JPH05271862A - Structural steel excellent in brittle fracture resistance and its production - Google Patents

Abstract

PURPOSE:To produce a structural steel excellent in brittle fracture propagation arresting characteristic at a low cost by subjecting, after rough rolling, the surface layer part alone to cooling down to a specific temp. or below and performing rolling while exerting recuperation at the time of hot-rolling a steel slab. CONSTITUTION:A slab of carbon steel of about 0.02-0.20wt.% C content is hot-rolled at a temp. not lower than the Ac3 point. In the course of the hot rolling, the range of the surface layer part of >=2% of the thickness of the hot rolled plate is cooled rapidly down to a temp. not higher than the Ar1 point at >=5 deg.C/sec cooling rate. Successively, rolling is resumed at the point of time when the surface layer part reaches a temp. not lower than the Ar3 point and rolling is finished at a temp. in the range between (Ac3 point -50) deg.C and the Ac3 point, and then, cooling is done down to at least the Ar1 temp. at >=2 deg.C/sec cooling rate without recuperation up to the Ac3 point. By this method, the structural steel plate having brittle fracture resistance, which has a structure composed essentially of ferrite or bainite structure of <=3mum average circle equivalent grain diameter over the range of >=2% of the thickness from the surface of the steel plate and constituted of spheroidal carbide phase of <=0.6mum average circle equivalent diameter and where the aspect ratio of texture colony having the same crystal orientation in the surface layer part structure is regulated to >=4, can be produced.

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 brittle fracture propagation stopping (arrest) performance, a manufacturing method described in Japanese Patent Laid-Open No. 59-47323, which is sufficiently reduced in a non-recrystallized region, or a positive method. The second phase particles, which tend to cause brittle fracture, are dispersed at the tip of the brittle crack to cause many microcracks at the tip to relax the stress state at the tip of the crack, and to prevent cracking by ductile fracture that occurs during coalescence between the microcrack and the main crack. 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 of 1/3 of the plate thickness, and without an external heat source, work ferrite is generated in the center part of the plate thickness and the toughness deteriorates. There is a high possibility. 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) at least a plate thickness of 2 from the surface of the steel plate.
%, Mainly composed of a ferrite structure or bainite structure having an average equivalent-circle grain size of 3 μm or less,
An aspect ratio (ratio of major axis / minor axis) of a textured colony having a structure composed of a spheroidal carbide phase having an average equivalent circle diameter of 0.6 μm or less and having the same crystallographic orientation of its surface layer structure is 4 The basic means is a steel plate characterized by the above.

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}
A region of (mm) or more is rapidly cooled to a temperature of Ar ₁ point or less at a cooling rate of 5 ° C./sec or more, and then the surface layer portion starts or restarts rolling at a temperature of Ar ₃ point or more, (Ac ₃ point- 50)
Rolling is completed within the range of ℃ to Ac ₃ ℃, then Ac ₃
The second means is to cool at least the Ar ₁ point at a cooling rate of 2 ° C./sec or more without recuperating to the point, and (3) a steel piece having a temperature of ₃ Ac points or more. Alternatively, when the plate thickness of the steel plate during water cooling during rolling is t ₀ , 0.02 × t ₀ (m
m) The above area 5 ° C. / sec. or more cooling rate in quenching below _a point Ar, then, the surface portion starts or resumes rolling from a temperature of more than ₃ points Ar, (Ac ₃ -50) The third means is to finish rolling in the range of ℃ to Ac ₃ ℃, and then perform accelerated cooling at a cooling rate of 5 ℃ / sec or more without reheat to the Ac ₃ point to manufacture (4) 550
The fourth means is to apply a tempering heat treatment at a temperature of not higher than ° C.

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]

[Function] Even if the ferrite grain size of the ferrite / pearlite steel sheet containing no Ni element is reduced to 5 μm or less, the vTrs, which is the toughness of the base material, is hardly improved as shown in FIG. Therefore, in order to investigate the mechanism in detail, various experiments were conducted using a general structural steel having the following chemical composition. 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
As a result, brittle fracture of the structure with a ferrite grain size of 5 μm or less occurred from the pearlite colony, and the size of the pearlite colony phase was almost unchanged at the ferrite grain size of 5 μm to 1.4 μm. It was Therefore, as a result of investigating the relationship between the average size of the carbide phase and the microscopic critical fracture stress necessary to cause brittle fracture by changing the morphology of the carbide phase by quenching and tempering heat treatment, as shown in FIG. It was found that the brittle fracture initiation characteristics greatly changed not only by the size of the carbide phase but also by its morphology.

In order to prevent brittle fracture at -165 ° C., considering the stress concentration at the crack tip, -165
Since the microscopic limit fracture stress of at least 3 times the yield strength at ℃ is required, the yield strength at -165 ℃ which can be achieved with ferritic / pearlitic steel is 100 kg / mm ²
The microscopic limit fracture stress is 300kg / mm
² or more is required. Therefore, in order to achieve this microscopic limit fracture stress level, it is not enough to limit the size of the carbide phase, and it is necessary to make the morphology spherical and to make the size equivalent to a circle equivalent diameter of 0.6 μm or less. It was discovered that

FIG. 3 shows the relationship between the ferrite grain size and the fracture toughness value Kc showing the brittle fracture initiation characteristic. If the ferrite grain size is 2 μm or less and the carbide phase is spherical,
Excellent fracture toughness value of 500 kg / mm ² or more even at ℃.

The present inventors have conducted the following studies for the purpose of preventing brittle fracture even at lower temperatures. 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 critical microscopic fracture stress of the steel sheet and a method of reducing the stress of the crack or the notch tip by some means are considered.

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 at low temperature, the yield point of the steel material rises and the plastic region at the crack tip becomes smaller, so that local deformation due to plastic anisotropy occurs between colonies with different crystal orientations necessary for the occurrence of separation. It is considered that this is because it is difficult, and the critical fracture stress of the steel sheet is reached at the crack tip before the occurrence of separation, resulting in brittle fracture. Therefore, the following experiment was conducted.

First, in order to quantify the structure morphology required 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 organization,
By applying the temper color method that utilizes the change in the thickness of the oxide film depending on the crystal orientation, colonies with the same crystal orientation are revealed, and the aspect ratio (ratio of major axis / minor axis) and the critical fracture stress in the plate thickness direction are calculated. evaluated. As a result, as shown in FIG. 4, when the aspect ratio is 4 or more, the critical fracture stress in the plate thickness direction is 1 when the aspect ratio is about 1 without texture.
It was found that it would be less than / 2.

Next, the aspect ratio is set to 4 or more by 2
The relationship between the ferrite grain size and the critical temperature at which separation occurs was investigated using a steel sheet subjected to phase rolling. The result is shown in FIG. In order to cause the separation even in the low temperature range of -170 ° C or less, the ferrite grain size is 3μ.
It was found to be m or less.

Since the intended brittle fracture propagation stopping performance is considered to be a microscopic generation characteristic at the crack tip during propagation, FIG. 6 shows the ferrite grain sizes with different aspect ratios and the impact at -196 ° C. Brittle fracture initiation toughness Ki under load
The relationship with d is shown. That is, it is a decisive factor to improve the propagation resistance of brittle fracture by developing the texture and reducing the ferrite grain size to 3 μm or less so that 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. Further, it was confirmed that the Kid value was dramatically improved in the structure in which the carbide phase was spheroidized and the average equivalent circle diameter was controlled to be 0.6 μm or less.

In order to realize the structure of the present invention, the surface of the steel sheet is cooled at a cooling rate of 5 ° C./sec or more during rolling, and Ar is cooled.
_By setting it to ₁ point or less, ferrite (bainite) transformation is once performed, and the sensible heat of the center part of the plate thickness where the temperature hardly decreases even if the surface layer is rapidly cooled is used to convert the ferrite (bainite) structure of the surface layer into carbides. Further rolling was performed while recuperating at a high rate of temperature so as not to cause coarsening.

When the air-cooled structure was observed after completion of rolling, some carbides were spheroidized, but some coarse phases of 0.6 μm or more were present, and ferrite grains were also coarsened. there were. Therefore, an experiment was carried out by changing the cooling rate up to Ar ₁ point where the ferrite transformation after rolling was completely completed, and when the cooling rate of 2 ° C./sec or more could be secured in the surface layer portion, coarsening of ferrite grains and carbides was observed. It was confirmed that it was possible to suppress and realize the desired organization.

This structure has a texture because the rolling is completed at the Ac ₃ point or less, and an ultrafine grain structure of 3 μm or less having a texture in the surface layer portion and a fine spherical shape of 0.6 μm or less. A structure composed of carbide was formed.

As a result of investigating the Kca performance of the steel sheet in which the thickness of the surface layer modified structure was changed by changing the water cooling conditions during rolling, the Kca property was improved by increasing the thickness of the surface layer modified structure and It has been found that there is a required thickness of the surface layer modified structure depending on the required Kca performance.

[0028]

[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.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

In Test Nos. 1 to 12 of the present invention and Test Nos. 13 to 16, 21, 22, and 24 of Comparative Examples, cooling is applied after rough rolling, and the steel sheet surface layer portion is made to have Ar ₁ point or less to undergo ferrite transformation. Test No. 1 of Comparative Example
Since Nos. 4, 21 and 22 had a low cooling rate, the temperature of the entire steel sheet was lowered, and the rolling after cooling was not 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, test numbers 14, 21, which are comparative examples,
The textures of the surface layers of Nos. 22 and 24 were not finely divided.

In the materials of these comparative examples, the entire plate thickness was rolled in the two-phase region, the base material toughness vTrs was deteriorated, and both the NDT characteristics and the arrest characteristics were deteriorated. Also,
In Comparative Examples 13 and 16, although predetermined cooling / rolling was carried out, the ferrite grain size was 3 μm due to air cooling after completion of rolling.
Since it was not less than m, Comparative Examples 15 and 16 reheated to Ac ₃ or more in the recuperating process after rolling, so that grain growth partially occurred and a predetermined structure could not be obtained. Therefore, in Comparative Examples, Test Nos. 13 to 20 and 23, both the temperature and the NDT characteristic showing Kca = 600 kgf / mm ^1.5 as the arrest performance 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 -80 ° C. or higher, and the temperature at which the arrest performance, Kca = 600 kgf / mm ^1.5 , was also a sufficient characteristic.

[0035]

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
If rolling is performed while recovering heat by sensible heat inside the plate thickness after r ₁ point or less, sufficient unrecrystallized crystal is not formed 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 due to the 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 chart showing a relationship between a ferrite grain size and vTrs indicating a base material toughness.

FIG. 2 is a table showing the relationship between the average equivalent circular grain size of a carbide phase and the microscopic limit fracture stress.

FIG. 3 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.

FIG. 4 (a) is a chart showing the relationship between the ratio of major axis / minor axis (aspect ratio) of colonies having the same orientation of the tissue revealed by the temper color method and the critical fracture stress in the plate thickness direction. (B) is a schematic diagram of an aspect ratio.

FIG. 5 is a chart showing a relationship between a ferrite grain size and a separation generation limit temperature.

FIG. 6 is a chart showing the relationship between the ferrite grain size and the Kc value, which is the brittle fracture initiation toughness at −196 ° C.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] December 4, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0031

[Correction method] Change

[Correction content]

[0031]

[Table 3]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0034

[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 -80 ° 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, Nishinosu 1 Otsu Works, Nippon Steel Co., Ltd.

Claims (4)

[Claims] 1. A spherical carbide having a ferrite structure having an average equivalent-circle grain size of 3 μm or less or a bainite structure as a main component in a surface layer portion of a steel sheet and having a mean equivalent circle diameter of 0.6 μm or less over a range of 2% or more of the plate thickness. The aspect ratio (ratio of major axis / minor axis) of a texture colony having a structure composed of phases and having the same crystal orientation of the surface layer structure is 4
A structural steel having excellent brittle fracture resistance characterized by the above. 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. Region is rapidly cooled to a temperature of Ar ₁ point or lower at a cooling rate of 5 ° C./sec or higher, and then the surface layer portion starts or restarts rolling at a temperature of Ar ₃ point or higher, and (Ac ₃ point −50) point C To the Ac ₃ point, the rolling is completed, and then the surface layer portion is kept at 2 ° C./at least until the Ar ₁ point without reheating to the Ac ₃ point or more.
Cooled at a cooling rate of sec or more, and mainly composed of a ferrite structure or bainite structure with an average circle-equivalent grain size of 3 μm or less over at least 2% of the plate thickness from the surface layer, and an average circle-equivalent diameter of 0.6 μm or less. Of the textured colony having the same crystallographic orientation of the surface layer structure, and having an aspect ratio (major axis / minor axis ratio) of 4 or more. A method for manufacturing a structural steel sheet having excellent brittle fracture characteristics. 3. A structural material with excellent brittle fracture resistance according to claim 2, wherein after the rolling is completed, accelerated cooling is performed at a cooling rate of 5 ° C./sec or more without reheating to Ac ₃ point or more. Steel plate manufacturing method. 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 excellent characteristics.