JP2000328176A - Steel sheet excellent in arresting characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel sheet excellent in arresting characteristics. SOLUTION: As to this steel sheet excellent in arresting characteristics, in a steel sheet whose microstructure is substantially composed of a ferritic structure and a pearlitic structure, on both surface parts of the steel sheet, pearlitic structures parallel to the sheet surfaces are present respectively over >=5% of the sheet thickness, the thickness in the sheet thickness direction of each pearlitic structure is <=5 μm, and also, the mutual distance of each pearlitic phase in the sheet thickness direction is <=5 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet having excellent arrest characteristics (brittle crack propagation stopping characteristics). According to the present invention, a steel sheet having excellent arrest characteristics, which is one of the important properties for ensuring the safety of a structure, is provided at low cost without adding a large amount of expensive elements such as Ni. This is extremely useful because

[0002]

2. Description of the Related Art Various methods for reducing the ferrite grain size in the surface layer have been proposed based on the viewpoint that the refinement of equiaxed ferrite grains in the surface layer of the sheet thickness is effective for enhancing the arrest characteristics of the steel sheet. Have been.

For example, Japanese Patent Application Laid-Open No. 3-44444 discloses that
A method for producing a steel material having excellent arrest characteristics by dispersing cementite having a length of 10 to 200 μm and a mutual distance of 5 to 50 μm in a ferrite structure in a layered manner is disclosed. However, even if the above method is adopted, the effect of increasing the arrest property is still not sufficient, and there is a strong demand for providing a steel sheet having further excellent arrest property.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a steel sheet having excellent arrest characteristics, and more specifically, to a steel sheet which has been difficult to achieve in the past.
It is an object of the present invention to provide a steel sheet that satisfies "arrest characteristics at 80 ° C .: 600 kgf / mm ^2/3 or more".

[0005]

Means for Solving the Problems A steel sheet excellent in arrest characteristics according to the present invention which has solved the above-mentioned problems is a steel sheet having a microstructure substantially composed of a ferrite structure and a pearlite structure. The pearlite structure parallel to the plate surface exists on both surface portions over 5% of the plate thickness, and the thickness of each pearlite structure in the plate thickness direction is 5 μm or less, and each pearlite phase in the plate thickness direction. The mutual distance is 5
It has a gist where it is less than μm.

In the present invention, the chemical components in steel are as follows: C: 0.03 to 0.2% by mass (the same applies hereinafter);
0.5% or less (excluding 0%), Mn: 1.8% or less (excluding 0%), Al: 0.01 to 0.1%,
N: 0.01% or less (excluding 0%) balance: Fe and unavoidable impurities are recommended. Further, Ti: 0.02% or less (excluding 0%), Nb:
0.03% or less (excluding 0%), V: 0.05
% Or less (not including 0%), B: 0.002% or less (not including 0%), and at least one selected from the group consisting of: Cu: 0.5% or less (0% And Ni:
Contains at least one selected from the group consisting of 0.5% or less (excluding 0%), Cr: 0.1% or less (excluding 0%), and Mo:
At least one selected from the group consisting of 0.1% or less (excluding 0%), Ca: 0.01% or less (excluding 0%), and Z
r: at least one selected from the group consisting of 0.01% or less (excluding 0%) is a preferred embodiment of the present invention.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have developed a steel sheet capable of achieving a severe arrest property, which was difficult to achieve conventionally, that is, "arrest property at -80 ° C: 600 kgf / mm ^2/3 or more". In order to provide, the intensive study has been made particularly focusing on pearlite which is the second phase structure of the steel sheet surface portion. This is because the steel sheet mentioned in the prior art also aims to improve the arrest characteristics by controlling the pearlite structure similarly to the present invention, but the steel sheet could not reach the target level. As a result, in order to obtain desired arrest characteristics, it is necessary to precisely control the thickness of the pearlite structure and the mutual distance between the pearlite structures in the surface layer of the steel sheet, and reduce the thickness of the pearlite structure in the thickness direction. In addition, the present inventors have found that it is effective to increase the distance between the pearlite phases in the sheet thickness direction, and have completed the present invention.

That is, the steel sheet of the present invention is a steel sheet whose microstructure is substantially composed of a ferrite structure and a pearlite structure, and must have a pearlite structure satisfying the following requirements.

A pearlite structure parallel to the plate surface is present on both surface portions of the steel plate over 5% or more of the plate thickness. The thickness of each pearlite structure in the thickness direction is 5 μm or less. The distance between the pearlite phases in the thickness direction is 5 μm or less.

The above-mentioned requirements are extremely important for obtaining a desired high arrest property, and the pearlite structure having the pearlite structure thickness and the pearlite phase mutual distance satisfying the above conditions is required to be formed over 5% or more of the sheet thickness. Only when it is present can the target level of "arrest characteristics at -80 ° C: 600 kgf / mm ^2/3 or more" set forth in the present invention be achieved.

In order to further clarify the reasons for limiting the above requirements in the present invention, FIGS.
The results obtained by examining the effect on the arrest characteristics (arrest characteristics at -80 ° C) by the double tensile test are shown in a graph. These data are extracted from the examples described below, and all are examples examined using steel type A and a test plate thickness of 30 mm in Table 1 (described later).

First, FIG. 1 shows a parlor in a surface layer of a steel sheet.
The thickness of the sheet structure in the thickness direction (corresponding to t in the figure) is arrested
4 is a graph showing an effect on characteristics. According to FIG.
When the thickness of the light structure is 5 μm or less (in the figure,
-80% when the light layer is more than 5% of the steel sheet
Arrest characteristics at 600 ° C are the target value of 600 kgf / mm^{3 / Two}
And it is extremely excellent in arrest characteristics
I understand. Preferably 4 μm or less, more preferably 3 μm
m or less.

FIG. 2 is a graph showing the influence of the mutual distance (corresponding to 1) of each pearlite phase in the sheet thickness direction on the surface layer of the steel sheet on the arrest characteristics. FIG. 2 shows that the arrest characteristic exceeds the target value when the pearlite phase mutual distance is 5 μm or less (（and Δ in the figure). Preferably it is 3 μm or less.

In the steel sheet of the present invention, the pearlite structure in which such fine pearlite phases are present densely with each other must exist on both surface portions of the steel sheet over 5% or more of the sheet thickness. This is because the desired high arrest property described in the present invention is effectively exhibited only when 5% or more of a pearlite structure satisfying the above requirements is present.

Next, the chemical composition of the steel sheet of the present invention will be described. As described above, the steel sheet of the present invention has
The most important point exists where the pearlite structure that satisfies the requirements of is specified, and the chemical components in the steel are not particularly limited, and are appropriately set within a range that does not impair the operation of the present invention. Preferred chemical components are as follows:

C: 0.03 to 0.2% C is preferably added in an amount of 0.03% or more in order to secure required strength. More preferably, it is 0.05% or more. However, since excessive addition deteriorates weldability and base metal toughness, the upper limit is preferably set to 0.2%. More preferably, it is 0.17% or less.

Si: 0.5% or less (excluding 0%) Si is an element useful as a material for increasing the strength of the base material and as a deoxidizing material for molten steel. For this purpose, 0.05% or more must be added. Is preferred. However, excessive addition deteriorates weldability and base metal toughness, so the upper limit is preferably set to 0.5%. More preferably, it is 0.45% or less.

Mn: 1.8% or less (excluding 0%) Mn is useful as an element for increasing the strength of the base material, and therefore, it is preferable to add 0.5% or more. It is more preferably at least 0.7%. However, excessive addition degrades weldability and base metal toughness, so the upper limit is preferably made 1.8% or less. It is more preferably at most 1.6%.

Al: 0.01 to 0.1% Al is not only useful as a deoxidizing agent, but also has a function of forming a nitride to refine the base material structure.
If it is less than this, such an effect cannot be exhibited effectively. More preferably, it is 0.015% or more. However,
If the addition exceeds 0.1%, the base material toughness deteriorates.
The upper limit is preferably set to 0.1%. More preferably, it is 0.05% or less.

N: 0.01% or less (excluding 0%) N forms a nitride with the above-mentioned additional elements such as Al, Ti, Nb, and V, and has an effect of refining the base material structure. To effectively exert such an effect, it is preferable to add 0.001% or more. However, an excessive addition exceeding 0.01% causes an increase in solid solution N, and particularly deteriorates the toughness of the welded portion. Therefore, the upper limit is preferably made 0.01%.

The steel sheet of the present invention preferably contains the above elements as essential components, and the balance: Fe and unavoidable impurities, but it is recommended to actively add the following elements for the purpose of imparting various characteristics. Is done.

Ti: 0.02% or less, Nb: 0.03%
Below, V: 0.05% or less, and B: 0.002% or less
At least one selected from the group consisting of
These elements do not contain austenitic grains at the time of heating the slab, suppress the austenite grain coarsening during the heating of the slab, promote the ferrite transformation nucleation after rolling, or suppress the austenite grain recrystallization. It is an element that has an effect. Specifically, Ti has the above-mentioned effect by the formation of nitrides.
It is preferable to add at least 04%. However, 0.02%
Even if it is added in excess of, the toughness of the base material will deteriorate,
It is preferable to set the upper limit to 0.02% or less.

Nb is an element effective for forming austenite grains during rolling and suppressing recrystallization by forming carbonitrides and effective for refining ferrite grains after rolling. In order to effectively exert the effect, it is preferable to add 0.002% or more. However,
If the content exceeds 0.03%, the weldability deteriorates. Therefore, it is preferable to set the upper limit to 0.03%.

V, like Nb, is an element effective for forming austenite grains during rolling and suppressing recrystallization by forming carbonitrides, and effective for refining ferrite grains after rolling. To effectively exert such an effect, it is preferable to add 0.002% or more. However, 0.
If the addition exceeds 0.05%, the weldability deteriorates, so the upper limit is preferably made 0.03%.

B is an element effective for improving the toughness of the heat affected zone (HAZ), and is preferably added in an amount of 0.0002% or more to effectively exert such an effect. However, if added in excess of 0.002%, the hardenability increases and the low-temperature toughness of the base material is degraded, so the upper limit is preferably made 0.002%.

Cu: 0.5% or less and Ni: 0.5% or less
At least one selected from the group consisting of
Elements are also not including 0%) These elements are all elements contributing to the improvement of the refinement of austenite grains and low-temperature toughness.

Specifically, Cu is an element effective for refining crystal grains, and it is preferable to add 0.2% or more to effectively exert such an effect. However, the addition of a large amount deteriorates the weldability of the base material, so the upper limit is preferably set to 0.5%.

Ni is an element effective for improving low-temperature toughness, but is expensive, so its upper limit is preferably made 0.5%.

These elements may be used alone,
Alternatively, they may be used in combination. However, since hot cracking may occur when Cu is added alone, it is preferable to add Ni at the same time to prevent hot cracking.

Cr: 0.1% or less and Mo: 0.1% or less
At least one selected from the group consisting of
Elements also do not contain 0%) Cr and Mo, both precipitated carbonitrides, is an element contributing to the increase in strength, in order to develop this function effectively, any of the elements 0. It is preferable to add at least 03%. However, excessive addition deteriorates weldability and base metal toughness, so the upper limit is preferably set to 0.1%.

Ca: 0.01% or less and Zr: 0.01
% Or less selected from the group consisting of
These elements do not include 0%). These elements have an effect of increasing the toughness of the base material by spheroidizing the inclusion morphology in the steel.

Of these, Ca has the effect of improving the toughness of the base metal by spheroidizing the form of inclusions in the steel. In order to exert such an effect effectively, 0.00
It is preferable to add at least 05%. However, an excessive addition adversely degrades the toughness of the base material.
Preferably, it is 1%.

Like Ca, Zr has the effect of improving the toughness of the base metal by making the form of inclusions in steel spheroid. In order to exert such effects effectively,
It is preferable to add 0.003% or more. However, since excessive addition degrades the toughness of the base material, the upper limit is preferably made 0.01%.

Next, a method for producing the steel sheet of the present invention will be described. As described above, in the steel sheet of the present invention, the pearlite structure parallel to the sheet surface exists on both surface portions of the steel sheet over 5% or more of the sheet thickness, and the thickness of each pearlite structure in the sheet thickness direction is 5 μm or less. And the distance between the respective pearlite phases in the thickness direction is 5 μm or less,
In order to manufacture a steel sheet having such a pearlite structure, basically, a rolling step in a grain refining temperature range (Ar ₃ transformation point to Ar ₃ transformation point + 60 ° C.) and a rolling step in a set temperature range. It is recommended that a two-stage rolling process be performed. First, in the first-stage rolling, rolling is performed in a grain-refining temperature range such that a range of 5% or more of the sheet thickness from the surface layer of the steel sheet has a fine-grained ferrite structure. It is recommended to control the temperature from the surface layer at a position of 5% or more of the plate thickness to the Ar ₃ transformation point or lower and the temperature at the center of the plate thickness to be in the temperature range for grain refining, thereby obtaining a desired structure. be able to. Preferably, in the rolling process of the first stage, a region corresponding to a thickness of 5% or more from the surface layer portion of the steel plate in the thickness direction is rolled at a temperature range from the Ar ₃ transformation point to Ar ₃ + 60 ° C. with a cumulative rolling reduction of 35% or more. The part corresponding to the thickness of 5% or more in the thickness direction from the surface layer of the steel sheet by the forced cooling or natural cooling is reduced to the Ar ₃ transformation point or lower, and the center part is changed from 25% (1/4) of the sheet thickness. Ar ₃ transformation point to Ar ₃ +6
This is a method of performing rolling at a cumulative rolling reduction of 50% or more in a second rolling process after controlling the temperature to 0 ° C.

Hereinafter, the present invention will be described in detail with reference to examples.
However, the following examples do not limit the present invention,
All modifications and alterations without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention.

[0036]

EXAMPLES Using the steel types A to I shown in Table 1, heating under the conditions shown in Tables 2 and 4, rolling in two stages, and cooling were carried out. A steel sheet having a structure was manufactured. The “rolling pattern 1” described in Tables 2 and 4 corresponds to the “rolling pattern 1” described in Tables 2 and 4 corresponding to the thickness from the surface layer portion to 5%. After the first stage of rolling is performed by controlling the portion to a temperature range of the grain refining temperature range (Ar _{3 to} Ar ₃ + 60 ° C.), it corresponds to the thickness from the surface layer portion to the sheet thickness of 5% by forced cooling. The second stage rolling is performed after controlling the temperature of the portion to be subjected to Ar ₃ points or less and the portion corresponding to the thickness of 1/4 to 1/2 of the plate thickness to the grain refining temperature range. "Rolling pattern 2"
In the above-mentioned rolling pattern 1, after the first stage of rolling,
Rolling was performed in the same manner as in Rolling Pattern 1 except that the rolls were naturally cooled. Each of these rolling patterns is shown in FIG.

With respect to the various steel plates thus obtained,
Tensile test piece A tensile test is performed using JIS No. 1B to evaluate the elongation property, and the arrest property (brittle crack propagation arrest property) at −80 ° C. is measured by a temperature gradient double tensile test specified by WES. did.

The results are shown in Tables 3 and 5.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

[0042]

[Table 4]

[0043]

[Table 5]

In Table 3, No. Nos. 1 to 25 are examples of the present invention satisfying the requirements of the present invention. While the arrest characteristics at -80 ° C exceed the target value of 600 kgf / mm ^2/3 , all of the requirements of the present invention are satisfied. No. in Table 5 that does not satisfy 1 to 25 indicate that the target level arrest characteristics cannot be achieved.

[0045]

The present invention is constituted as described above, and by controlling the surface layer of the steel sheet to a fine and dense pearlite structure, even if a brittle fracture occurs, the propagation of the crack is stopped. This is extremely significant in that it was possible to provide a steel sheet having excellent performance (ie, excellent arrest characteristics) without adding an expensive alloy element. According to the present invention, the arrest characteristic at −80 ° C. is 600 kgf
/ Mm ^2/3 and extremely excellent arrest characteristics can be obtained, so that the safety of the structure can be improved and the industrial applicability is extremely high.

[Brief description of the drawings]

FIG. 1 is a graph showing the influence of the thickness of a pearlite structure in the thickness direction on the surface layer of a steel sheet on arrest characteristics.

FIG. 2 is a graph showing the effect of the mutual distance of each pearlite phase in the sheet thickness direction on the surface layer of a steel sheet on arrest characteristics.

FIG. 3 is a graph showing an outline of a rolling pattern used in Examples.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Satoshi Takeshita 1 Kanazawacho, Kakogawa City, Hyogo Prefecture Inside the Kobe Steel Works Kakogawa Works (72) Inventor Toru Yamashita 1 Kanazawacho, Kakogawa City, Hyogo Prefecture Kobe Corporation Inside the steel mill Kakogawa Works

Claims (6)

[Claims] 1. A steel sheet whose microstructure is substantially composed of a ferrite structure and a pearlite structure, wherein a pearlite structure parallel to the sheet surface is present on both surface portions of the steel sheet over 5% or more of the sheet thickness, respectively. A steel sheet having excellent arrest properties, wherein the steel sheet has a thickness in the thickness direction of each pearlite structure of 5 μm or less and a mutual distance of each pearlite phase in the thickness direction of 5 μm or less. 2. The chemical composition in steel is expressed by mass% (the same applies hereinafter): C: 0.03 to 0.2%, Si: 0.5% or less (0
%), Mn: 1.8% or less (excluding 0%), Al: 0.01 to 0.1%, N: 0.01% or less (excluding 0%), balance: Fe and The steel sheet according to claim 1, which is an unavoidable impurity. 3. Ti: 0.02% or less (excluding 0%), Nb:
0.03% or less (excluding 0%), V: 0.05
% Or less (not including 0%), and B: 0.002%
The steel sheet according to claim 2, wherein the steel sheet contains at least one selected from the group consisting of the following (not including 0%). 4. Further, Cu: 0.5% or less (excluding 0%), and Ni:
The steel sheet according to claim 2, comprising at least one selected from the group consisting of 0.5% or less (excluding 0%). 5. 5. Further, Cr: 0.1% or less (excluding 0%), and Mo:
The steel sheet according to any one of claims 2 to 4, comprising at least one selected from the group consisting of 0.1% or less (excluding 0%). 6. Ca: 0.01% or less (excluding 0%), and Zr:
The steel sheet according to any one of claims 2 to 5, comprising at least one selected from the group consisting of 0.01% or less (not including 0%).