JPH1088280A - Steel sheet for structural purpose excellent in brittle fracture resistance after plastic deformation and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To economically produce a steel sheet free from drastical deterioration in brittle fracture resistance which is one of the important capacities of a steel sheet compared to the arrest capacity at the time of being the stock in which plastic deformation has not been applied even if large structures such as ships are applied with plastic deformation on a large scale by their collision with each other or the like with high productivity. SOLUTION: This steel sheet for structural purpose excellent in brittle fracture propagation stopping characteristics after plastic deformation is the one in which, in a steel sheet having a compsn. contg., by weight, 0.04 to 0.08% C, 0.05 to 0.5% Si, 0.3 to 2.0% Mn, 0.005 to 0.1% Al and <=20PPM free N, furthermore contg., at need, one or >=two kinds among 0.005 to 0.10% Ti, 0.05 to 0.5% Cu, 0.05 to 0.5% Ni, 0.005 to 0.05% Nb, 0.005 to 0.05% V, 0.05 to 0.5% Mo and 0.0002 to 0.0015%, and the balance Fe with inevitable impurities, the X-ray plane intensity ratio in the (110) plane showing a texture depeloping degree is regulated to >=2, and the area ratio of coarse grains with >=20μm diameter equivalent to a circle in the crystal grains constituting it is regulated to <=10%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a steel plate having a large arrest performance which is particularly important in brittle fracture resistance even when large structures such as a hull undergo large-scale plastic deformation due to collision or the like. TECHNICAL FIELD The present invention relates to a structural steel sheet capable of ensuring safety against brittle fracture in an emergency such as a collision without significantly deteriorating as compared with a case where the steel sheet has not been subjected to heat, and a method for manufacturing the same.

[0002]

2. Description of the Related Art It is generally known that when a steel sheet is plastically deformed, the mechanical properties change, particularly the toughness of the steel sheet is deteriorated. In general, a strain aging Charpy test and the like are widely performed to evaluate the use performance of the steel sheet during plastic deformation. Have been. However, there are still many unclear factors governing this strain aging characteristic,
This is a serious problem from the viewpoint of ensuring safety in the event of a collision between structures. In general, the strain aging property is such that the C element or N element dissolved in steel interacts with dislocations introduced by applying strain and hinders the movement of the dislocations, so that the yield point rises. Is described as embrittlement.

[0003] In addition, the arrest performance, which is the ability to prevent the propagation of brittle fracture, significantly deteriorates when a steel sheet undergoes large-scale plastic deformation, according to the Ship End Ocean's 1994 report. It is possible to provide higher arrest performance than normal design for plastic damage caused by unexpected collision between large hulls, etc.
Steel materials that can ensure the minimum necessary safety even when arrest performance deteriorates due to plastic deformation are introduced. In order to ensure safety in an emergency such as undergoing large-scale plastic deformation, a method of providing high brittle fracture resistance in consideration of the degradation allowance as described above, and even if subjected to large-scale plastic deformation There are two methods of providing a steel material that is hard to deteriorate. However, especially with regard to arrest performance, a method for improving the arrest performance is finally being clarified, and a technique for minimizing the deterioration phenomenon after plastic deformation has not yet been clarified.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel sheet and a method for manufacturing the same, which minimize the deterioration of brittle fracture resistance due to plastic deformation as much as possible.

[0005]

Means for Solving the Problems The present invention has been made to achieve the above object, and the means are as follows.

(1) By weight%, C: 0.04 to 0.08
%, Si: 0.05 to 0.5%, Mn: 0.1 to 2.0
%, Al: 0.005 to 0.1%, Free N: ≦ 20P
In a steel sheet composed of PM, the balance of Fe and unavoidable impurities, the (110) plane exhibiting the degree of texture development has an X-ray plane intensity ratio of 2 or more, and the equivalent circle diameter of the constituting crystal grains is 2
A structural steel sheet having excellent brittle fracture resistance after plastic deformation, characterized in that the area ratio of coarse grains of 0 μm or more is 10% or less.

(2) C: 0.04 to 0.08 by weight%
%, Si: 0.05 to 0.5%, Mn: 0.1 to 2.0
%, Al: 0.005 to 0.1%, Free N: ≦ 20P
PM, further Ti: 0.005 to 0.10%, Cu: 0.
05-0.5%, Ni: 0.05-0.5%, Nb:
0.005 to 0.05%, V: 0.005 to 0.05
%, Mo: 0.05-0.5%, B: 0.0002-
A structural steel sheet having a structure having the characteristics described in (1) above and having excellent brittle fracture resistance after plastic deformation, comprising 0.0015% of one or more kinds.

(3) A steel strip or a steel sheet containing the chemical component described in (1) or (2) above is heated to a temperature of ₃ or more Ac and rolled in a recrystallization temperature range by 20% or more. Rolling of 30% or more in a non-recrystallization temperature range, and subsequently rolling of 30% or more at a temperature of ₃ points or less of Ar and ₁ point or more of Ar, characterized in that the brittle fracture propagation arrest property after plastic deformation is reduced. Manufacturing method of excellent structural steel sheet.

(4) In the manufacturing method described in the above (3), after the rolling is completed, the steel sheet is further cooled at a cooling rate of 1 ° C./sec or more until the surface temperature of the steel sheet reaches at least Ar ₁ point. A method for producing a structural steel sheet having excellent brittle fracture arrestability after plastic deformation.

(5) In the manufacturing method described in the above (4), after the cooling is completed, a tempering treatment is further performed at a temperature of 500 ° C. to 600 ° C., which is excellent in brittle fracture propagation stop characteristics after plastic deformation. Method of manufacturing a structural steel sheet.

The components of the present invention and the reasons for their addition are as follows.

C is generally used as an effective component for improving the strength of steel.
In the present invention, 0.08% is added in order to suppress the deterioration of arrest performance after plastic strain.
It is regulated as follows.

Si is necessary as a deoxidizing element of molten steel,
It is useful as a strength increasing element, but if it exceeds 0.5%, the workability of steel or the toughness of the welded portion deteriorates, and if it is less than 0.05%, the deoxidizing effect is insufficient, so the addition amount is 0.05%. ~ 0.5
Regulate to%.

Mn needs to be added in an amount of 0.3% or more as a component for improving the strength of the steel material. However, the addition of Mn lowers the transformation temperature. Is too low, deformation resistance increases, and rolling becomes difficult, so the upper limit is 2.0%.

Al is combined with N in steel to form an Al nitride, which has an effect of reducing free nitrogen, which interacts with dislocations introduced by plastic strain and hinders the movement of dislocations. If added too much, the toughness of the material is degraded, so the content is restricted to 0.005 to 0.10%.

Although N is contained as an unavoidable impurity, free N interacts with dislocations introduced by plastic deformation to cause a phenomenon called Cottrell atmosphere which hinders deformation. Therefore, free N is restricted to 20 ppm or less.

The above are the basic components of the steel targeted by the present invention. In order to increase the base metal strength, Cu: 0.05 to 0.5% Ni: 0.05 to 0.5% Nb: One or more of 0.005 to 0.05% V: 0.005 to 0.05% Mo: 0.05 to 0.5% may be used. When the above elements are contained, there is a concern that deterioration of the joint toughness may occur. To improve the joint toughness, Ti: 0.005 to 0.10% B: 0.0002 to 0.0015% Species or two can be used.

The lower limit of the above-mentioned additive element is the minimum amount required for exhibiting the effect of improving the strength or the joint toughness, and if the upper limit is more than that, the joint toughness and the base metal toughness are reduced. It is regulated to deteriorate.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors studied experimentally the change in mechanical properties of a steel material when plastic strain was imparted to a steel sheet, clarified the mechanism thereof, and degraded the arrest performance due to the plastic strain. Factors for producing a steel sheet with a small cost were studied.

It is widely known that the aging treatment after the plastic strain is applied to the steel sheet causes the toughness to be deteriorated by the V-notch Charpy impact test. However, the mechanism regarding the toughness degradation phenomenon of the steel sheet without aging treatment is not clarified.

Further, a one-to-one correlation is not recognized between the KCa value obtained by the temperature gradient type ESSO test which is an index of the arrest performance and the result of the V notch Charpy striking test. The dominant factors are also known to be different.

First, as a result of a detailed observation of a fracture surface in which a brittle crack propagated in a steel plate subjected to plastic strain, a tear ridge formed around a facet defined as a unit of a wall fracture surface (a ductile fracture) was observed. (Referred to as the area of the fracture surface around the facet present) and the ductile fracture area around the facet was reduced. This phenomenon can be interpreted as a decrease in ductility in a microscopic region under a situation where deformation is constrained in a narrow region between facets that have undergone open-wall fracture. Therefore, a notched tensile test piece was provided from a steel plate subjected to plastic strain, and the relationship between chemical components and ductility characteristics after plastic strain was applied was investigated. As a result, the contribution of the ductility characteristics of free N and C is large, the free N amount is 20 ppm or less, and the C amount is 0.1 ppm.
It was found that when the content is 08% or less, a remarkable decrease in elongation does not occur as shown in FIG. In the experiment, free N was 3
Even if it is 0 ppm, when the C content is 0.03%, the elongation does not decrease, but when the C content is 0.03%, a large amount of a reinforcing element such as Ni must be added to secure the strength of the steel sheet. It is not economical.

Next, a steel sheet containing 0.06% of C and 15 ppm of free N, which are components that do not cause microscopic decrease in ductility, was prototyped and subjected to a temperature gradient type ESSO test. The value was found to decrease. Considering the reason, it is assumed that although the ductility does not decrease, the yield strength increases due to the application of plastic strain, and the stress at the tip of the crack reaches the critical microscopic fracture stress, thereby causing brittle fracture. Therefore, it has been clarified that it is important to reduce the stress at the crack tip by some means in order to suppress the brittle fracture after the plastic strain is applied.

Therefore, by developing a texture and causing a vertical crack called separation at the crack tip in parallel with the thickness direction of the steel sheet, the constraint on the crack or the notch tip is released, and the stress at the crack tip is reduced. We decided to use the method.

In order to generate separation before the local stress reaches the critical microscopic fracture stress, 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, separation occurs prior to fracture in a temperature region where plastic deformation is dominant, but brittle fracture occurs in a low temperature region.

This is because, at low temperatures, the yield point of the steel material increases, and the plastic region at the tip of the crack becomes smaller, so that local deformation occurs due to plastic anisotropy between Yolonies having different crystal orientations necessary for the generation of separation. It is thought that it is not. Therefore, various experiments were performed using general structural steels shown in Tables 1 and 2.

[0027]

[Table 1]

[0028]

[Table 2] First, in order to quantify the texture morphology of the texture required to generate separation, steel sheets having various crystal orientations and different texture levels were manufactured by changing the two-phase rolling conditions. Then, various texture forms were measured for surface intensity ratio by X-ray, and as shown in FIG.
There is a correlation between the surface strength ratio of the (0) plane and the critical fracture stress in the thickness direction, and the critical fracture stress of the texture having the (110) plane strength ratio of 2 or more has no texture (( 110)
It was found that the surface intensity ratio was 以下 or less of that of about 1).

From these results, it can be seen that the development of a texture having a (110) plane crystal orientation is effective for lowering the critical fracture stress in the thickness direction.

If the critical fracture stress in the thickness direction is always higher than the critical fracture stress in the thickness direction, microfracture occurs in the thickness direction before a crack existing in the thickness profile causes brittle fracture (separation). As a result, it is possible to reliably prevent the crack in the plate thickness section from shifting to brittle fracture.

However, if the (110) plane strength ratio is too large, the toughness in the cross-section in the thickness direction will be too low, and when applied to a structure in which stress acts in the cross-section in the thickness direction, the steel plate will peel off. Is also a concern. Therefore,
It is necessary to improve the critical fracture stress in the thickness direction.

In the two-phase zone rolled material used for developing the texture, unless the austenite structure before the two-phase zone rolling is refined, local grain growth occurs during the two-phase zone rolling, resulting in coarseness. A phenomenon in which various crystal grains are mixed is known.

As a result of various investigations on the relationship between the critical fracture stress and the microstructure in the cross section direction of the sheet thickness, it was newly found that this coarse grain was the cause of the decrease in the critical fracture stress, and as shown in FIG. It has been clarified that it is necessary to suppress the presence of coarse particles having a circle equivalent diameter of 20 μm or more in an area ratio of 10% or less.

As a production condition for suppressing the presence of coarse grains having a circle equivalent diameter of 20 μm or more to an area ratio of 10% or less, ferrite formed by transformation by reducing the austenite grain size before the two-phase region rolling is performed. A method for refining grains was studied. First, in the rough rolling stage, sufficient recrystallization was promoted to refine the grains, and furthermore, various conditions for sufficiently introducing dislocations by rolling in the non-recrystallized region were examined. However, it was found that the total rolling reduction could be reduced, and it was found that the rolling reduction in the non-recrystallization zone rolling was required to be 20% or more. Under these conditions, if the unrecrystallized zone rolling is performed by 30% or more, even in the rolling in which the two-phase zone rolling reduction required to obtain the required surface strength ratio is 30% or more,
It was found that coarse ferrite was not formed.

[0035]

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.

The method of applying a plastic strain of 10% is based on the ESSO test.
Pieces (width 500mm x length 500mm x plate thickness) can be collected
Used when conducting the ESSO test
Plastic strain is applied by applying a tensile load with a large-sized tensile testing machine
During loading, strain is monitored by an extensometer attached to the test piece.
After applying the load, draw a mark on the test piece after loading.
The amount of plastic strain was determined from the change in the interval before and after tension loading.
Performed V-notch Charpy impact test of plastic strain imparting material and material
And the vTr values were compared. Also, almost the same
An ESSO test piece loaded with plastic strain was subjected to three temperature gradient type brittleness.
Crack propagation arrest test, the crack arrest temperature and the time
The relationship between Kca value and temperature is determined from the K value of
Is 6000 N / mm^1.5 Was determined.

The chemical components of Test Nos. 1 to 12 of the present invention and Test Nos. 13 to 15 of the comparative example were steel types 1 to 1, respectively.
7, the C content is 0.08% or less, and the free N content is 2
0 ppm or less, which is within the regulation of the present invention. On the other hand, the test numbers 16 to 19 of the comparative examples use steel types 8 to 11, respectively, and the steel types 8 and 9 have a C amount equal to or more than a required amount, and the steel type 10 has a free N amount equal to or more than a predetermined amount, Class 1
1 is a predetermined amount or more in both the C amount and the N amount, both of which deviate from the component specified amounts of the present invention.

Test Nos. 1 to 12 and 16 to 19 were carried out under predetermined rolling conditions for rough rolling, unrecrystallized zone rolling, and two-phase zone rolling.
19 is a change in vTrs due to plastic strain, Kca = 600
The change in temperature indicating 0 N / mm ^1.5 was larger than those of Test Nos. 1 to 12 of the present invention.

Test Nos. 13, 14, and 15 used test steels having a predetermined chemical composition range. Test No. 13 was a case where the two-phase rolling was not applied. The zone rolling was performed, but the preceding unrecrystallized zone rolling was insufficient. Test No. 15 applied only the unrecrystallized zone rolling without rough rolling. In these three examples, vTrs and Kc due to plastic strain are larger than those of the present invention.
The change in temperature indicating a = 6000 N / mm ^1.5 was large.

[0040]

The present invention utilizes the above-described means and functions, and applies predetermined rough rolling, unrecrystallized zone rolling, and two-phase zone rolling to a steel sheet having a predetermined range of C and N contents. By doing so, it is possible to provide a structural steel sheet with little deterioration of arrest performance even when it receives plastic strain.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between chemical components and the results of a restricted tensile test after the application of plastic strain.

FIG. 2 is a diagram showing a relationship between a (110) plane strength ratio by X-rays and a critical fracture stress in a sheet thickness direction.

FIG. 3 is a diagram showing the relationship between the area ratio of coarse grains and the critical fracture stress in the thickness direction of the cross section.

Claims (5)

[Claims] 1. Weight%: C: 0.04 to 0.08% Si: 0.05 to 0.5% Mn: 0.3 to 2.0% A1: 0.005 to 0.1% Free N: ≦ 20 PPM In a steel sheet comprising the balance of Fe and inevitable impurities, the X-ray plane intensity ratio of the (110) plane showing the degree of texture development is 2 or more, and the equivalent circle diameter of the constituting crystal grains is 20 μm.
A structural steel sheet having excellent brittle fracture arrestability after plastic deformation, characterized in that the area ratio of coarse grains of m or more is 10% or less. 2. In weight%, C: 0.04 to 0.08% Si: 0.05 to 0.5% Mn: 0.3 to 2.0% Al: 0.005 to 0.1% Free N: ≦ 20 PPM Further, Ti: 0.005 to 0.10% Cu: 0.05 to 0.5% Ni: 0.05 to 0.5% Nb: 0.005 to 0.05% V: 0. 005-0.05% Mo: 0.05-0.5% B: 0.0002-0.0015% In a steel sheet containing one or more of the following, the balance being Fe and unavoidable impurities, the degree of texture development Plastic deformation characterized in that the (110) plane has an X-ray plane intensity ratio of 2 or more, and the area ratio of coarse grains having an equivalent circle diameter of 20 μm or more of the constituting crystal grains is 10% or less. Structural steel sheet with excellent brittle fracture propagation arrestability afterwards. 3. A steel slab or a steel sheet containing the chemical component according to claim 1 or 2, and a steel slab or a steel sheet having a temperature of ₃ points or more of Ac in a recrystallization temperature range of 20% or more. Rolling, and a rolling reduction of 30% in the non-recrystallization temperature range
The production of a structural steel sheet having excellent brittle fracture resistance after plastic deformation, characterized in that the above-mentioned rolling is carried out and the rolling is performed at a rolling reduction of 30% or more at a temperature of Ar ₃ or less and Ar ₁ or more. Method. 4. The production method according to claim 3, wherein after completion of the rolling, the steel sheet surface temperature is at least Ar.
_A method for producing a structural steel sheet having excellent brittle fracture resistance after plastic deformation, characterized by cooling at a cooling rate of 1 ° C./sec or more to _one point. 5. A structure having excellent brittle fracture resistance after plastic deformation, characterized by further performing a tempering treatment at a temperature of 500 ° C. to 600 ° C. after cooling is completed. Manufacturing method for steel sheet.