JP2807592B2 - Method for producing structural steel sheet with good brittle fracture resistance - 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 method for manufacturing a steel sheet which is dramatically improved without relying on the steel sheet.

[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.

However, these proposals modify the structure at the center of the sheet thickness and improve the brittle crack propagation stopping performance. The NDT characteristics in a drop load test mainly determined by the structure of the surface layer of the sheet thickness are proposed. Is not necessarily improved. Further, when the thickness of the steel sheet is increased, the grain refinement at the center of the thickness 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, and the surface layer is specified to have a structure of 5 μm or less. "Materials and Processes" 6 (1990),
As described on page 1796, even ferrite grains of 3 μm or less easily cause brittle fracture at −120 ° C. or less, and there is a limit to a method of improving arrest performance for forming a fine grain structure in the surface layer.

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 must be realized over a wide range up to 1/3 of the sheet thickness, and without an external heat source, the center of the sheet thickness may form processed ferrite and the toughness may be degraded. Great nature. In addition, 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. An object of the present invention is to provide a method for manufacturing a steel sheet having good arrest performance without adding an expensive Ni element or the like to be made.

[0007]

In order to achieve the above object, the present invention provides a ferrite structure or a bainite structure having an average equivalent circle diameter of 3 μm or less from the surface layer to at least 2% of the plate thickness. A method for producing a structural steel sheet having good brittle fracture resistance, characterized in that the ratio of the major axis / minor axis of textured colonies having the same crystallographic orientation of the surface layer structure is 4 or more. .

According to the present invention, (1) when a steel slab or a steel sheet having a temperature of not less than ₃ points of Ac is water-cooled during rolling and the thickness is set to t ₀ , at least 0.02 × t from the surface layer in the thickness direction.
_In the region above ₀ (mm), the ferrite fraction is 5 at 2 ° C / sec.
After rapidly cooling to a temperature of 0% or more, the surface layer starts or restarts rolling from a temperature of ₁ point or more of Ar, and finishes rolling in a range of (Ac ₃ point−100) ° C. to Ac ₃ points. After that, without reheating to Ac ₃ points,
The basic means is to cool to at least Ar ₃ point at 1 ° C./sec or more.

Further, the present invention relates to (2) a method in which a steel slab or a steel plate having a temperature of ₃ or more Ac is subjected to water cooling during rolling to a thickness t _0.
, At least 0.02 × t from the surface layer in the thickness direction
_In the region above ₀ (mm), the ferrite fraction is 5 at 2 ° C / sec.
After rapidly cooling to a temperature of 0% or more, the surface layer starts or restarts rolling from a temperature of ₁ point or more of Ar, and finishes rolling in a range of (Ac ₃ -100) ° C. to Ac ₃ ° C. Then, the second means is to accelerate the cooling at a cooling rate of 5 ° C./sec or more without reheating to Ac ₃ points, and (3) to apply the tempering heat treatment at 550 ° C. or less. Means.

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.

[0011] These constituent elements, 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%.

Si is necessary 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 for the purpose of refining steel by solid solution and precipitation in the rolling process in addition to refining steel by Al nitride, but it is effective when the addition amounts are small. 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 for securing the toughness of the base material.
They are respectively 0.01% or less and 0.01% or less.

The above are the basic components of the steel which is the subject of the present invention. For the purpose of increasing the strength of the base metal or improving the toughness of the joint, the case where an alloy element is added in accordance with the required properties. If the transformation temperature is too low, deformation resistance in the two-phase region increases and rolling becomes difficult. Therefore, Ni, Cr, Mo, Cu, W, P, Co, V, Nb, T
i, Zr, Ta, Hf, rare earth element, Y, Ca, Mg,
One or more of Te, Se, and B can be used, but the total amount thereof is regulated to 4.5% or less. 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.

[0016]

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, a study on the resistance to brittle fracture necessary to improve the toughness of the steel plate and experiments were performed.

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, a crack or a constraint at a notch tip is released to reduce stress. 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. Therefore, local deformation due to plastic anisotropy between colonies having different crystal orientations required for the occurrence of separation occurs. Therefore, various experiments were carried out using a general structural steel having the chemical components 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 to 0.008%, S: 0.001 to 0.003%.

First, in order to quantify the structural morphology necessary for causing separation by texture, steel sheets having different texture levels were manufactured by changing various two-phase region rolling conditions. To quantify the texture on the organization,
By applying the temper color method using the change in the thickness of the oxide film depending on the crystal orientation ,
A colony as an aggregate was made to appear, and the ratio of the major axis / minor axis of the colony and the critical fracture stress in the plate thickness direction were evaluated. As a result, as shown in FIG. 1, colonies having the same crystal orientation
It has been found that when the ratio of the major axis / minor axis is 4 or more, the critical fracture stress in the sheet thickness direction is 1/2 or less of the case of about 1 having no texture. Fig. 2 shows colonies having the same crystal orientation .
FIG. 3 is a schematic diagram of a ratio of a major axis / a minor axis .

Next , the major axis of the colony having the same crystal orientation
Using a steel sheet that had been subjected to two-phase region rolling so that the ratio of / minor diameter was 4 or more, the relationship between the ferrite grain size and the critical temperature for the occurrence of separation was investigated. The result is shown in FIG. It has been found that the ferrite grain size is 3 μm or less in order to cause separation even in a low temperature range of −170 ° C. or less.

FIG. 4 shows that colonies having the same crystal orientation
The relation between the ferrite particle diameters having different ratios of the major axis / minor axis and the brittle fracture initiation toughness Kc is shown. 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.

In order to achieve this structure, for example, if a certain amount of processing is applied to the ferrite during the heating process and the reverse transformation to austenitization is prevented, the dislocations introduced into the processed ferrite will recover. The microstructure of the present invention is achieved by causing rearrangement, improving the critical microscopic fracture stress by ultrafine graining of ferrite, and leaving the texture developed by the processing given to ferrite as it is. I learned that I can do it.

Therefore, during rolling, the surface of the steel sheet is water-cooled and once transformed into ferrite, and the sensible heat at the center of the sheet thickness where the temperature hardly decreases even by cooling is used to raise the temperature of the ferrite structure in the surface layer. Further rolling was performed to form an ultrafine grained structure of 3 μm or less having a texture only in the surface layer.

At this time, in the case of air-cooled after the rolling, the temperature after the rolling is high, so that part of the ultrafine grain structure may cause grain growth, and the desired structure may not be obtained. Then, in order to suppress the grain growth after rolling, it was cooled to Ar ₁ point.
It was confirmed that a predetermined structure could be obtained at a surface layer cooling rate of at least ° 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 modified structure was changed, it was found that Kc was increased by increasing the thickness of the surface modified structure.
It has been found that the a-characteristics are improved, and the required thickness of the surface-modified structure exists according to the Kca performance required for the steel sheet.

[0027]

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. Test steel
Each of the obtained data was obtained using the temper color method.
Colonies in units of aggregates of crystal grains with the same position
And crystal grains having different crystal orientations. Then
The major axis and minor axis of an aggregate of crystal grains having the same orientation were measured.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

[Table 3]

Test Nos. 1 to 12 of the examples of the present invention and Test Nos. 13 to 16, 21, 22, and 24 of the comparative examples are those in which cooling is applied after rough rolling and the surface layer of the steel sheet is transformed into ferrite. In Test Nos. 14, 21 and 22 of the comparative examples, the cooling rate was slow, so that the temperature of the entire steel sheet was lowered, and the rolling after cooling did not become the temperature raising processing. Further, in Test No. 24 of the comparative example, the elapsed time after cooling was too long, and the required conditions for rolling after cooling could not be satisfied. For this reason, the structure of the surface layer portion of Test Nos. 21, 22, and 24, which are comparative examples, did not become fine.

In the materials of these comparative examples, the entire plate thickness was subjected to two-phase rolling, the base material toughness vTrs was deteriorated, and both the NDT characteristics and the arrest characteristics were deteriorated. Also,
Test No. Comparative Example 17～20,23 are both not implemented cooling after rough rolling, since the finish rolling temperature was aimed directly above Ar ₃ point in the thickness average, the temperature of the surface is Ar ₃
The abnormal rolling of the ferrite particles caused the rolling, and as a result, the ferrite grain size in the surface layer became coarse.

In Comparative Examples 13 and 16, although the predetermined cooling / rolling was performed, the ferrite grain size did not become 3 μm or less because of air cooling after the completion of the rolling.
In the heat recovery process after the rolling, the heat recovered to Ac ₃ or more, so that partial grain growth occurred, and a predetermined structure could not be obtained. Therefore, Test Nos. 13 to 20, 23, which are comparative examples,
Did not reach −60 ° C. in both the temperature and the NDT characteristic showing Kca = 600 kgf / mm ^1.5 as the arrest performance.

On the other hand, Test Nos. 1 to 12
The material of satisfies the required manufacturing conditions as shown in Table 2,
In addition to satisfying the target strength and toughness, the temperature at which the NDT temperature, which is the target of the present invention, is -70 ° C. or more, and the arrest performance, ie, Kca = 600 kgf / mm ^1.5 , was also satisfactory. Further, the fatigue properties of the example of the present invention were good.

[0035]

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
After the r ₁ point or less, if rolling is performed while recuperating by sensible heat inside the sheet thickness, sufficient unrecrystallized in the center part of the sheet thickness without generating an embrittlement structure in the surface layer that deteriorates NDT characteristics. NDT, which is an arrest performance,
It is possible to achieve both characteristics and Kca characteristics,
It has a great effect on related fields as well as the field of business.

[Brief description of the drawings]

FIG. 1 is a table showing the relationship between the ratio of the major axis / minor axis of colonies having the same crystal orientation of a tissue revealed by the temper color method and the critical fracture stress in the plate thickness direction.

FIG. 2 is a schematic diagram of the ratio of the major axis / minor axis of colonies having the same crystal orientation .

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

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

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Toshiaki Hashi 1 Nishinosu, Oita, Nippon Steel Corporation Inside Oita Works (72) Inventor Hidesato Mabuchi 1 Nishinosu, Oita, Nippon Steel (56) References JP-A-5-202444 (JP, A) JP-A-5-271862 (JP, A) JP-A 5-271862 (JP, A) 64413 (JP, A) JP-A-5-9651 (JP, A) JP-A-61-235534 (JP, A) JP-A-3-2322 (JP, A) JP-A-62-4826 (JP, A) JP-A-58-19432 (JP, A) Masaki Kikuchi, Tamaki Nishio Kazunori Yano "Study on Ductile Fracture of Surface Cracks (2nd Report: Effect of Aspect Ratio)" Transactions of the Japan Society of Mechanical Engineers (A) Japan Society of Mechanical Engineers March 25, 1989 Vol. 55 No. 511 P560 -567 (58) investigated the field (Int.Cl. ^6, DB name) C21D 8/02

Claims (3)

(57) [Claims] 1. A steel slab or a steel plate having a temperature of ₃ or more Ac is 0.02 × t ₀ (mm) or more in the thickness direction at least from the surface layer when the thickness of the slab or the water-cooled plate during the rolling is t _0. and quenching the area at 2 ° C. / sec or more cooling rate until a ferrite fraction is 50% or more, then, the surface portion starts or resumes rolling from a temperature of more than ₁ point Ar, (Ac ₃ point −
100) Rolling is completed in the range of from ₃ ° C. to ₃ points of Ac, and then the surface layer is cooled at a cooling rate of 1 ° C./sec or more to at least ₁ point of Ar without reheating to ₃ points or more of Ac. And a major axis / minor axis of a textured colony having a ferrite structure or bainite structure having an average equivalent circle diameter of 3 μm or less over a range of at least 2% or more of the plate thickness and having the same crystal orientation of the surface layer structure. The method for producing a structural steel sheet having good brittle fracture resistance, wherein the ratio is 4 or more. 2. The method according to claim 1, wherein the rolling is completed, and thereafter, the cooling is performed at a cooling rate of 5 ° C./sec or more without reheating to _three or more points of Ac. Manufacturing method of structural steel sheet. 3. The brittle fracture resistance according to claim 1, 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 or more points, and further tempering heat treatment is performed. A method for producing a structural steel sheet having good characteristics.