JP3348359B2 - Structural steel with excellent arrest performance and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to the addition of expensive alloy elements such as Ni elements to the arrest performance of brittle fracture propagation, which is one of the important properties of steel sheets for ensuring the safety of structures. The present invention relates to a steel plate and a method for manufacturing the same, which are dramatically improved without relying on steel.

[0002]

2. Description of the Related Art As a means for improving the arrest performance of brittle fracture propagation, a manufacturing method of sufficiently reducing pressure in an unrecrystallized region as described in JP-A-59-47323, or an active method. Disperse the second phase particles which are apt to cause brittle fracture in the micro cracks to generate many micro cracks at the tip of the brittle crack to relax the stress state at the tip of the crack, and to stop the crack by the ductile fracture that occurs when the micro crack and the main crack are united. Methods have been proposed to facilitate this.

[0003] However, these proposals modify the structure at the center of the sheet thickness and improve the brittle crack propagation stopping performance, and the NDT characteristics in a drop weight test mainly determined by the structure of the surface layer of the sheet thickness. Is not necessarily improved. Further, when the thickness of the steel sheet is increased, the grain refinement at the central portion of the steel sheet as described above may not be achieved. In particular, development of a technique for improving the arrest performance of a steel sheet having a thickness of 25 mm or more is desired.

On the other hand, a method for producing a steel sheet having a fine-grained structure in the surface layer of a steel sheet is described in Japanese Patent Application Laid-Open No. 61-235534, which defines the surface layer as having a structure of 5 μm or less. Materials and Processes, 6 (1990), p.
As described in 1796, even ferrite grains of 3 μm or less easily cause brittle fracture at −120 ° C. or less, and there is a limit in a method of improving arrest performance for forming a fine grain structure in a surface layer portion.

Further, Japanese Patent Application No. 02-24509 discloses a technique of cooling and reheating a surface portion up to 1/3 of the plate thickness to achieve high arrest by improving the structure of the surface portion. ing. However, in this method, cooling recuperation has to be realized over a wide range up to 1/3 of the plate thickness, and without an external heat source, processed ferrite is generated at the center of the plate thickness and toughness is deteriorated. Great potential. Further, although arrest performance can be improved by such a manufacturing method, the organization required for arrest performance improvement is not clear, and the surface layer structure necessary for efficiently improving arrest performance, and its required thickness are unknown. .

[0006]

SUMMARY OF THE INVENTION The present invention clarifies the required structure and required thickness for improving the arrest performance of the Kca characteristic and the NDT characteristic by modifying the structure of the surface layer, thereby significantly increasing the manufacturing cost. It is an object of the present invention to provide a steel plate having good arrest performance without adding expensive Ni element or the like, deteriorating the toughness of the central portion of the sheet thickness and causing no separation at the central portion of the sheet thickness, and a method of manufacturing the same. .

[0007]

SUMMARY OF THE INVENTION The present invention has the following features to attain the above object . (1) C: 0.02 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0 %, Al: 0.001 to 0.20% by weight % , N: 0.020% or less, P: 0.01% or less, S: 0.01% or less, and the balance of Fe and unavoidable impurities .
The surface layer portion, 3 [mu] m following ferrite structure with Rights <br/> average equivalent circle diameter I respectively cotton in the range of 2% or more of the plate thickness, or having a bainite structure, and its surface layer portion tissues, adjacent
A texture having an aspect ratio (ratio of major axis / minor axis) of 4 or more of textured colonies having the same crystal orientation among crystal grains is
Ares characterized by occupying 30 to 86% in the surface layer
Structural steel with excellent performance . (2) The steel sheet of the above-mentioned component further contains Ni, C
r, Mo, Cu, W, Co, V, Nb, Ti, Zr, T
a, Hf, rare earth element, Y, Ca, Mg, Te, Se,
B containing at least one kind of B in a total amount of 4.5% or less.
The structural steel having excellent arrest performance according to claim 1.

( 3 ) Ac When a steel slab or a steel plate having a temperature of ₃ points or more is subjected to water cooling during rolling and the thickness is set to t ₀ , at least 0.02 × t ₀ (mm) from the surface layer in the thickness direction. and rapidly cooled to below the Ar ₃ point or more regions at 2 ° C. / sec or more cooling rate, then the surface layer portion, ferrite or more ₁ point Ar
The rolling was resumed from the two-phase temperature of
c Maximum recuperation temperature in the range from more than ₃ points to (Ac ₃ +60) ° C
The rolling exit to reach the up then Ar ₁ point and cool the surface layer portion at 1 ° C. / sec or more, less from the surface of the steel sheet
Both have an average circle equivalent diameter of 3 over the range of 2% or more of the plate thickness.
μm or smaller ferrite structure or bainite structure
Having the same texture between adjacent crystal grains
Aspect ratio of textured colonies with single crystal orientation
The tissue having a ratio of (major axis / minor axis) of 4 or more was treated with 30
~ 86%, excellent arrest performance
Manufacturing method of structural steel sheet. ( 4 ) After the rolling is completed , the surface layer is kept at 5 ° C until Ar ₁ point.
(3) characterized in that the cooling is accelerated at a rate of / sec or more.
A method for producing a structural steel sheet having excellent arrest performance as described above. ( 5 ) After the rolling is completed, the cooling rate is 5 ° C / sec or more.
It is characterized by rapid cooling and tempering heat treatment.
The production of the structural steel sheet having excellent arrest performance according to the above (3).
Construction method.

In the present invention, the structural steel of interest is:
For example, as described in the above-mentioned Japanese Patent Publication No. 58-14849, as described below, in order to obtain a required material of a normal structural steel, an operation and an effect which have been conventionally confirmed in the field of use in the field of the art. The same action and effect can be obtained by using the type and amount of the additional element determined based on the relationship in the same manner. Accordingly, the present invention includes steels containing these elements as target steels.

[0010] The components, the reasons for their addition, and the amounts thereof are as follows. C is added in an amount of 0.02% or more as an effective component for improving the strength of steel.
Excess content exceeding 0.2% not only increases the deformation resistance during two-phase rolling, making rolling difficult, but also causes precipitation of island-like martensite in the weld and significantly deteriorates the toughness of the steel. Restrict to 02% to 0.20%.

[0011] Si is required as a deoxidizing element of molten steel,
It is useful as a strength increasing element, but if it exceeds 1.0%, the workability of the steel decreases, the toughness of the welded portion deteriorates, and
%, The deoxidizing effect is insufficient.
Regulate to ~ 1.0%.

Mn needs to be added in an amount of 0.3% or more as a component for improving the strength of steel. However, the addition of Mn lowers the transformation temperature. Rises to 2.0%.

Al and N are added to refine the steel by Al nitride, to adjust the crystal orientation of the steel by solid solution and precipitation during the rolling process, and to recrystallize the steel. And excessive addition deteriorates the toughness of the steel, so that Al is 0.001 to 0.20% and N is 0.020%.
% Or less.

P and S are set to 0.01% or less and 0.01% or less, respectively, in order to secure the toughness of the base material. The above are the basic components of the steel targeted by the present invention. For the purpose of increasing the strength of the base metal or improving the joint toughness, when adding an alloy element according to the required properties, If the temperature is lowered too much, the deformation resistance in the two-phase region increases and rolling becomes difficult, so Ni, Cr, M
o, Cu, W , Co, V, Nb, Ti, Zr, Ta, H
f, rare earth element, Y, Ca, Mg, Te, Se, B
More than one kind can be used, but the added amount is 4.5% in total
It is regulated as follows. Note that the average circle equivalent particle size is obtained by averaging the diameters of circles that are assumed to have the same area and focusing on individual particles of the relevant tissue.

[0015]

The present inventors have paid attention to the fact that, even when the ferrite grain size of a ferrite-pearlite steel sheet containing no Ni element is reduced to 5 μm or less, the base metal toughness, vTrs, hardly improved. Through the elucidation of the mechanism, we devised and conducted experiments on the resistance to brittle fracture necessary to improve the toughness of the steel sheet.

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

The above-mentioned method is a phenomenon that a texture is developed to cause a vertical crack called separation in a direction parallel to the thickness of the steel sheet, and as a result, the crack or the notch tip is released and the stress is reduced. It has been known. That is, before the local stress reaches the critical microscopic fracture stress,
It is understood that the separation only needs to occur. For that purpose, it is necessary that the critical fracture stress of the steel sheet is higher than the separation initiation stress. However, in an actual steel sheet rolled in the ferrite-austenite two-phase region,
At temperatures predominant in plastic deformation, separation occurs prior to fracture, but at low temperatures, brittle fracture occurs.

This is because, at low temperatures, the yield point of the steel material rises, and the plastic region at the tip of the crack becomes smaller, so that local deformation occurs due to plastic anisotropy between colonies having different crystal orientations necessary for generation of separation. Therefore, various experiments were conducted using a general structural steel having the chemical components shown below. C: 0.04-0.15% Si: 0.15-
0.25% Mn: 0.4 to 1.6% Al: 0.01 to
0.05% P: 0.005 to 0.008% S: 0.001
First, steel plates with different texture levels were manufactured by changing various two-phase region rolling conditions in order to quantify the texture morphology necessary for causing separation by texture. In order to quantify the texture on the structure, a temper color method using a change in the thickness of the oxide film depending on the crystal orientation is applied to reveal colonies having the same crystal orientation and the aspect ratio (major axis / minor axis). And the critical fracture stress in the thickness direction were evaluated. As a result, as shown in FIGS. 1 and 2, when the aspect ratio is 4 or more, the critical fracture stress in the sheet thickness direction is 1/1 of the case where the aspect ratio is about 1 without texture.
2 or less.

Next, 2 so that the aspect ratio becomes 4 or more.
The relationship between the ferrite grain size and the separation limit temperature was investigated using a steel plate that had been subjected to phase zone rolling.
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 must be 3
It was found to be 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 decisive to improve the brittle fracture resistance by reducing 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, is ultra-fine-grained, the critical microscopic fracture stress is increased, and a texture that can generate separation is developed.

A texture having an average circle equivalent diameter of 3 μm or less and having an aspect ratio of a textured colony composed of adjacent crystal grains having the same orientation of 4 or more has extremely excellent brittle fracture resistance as described above. However, in order to realize such a structure, extremely precise control of manufacturing conditions is required, while on the other hand, the characteristics of the steel material do not necessarily need to be dramatically improved according to the use temperature.

Therefore, the relationship between the brittle fracture resistance and the brittle fracture characteristics was investigated by a Charpy test with a fatigue crack while changing the occupancy of the structure revealed in the present invention. As a result, when the structure has an occupancy of 30% or less, the brittle fracture resistance is dominated by the remaining structure, and when the structure is 30% or more, the brittle fracture resistance improves according to the occupancy. Found (FIG. 5).

In order to achieve the structure according to the present invention, for example, if a given amount of processing is given to the ferrite during the temperature raising process and the reverse transformation to austenitization is prevented, the ferrite is introduced into the processed ferrite. Dislocations recover and cause rearrangement, the limit of microscopic fracture stress is improved by ultra-fine graining of ferrite, and the texture developed by the processing given to ferrite is left as it is, thereby the present invention. We learned that the organization can achieve it.

Therefore, the surface of the steel sheet is water-cooled during rolling, and Ar
_By reducing the temperature to ₃ points or less, the ferrite transforms once, and the temperature is hardly reduced even by cooling. 3μ with only part texture
m or less was formed. At this time, if the maximum recuperation temperature after rolling was not more than Ac ₃ , the finishing temperature at the center of the sheet thickness was low, and generation of separation was observed in the Charpy test.

[0025] As a result of going by increasing the maximum recuperation temperature after the end of rolling, Ac ₃ point super Ac ₃ as shown in FIG. 6
If the temperature is + 60 ° C. or lower, the present inventors have found manufacturing conditions that ensure that the invention structure in the surface layer portion is 30% or more and do not cause separation in the V-notch Charpy impact test at the center of the sheet thickness. Further, when held in rolling end after temperature rise to fine grain structure super grain growth, no target tissue is obtained, in order to suppress rolling after grain growth, 1 ° C. / sec or more surface portion cooled to _a point Ar It was confirmed that a predetermined tissue could be obtained if the speed was high.

[0026]

EXAMPLES The components of the test steels of the examples are shown in Table 1, and the production conditions and the obtained materials are shown in Table 2 together with comparative examples.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

Test Nos. 1 to 12 of the present invention and Test Nos. 13 to 19, 21, 22, and 24 of the comparative example apply cooling after rough rolling.
In 7,21,22, since the cooling rate was slow, the temperature of the whole steel sheet was lowered, and the rolling after cooling did not become the temperature raising processing. In addition, in Test Nos. 22 and 24 of the comparative examples, the elapsed time after cooling was too long and the required conditions for rolling after cooling could not be satisfied. Therefore, the test numbers 21, which are comparative examples,
The structure of the surface layer of No. 24 was not refined.

In the materials of these comparative examples, the entire plate thickness was subjected to two-phase rolling, the toughness of the base material, vTrs, also deteriorated, and both the NDT characteristics and the arrest characteristics deteriorated. Also,
In Test Nos. 20 and 23 of the comparative examples, cooling after the rough rolling was not performed, and the finish rolling temperature was Ar
Because the target was just above the _three points, the surface temperature was lower than the Ar _three points, and the rolling at this point caused abnormal grain growth of the ferrite, and as a result, the ferrite grain size in the surface layer became coarse, and furthermore, it gathered even in the center of the sheet thickness. Separation occurred in the Charpy test due to tissue development.

Comparative Examples 13, 14, 16, 17, 1
In No. 9, although the cooling was performed during the predetermined rolling, the rolling end temperature after the rolling became higher than Ac ₃ + 60 ° C., and the area ratio of the ultrafine grain structure portion became 30% or less. Therefore, these comparative examples have Kca = 600 kgf / mm ^1.5
And the NDT characteristics do not reach −65 ° C. or less. In Comparative Examples 15 and 18, the rolling finish temperature was A
For the following c ₃ points, although the predetermined surface layer structure is obtained, inside the finish rolling temperature becomes below ₃ points Ar, temperature indicating the Kca = 600kgf / mm ^1.5, NDT properties -70 ° C. or less Although separation was achieved, separation occurred.

On the other hand, Test Nos. 1 to 12
The material of satisfies the required manufacturing conditions as shown in Table 2,
While satisfying the strength and toughness of the target, aimed at a NDT temperature of the present invention, to achieve a -60 ° C. or less with temperature showing a Kca = 600kgf / mm ^1.5, occurrence of separation of the center of plate thickness was also suppressed. In addition, the fatigue properties of the example of the present invention are good.
Met.

[0034]

According to the present invention, since the above-mentioned action is utilized by using the above-mentioned means, after rough rolling, only the surface layer portion is cooled and A
rolling with heating condensate carried by the plate thickness inside the sensible heat after less r ₃ points, without generating brittle tissue of the surface layer portion to deteriorate the NDT characteristics, sufficient non-recrystallization region rolling in the plate thickness center This makes it possible to achieve both the arrest performance of the NDT characteristic and the Kca characteristic, and has great effects not only in the technical field but also in related fields.

[Brief description of the drawings]

FIG. 1 is a chart showing a relationship between an aspect ratio and a critical breaking stress in a thickness direction.

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

FIG. 3 is a table showing a relationship between a ferrite grain size and a separation occurrence limit temperature.

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

FIG. 5 shows the relationship between the occupancy of the structure (structure having a ferrite particle size of 3 μm or less and the texture colony aspect ratio of 4 or more) and the brittle-ductile fracture surface transition temperature vTrs in the Charpy test with fatigue cracking. It is a chart.

FIG. 6 shows a relationship between the maximum average recuperation temperature in a region (surface layer) having reached a temperature of Ar ₃ or less due to intermediate cooling, the tissue occupancy in the surface layer, and SImax in a V-notch Charpy impact test at the center of thickness. FIG.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshiaki Hashi 1 Nishinoshima, Oita-shi, Nippon Steel Corporation Inside the Oita Works (72) Inventor Yuji Funatsu 1 Nishi-nosu, Oita-shi Oaza, Nishi-Nosu 1 Nippon Steel Corporation (56) References JP-A-63-20414 (JP, A) JP-A-61-235534 (JP, A) JP-A-5-148540 (JP, A) JP-A-5-148541 ( JP, A) JP-A-5-148542 (JP, A) JP-A-5-202444 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/00-8/10

Claims (5)

(57) [Claims] 1. A weight%, C: 0.02~0.20%, Si : 0.01~1.0%, Mn: 0.3~2.0%, Al: 0.001~0. 20%, N: 0.020% or less, P: 0.01% or less, S: 0.01% or less, and contains, in the surface layer portion of the steel sheet and the balance Fe and unavoidable impurities, respectively thickness 2 a percent of the average circle equivalent diameter 3μm following ferrite structure over a range, or a bainite structure, and its surface layer portion tissues, adjacent
A structure with excellent arrest performance, characterized in that the texture of colonies having the same crystal orientation among crystal grains and having an aspect ratio (ratio of major axis / minor axis) of 4 or more occupies 30 to 86% of the surface layer. For steel. 2. The steel sheet of the above component further comprises N
i, Cr, Mo, Cu, W, Co, V, Nb, Ti, Z
r, Ta, Hf, rare earth element, Y, Ca, Mg, Te,
The structural steel having excellent arrest performance according to claim 1, wherein one or more of Se and B are contained in a total amount of 4.5% or less. _3. A steel slab or a steel sheet having a temperature of ₃ or more Ac is 0.02 × t ₀ (mm) or more at least in the thickness direction from the surface layer, when the sheet thickness during water cooling during rolling is t _0. Region was rapidly cooled to a temperature of Ar ₃ or less at a cooling speed of 2 ° C./sec or more.
After that, the surface layer is made of ferrite with _one or more Ar points and ferrite.
Resume rolling from 2-phase region temperature of austenite, finished rolling up from Ac ₃ point than reached (Ac ₃ +60) maximum recuperation temperature in the range of ° C., the up then Ar ₁ point the surface layer portion 1 Cooling at a temperature of at least 2 ° C./sec, and having a ferrite structure or a bainite structure having an average circle equivalent diameter of 3 μm or less over a range of at least 2% of the sheet thickness from the steel sheet surface,
In the surface layer structure , the aspect ratio of the textured colony having the same crystal orientation between adjacent crystal grains (major axis /
The ratio of the minor axis is 4 or more to 30-86% of the surface layer
A method for producing a structural steel sheet having excellent arrest performance. 4. After completion of the rolling, a method of manufacturing superior structural steel arrest performance according to claim 3, wherein the up _one point Ar characterized by accelerated cooling the surface layer portion at 5 ° C. / sec or higher. 5. The method for producing a structural steel sheet having excellent arrest performance according to claim 3 , wherein after the rolling is completed, accelerated cooling is performed at a cooling rate of 5 ° C./sec or more, and tempering heat treatment is further performed.