improve the toughness of the steel sheet,
A method of improving the limit microscopic fracture stress of a steel plate,
It is conceivable to reduce the stress at the crack or notch tip by some means.

As the above method, a phenomenon in which a texture is developed to cause vertical cracks called separation in the direction parallel to the plate thickness of the steel sheet, and as a result, cracks or notch tips are released and stress is reduced. It has been known. That is, before the local stress reaches the critical microscopic fracture stress,
It turns out that it suffices that separation occurs. For that purpose, it is necessary that the critical fracture stress of the steel sheet is higher than the separation generation stress. However, in the actual steel sheet rolled in the ferrite-austenite two-phase region,
Separation occurs prior to fracture at the temperature at which plastic deformation prevails, but brittle fracture occurs at low temperatures.

This is because the yield point of the steel material rises at low temperatures and the plastic region at the crack tip becomes smaller, so that local deformation due to plastic anisotropy between colonies with different crystal orientations necessary for the occurrence of separation is caused. It is considered that this is because it does not occur, so various experiments were conducted using general structural steels having the chemical compositions shown below. C: 0.04 to 0.15% Si: 0.15 to
0.25% Mn: 0.4 to 1.6% Al: 0.01 to
0.05% P: 0.005-0.008% S: 0.001
~ 0.003% First, in order to quantify the microstructure morphology necessary for generating separation by the texture, various two-phase rolling conditions were changed to manufacture steel sheets having different texture levels. In order to quantify the texture on the texture, a temper color method that uses the change in the thickness of the oxide film depending on the crystal orientation was applied to reveal colonies with the same crystal orientation, and the aspect ratio (major axis / minor axis) Ratio) and the critical fracture stress in the plate thickness direction were evaluated. As a result, as shown in FIG. 1, it has been found that when the aspect ratio is 4 or more, the critical fracture stress in the plate thickness direction is 1/2 or less of the case where the aspect ratio is about 1 without texture. FIG. 2 is a schematic diagram of the aspect ratio.

Next, the aspect ratio becomes 2 or more so that it becomes 4 or more.
The relationship between the ferrite grain size and the critical temperature at which separation occurs was investigated using a steel sheet that had been phase-rolled.
The result is shown in FIG. In order to cause separation even in a low temperature range of −170 ° C. or less, the ferrite grain size is 3
It was found that it was less than μm.

FIG. 4 shows the relationship between the ferrite grain sizes having different aspect ratios and the brittle fracture initiation toughness Kc. That is, it is a decisive factor for improving the brittle fracture resistance to reduce the ferrite grain size to 3 μm or less so that the texture is developed and the separation is generated even at an extremely low temperature. This is because ferrite, which is a matrix structure, was made into ultrafine grains to enhance the critical microscopic fracture stress and to develop a texture structure capable of generating separation.

In order to achieve this structure, for example, if the ferrite is subjected to a necessary amount of working during the temperature rising process and the reverse transformation to austenite is prevented, the dislocations introduced into the worked ferrite are recovered. , The rearrangement is caused, the ultimate microscopic fracture stress is improved by the ultra-fine graining of ferrite, and the texture developed by the processing given to ferrite achieves the texture of the present invention by leaving it as it is. I found that I could do it.

Therefore, during rolling, the surface of the steel sheet is water-cooled to transform it into ferrite, and the sensible heat of the center of the sheet thickness where the temperature hardly drops even by cooling is used to raise the ferrite structure of the surface layer portion. While further rolling, an ultrafine grain structure of 3 μm or less having a texture only in the surface layer portion was formed.

At this time, in the case where the material was air-cooled after rolling, the temperature after rolling was high, so that a part of the ultrafine grain structure caused grain growth, and the target structure could not be obtained. Therefore, in order to suppress the grain growth after rolling, when cooled to the Ar ₁ point, 1
It was confirmed that a predetermined structure could be obtained if the cooling rate of the surface layer portion was not less than ° C / sec.

Also, by changing the water cooling conditions during rolling,
As a result of investigating the Kca performance of the steel sheet in which the thickness of the surface layer modified structure is changed, Kc is increased by increasing the thickness of the surface layer modified structure.
It has been found that the a-characteristics are improved, and that the required thickness of the surface-modified structure exists depending on the Kca performance required for the steel sheet.

[0025]

[Examples] Table 1 shows the components of the sample steels of Examples, and Table 2 shows the manufacturing conditions and the obtained materials together with Comparative Examples.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

The test numbers 1 to 12 of the present invention and the test numbers 13 to 16, 21, 22, and 24 of the comparative examples are those in which cooling was applied after rough rolling and the surface layer of the steel sheet was transformed into ferrite. In the test numbers 14, 21, and 22 of the comparative examples, the cooling rate was slow, so that the temperature of the entire steel sheet decreased, and the rolling after cooling did not become the temperature rising working. Moreover, in the test number 24 of the comparative example, the elapsed time after cooling was too long to satisfy the requirements for rolling after cooling. Therefore, the microstructures of the surface layer portions of test numbers 21, 22, and 24, which are comparative examples, were not made fine.

In the materials of these comparative examples, the entire plate thickness was rolled in the two-phase region, the vTrs which is the toughness of the base material was deteriorated, and both the NDT characteristics and the arrest characteristics were deteriorated. Also,
In each of the test numbers 17 to 20 and 23 of the comparative examples, cooling after rough rolling was not carried out, and the finish rolling temperature was aimed at just above the Ar ₃ point in the plate thickness average, so that the surface temperature was Ar ₃
Below the point, rolling caused abnormal ferrite grain growth, and as a result, the ferrite grain size in the surface layer was coarsened.

Further, in Comparative Examples 13 and 16, although the predetermined cooling / rolling was carried out, the ferrite grain size did not become 3 μm or less because of air cooling after the completion of rolling, and Comparative Examples 15 and 16
In the recuperating process after rolling, the alloy recuperated to Ac ₃ or more, so that grain growth partially occurred and a predetermined structure could not be obtained. Therefore, test numbers 13 to 20 and 23 which are comparative examples
As for the arrest performance, both temperature and NDT characteristics showing Kca = 600 kgf / mm ^1.5 did not reach -60 ° C.

On the other hand, test numbers 1 to 12 of the examples of the present invention
The material of the material satisfies the required manufacturing conditions, as shown in Table 2,
In addition to satisfying the target strength and toughness, the NDT temperature, which is the aim of the present invention, was -70 ° C. or higher, and the temperature at which the arrest performance, Kca = 600 kgf / mm ^1.5 , was also a sufficient characteristic.

[0033]

Since the present invention utilizes the above-mentioned action by using the above-mentioned means, after the rough rolling, only the surface layer portion is cooled to
After r ₁ or less, if rolling is performed while recovering heat by sensible heat inside the plate thickness, sufficient unrecrystallized in the center part of the plate thickness without generating an embrittlement structure in the surface layer portion that deteriorates NDT characteristics. NDT, which has arrest performance for rolling area rolling
It is possible to achieve both characteristics and Kca characteristics,
It has a great effect not only in the field of art but also in related fields.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a ratio of major axis / minor axis (aspect ratio) of a colony having the same orientation of a tissue developed by a temper color method and a critical fracture stress in a plate thickness direction.

FIG. 2 is a schematic diagram of an aspect ratio.

FIG. 3 is a chart showing a relationship between a ferrite grain size and a separation generation limit temperature.

FIG. 4 is a chart showing the relationship between the ferrite grain size and the Kc value, which is the toughness at which brittle fracture occurs at −165 ° C.

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[Procedure amendment]

[Submission date] December 4, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0028

[Correction method] Change

[Correction content]

[0028]

[Table 3]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0032

[Correction method] Change

[Correction content]

On the other hand, test numbers 1 to 12 of the examples of the present invention
The material of the material satisfies the required manufacturing conditions, as shown in Table 2,
In addition to satisfying the target strength and toughness, the NDT temperature, which is the aim of the present invention, was -70 ° C. or higher, and the temperature at which the arrest performance, Kca = 600 kgf / mm ^1.5 , was also a sufficient characteristic. The fatigue characteristics of the inventive examples were also good.

Front page continuation (72) Inventor Toshiaki Hajishi Oita-shi Osamu Nishinosu 1st Nippon Steel Co., Ltd. Oita Steel Co., Ltd. Inside the ironworks

Claims (4)

[Claims] 1. The front and back layers of a steel sheet each have a ferrite structure or bainite structure with an average equivalent circle diameter of 3 μm or less over a range of 2% or more of the plate thickness, and have the same crystal orientation of the surface layer structure. A structural steel having good brittle fracture resistance, characterized in that the texture colony has an aspect ratio (ratio of major axis / minor axis) of 4 or more. 2. A steel plate or a steel plate having a temperature of Ac ₃ points or more is 0.02 × t ₀ (mm) or more from the surface layer at least in the plate thickness direction when the plate thickness during water cooling during rolling is t _0. Area is rapidly cooled at a cooling rate of 2 ° C./sec or more until the ferrite fraction reaches 50% or more, and then the surface layer portion starts or restarts rolling at a temperature of Ar ₁ point or more, and (Ac ₃ point −
Rolling is completed within the range of 100) ° C. to Ac ₃ points, and then the surface layer portion is cooled at a cooling rate of 1 ° C./sec or more until at least the Ar ₁ point without reheating to the Ac ₃ point or more, and then the surface layer portion To at least 2% of the plate thickness, the aspect ratio (major axis) of a textured colony having a ferrite structure or bainite structure with an average circle-equivalent grain size of 3 μm or less and having the same crystallographic orientation of its surface layer structure /
A ratio of minor axis) is 4 or more, and a method for producing a structural steel sheet having good brittle fracture resistance. 3. The brittle fracture resistance is good according to claim 2, wherein the rolling is terminated, and thereafter, accelerated cooling is performed at a cooling rate of 5 ° C./sec or more without reheat to the Ac ₃ point or more. Method for manufacturing structural steel sheet. 4. The embrittlement-resistant fracture according to claim 2, wherein after the rolling is finished, accelerated cooling is performed at a cooling rate of 5 ° C./sec or more without reheating to Ac ₃ point or more, and further tempering heat treatment is performed. A method for manufacturing a structural steel sheet having good characteristics.