JPH09176782A - Building use high tensile strength steel excellent in fracture resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce the objective steel by forming a specified compsn. in which the contents of P, S and 0 are reduced and elements such as Al, Ti or the like forming nitrides and having effect of fixing N are incorporated to reduce the content of solid solution N and imparting an ultrafine-grained structure on the surface layer. SOLUTION: This steel has a compsn. contg., by weight, 0.01 to 0.15% C, 0.01 to 1.0% Si, 0.1 to 2.0% Mn, 0.003 to 0.1% Al and 0.001 to 0.005% N, in which the contents of P, S and O as impurities are regulated to <=0.01% P, <=0.01% S and <=0.006% O, and, also, N(%)-Al(%)/3.0<=0 is satisfied, and the balance iron. Then, at least two outer surfaces among the outer surfaces thereof has an ultrafine-grained structure in which the average ferrite grain size in the range from the surface layer to 10 to 33% of the total thickness is regulated to <=3μ. Moreover, prescribed amounts of one or more kinds among Ti, Zr, Nb, Ta, V and B may be used instead of Al, and, furthermore, the above steel may be incorporated with one or more kinds among Cr, Mo, Ni, Cu and W or one or more kinds among Mg, Ca and rare earth metals.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material for a strength member used for a structure which is subjected to a large and repeated plastic strain due to a large earthquake or the like during use and a method for manufacturing the same. For example, steel materials manufactured by this method can be used for general welded steel structures such as marine structures, pressure vessels, shipbuilding, bridges, buildings, line pipes, etc., but especially for buildings requiring earthquake resistance, bridges, etc. It is useful as a steel material for structures. The form of the steel material is not particularly limited,
Steel plate used as a structural member and requiring low temperature toughness,
It is especially useful for thick plates, steel pipe materials or shaped steel.

[0002]

2. Description of the Related Art In recent years, structures have been increasing in size as seen in higher-rise buildings and larger spans of bridges. Steel materials used in such applications include structures caused by earthquakes, typhoons, and the like. Securing performance to prevent material collapse is an important issue. In particular, from the experience of the Great Hanshin Earthquake, if the safety of a structure is to be ensured by the performance of steel materials without special consideration in design and construction, steel materials with high safety in both ductile fracture and brittle fracture are required. It is being recognized.

Recently, the use of low yield ratio steel (low YR steel) or high uniform elongation steel in consideration of ductile fracture performance has been studied for high-rise building steel. If the collapse of a structure due to an earthquake is caused only by ductile fracture of the material, the use of such steel materials will improve the safety of the structure.

However, in a huge earthquake such as the Great Hanshin Earthquake, the steel material is not necessarily ductile fracture and finally collapses. The ductile fracture is followed by brittle fracture, and the brittle crack propagates throughout. Various post-earthquake studies have shown that this may cause the ultimate collapse of structures.

[0005] In addition, deformation due to an earthquake is not simple, and it is reasonable to think that a large and continuous vibration, especially in the case of a huge earthquake, repeatedly applies a large force at such a level as to cause plastic deformation of the steel material. is there.

From the above viewpoint, in order to prevent the structure from collapsing due to a huge earthquake or huge typhoon such as once every several decades to hundreds of years, among the additional characteristics of the steel material, It is important to add the following characteristics of.

[0007] Due to the improvement of the ductility characteristics, the ductile cracking ability capable of absorbing the energy of the earthquake is excellent, and the generation and propagation resistance of the ductile crack is large. Little deterioration in toughness due to repeated plastic deformation. Even if a brittle crack occurs, it stops halfway and does not lead to destruction or collapse of the member or the entire structure. Conventionally, techniques for improving the above characteristics have been developed in various fields in some fields.

Regarding the ductile fracture capacity, many means for lowering the yield ratio have been proposed, and further, the cementite is stabilized by controlling the quenching stop temperature to directly obtain the ductility characteristic (uniform elongation). A technique for improving the ductile fracture capability is also disclosed in JP-A-6-25737.

It has been conventionally known that the inclusion of Ni is effective for stopping brittle cracks. Further, recently, a technique has been developed to improve the propagation stopping property of a brittle crack without increasing the amount of Ni by imparting an ultrafine grain structure to the surface layer portion as disclosed in JP-A-4-141517. There is.

[0010]

However, before the experience of the Great Hanshin Earthquake, the above-mentioned characteristics (1), which had not been imagined as necessary steel material characteristics, in particular, were not fully recognized, and therefore, Satisfying the above characteristics at the same time, without damaging the structure, even if a huge earthquake or huge typhoon such as once every several decades to hundreds of years is encountered without any special consideration in design and construction. It cannot be said that the steel material excellent in fracture resistance and its manufacturing technology have been completely established at this stage.

From the viewpoint of ensuring safety as a structure, it is of course the first consideration to suppress the occurrence of brittle fracture. However, from the viewpoint of ensuring safety in the event of a huge earthquake, which is rarely seen, it is necessary to further suppress the initiation and growth of ductile cracks that facilitate the initiation of brittle fracture, and to prevent crack tip growth after ductile crack growth. A new problem imposed on the steel sheet is that the large deterioration of toughness in the plastic region and the large deterioration of toughness after repeated plastic deformation do not occur.

However, the structure has a welded portion with deteriorated properties, and it is impossible to completely eliminate the existence of a defect which becomes a starting point of brittle fracture. In that case, it is extremely difficult to completely suppress the occurrence of brittle fracture itself, and it is also extremely economically disadvantageous.

Therefore, it is necessary to allow the brittle fracture in the unlikely event and make the propagation stopping characteristic of the brittle crack capable of inhibiting the propagation of the crack compatible with the ductile fracture characteristic and the toughness after plastic deformation.

A technique for forming a superfine grain structure in the surface layer to improve the propagation stopping property of a brittle crack is disclosed in Japanese Patent Application Laid-Open No. 4-141.
As disclosed in Japanese Laid-Open Patent Publication No. 517, etc., in the case where the present invention is subject to large plastic deformation due to a large earthquake or the like, toughness and ductility can be obtained only by forming ultrafine grain plasticity in the surface layer portion. However, it is difficult to secure resistance to fracture such as brittle crack propagation stopping characteristics, and as will be described below, it is achieved only by strict control of the impurity element and devising a component for fixing N.

[0015]

According to the present invention, it is necessary to reduce P, S and O as impurities as much as possible in order to improve the characteristics of ductile fracture initiation and crack propagation arrest characteristics, and even after repeated plastic deformation. It is important to reduce the amount of solute N in order to maintain the characteristics, and it is necessary to reduce the amount of solute N in the steel in order to suppress deterioration of toughness due to plastic strain such as repeated plastic deformation. Al, Ti, Zr, Nb, Ta, V, which has the effect of forming a nitride to fix N,
It was discovered by detailed experimental analysis that it is important to properly contain B.

Further, as a means for making ductile fracture characteristics, brittle fracture initiation characteristics (toughness) and brittle crack propagation arresting characteristics compatible with each other regardless of steel type and composition range, by imparting an ultrafine grain structure to the surface layer. It was found that it is most preferable to improve the propagation stopping property of brittle cracks.

In particular, the average ferrite grain size is 3 μm.
m or less, even when subjected to severe plastic deformation such that the plastic strain exceeds 10%,
It has been clarified that the deterioration of the Charpy impact property and the propagation arrest property of brittle cracks is significantly smaller than that of a normal structure having a ferrite grain size of 5 μm or more.

Further, it has been experimentally confirmed that the reduction of the amount of N, the fixing of N by the nitride forming element, and the addition of the ultrafine grain layer to the surface layer portion are also effective for improving the toughness and ductility of the welded joint. . The present invention, by comprehensively analyzing the above findings, due to the limitation of chemical components, in the steel for which the ductility is improved and the toughness is deteriorated due to plastic deformation, the presence of the surface ultrafine grain structure further improves The present invention has established and invented a technique capable of improving the fracture resistance, and the gist thereof is as follows.

(1)% by weight, C: 0.01 to 0.15
%, Si: 0.01 to 1.0%, Mn: 0.1 to 2.0
%, Al: 0.003 to 0.1%, N: 0.001 to
0.005% is contained, and the content of P, S, O as impurities is P: 0.01% or less, S: 0.01% or less, O:
0.006% or less and N (%)-Al (%) /
A steel material having 3.0 ≦ 0 and a balance of iron and unavoidable impurities, and an average ferrite grain in a range of 10 to 33% of the total thickness from the surface layer with respect to at least two outer surfaces of the outer surfaces constituting the steel material. A high-strength steel material for construction with excellent fracture resistance, which has an ultrafine grain structure with a diameter of 3 μm or less.

(2) Ti: 0.003 to 0.
020%, Zr: 0.003 to 0.10%, Nb: 0.
002 to 0.050%, Ta: 0.005 to 0.20
%, V: 0.005 to 0.20%, B: 0.0002 to
Contains 0.003% of 1 or 2 or more, N (%)
-Al (%) / 3.0-Ti (%) / 3.4-Zr
(%) / 6.5-Nb (%) / 13.2-Ta (%) /
25.8-V (%) / 10.9-B (%) / 2.0 ≦ 0
The high-strength steel material for construction having excellent fracture resistance as described in (1) above.

(3) Cr: 0.01-2.0 by weight%
%, Mo: 0.01-2.0%, Ni: 0.01-4.
0%, Cu: 0.01 to 2.0%, W: 0.01 to 2.
The high-strength steel material for construction having excellent fracture resistance as described in (1) or (2) above, which contains 0% of one or more kinds.

(4) Mg: 0.0005% by weight
0.01%, Ca: 0.0005-0.01%, RE
M: 0.0005 to 0.10%, and one or more of them is contained in a total amount of 0.0005 to 0.10%. (1) to (3) High-strength steel materials for construction with excellent fracture resistance described in.

(5) The steel slab is made to have an Ac ₃ transformation point or higher and 1250
After heating to a temperature of ℃ ℃ or less, and rough rolling with a cumulative reduction of 10 to 50% in the austenite region at 950 ° C or less, it corresponds to 10% to 33% of the total thickness of the billet at that stage. To start cooling at least two surface layer regions of the outer surface from a temperature not lower than the Ar ₃ transformation point at a cooling rate of 2 to 40 ° C./s and stop cooling at a temperature not higher than the Ar ₃ transformation point to reheat. In the process of passing through one or more times, finish rolling with a cumulative rolling reduction of 20 to 90% is completed until the recuperation after the last cooling is completed, and the surface layer area of the steel material after completion of the rolling (( Ac ₁ transformation point −50 ° C.) to (Ac ₃ transformation point + 50 ° C.)
The steel material according to any one of (1) to (4) above is reheated to a temperature range to produce a high tensile strength steel material for construction having excellent fracture resistance.

(6) 5-40 ° C./s
The method for producing a high-strength steel material for construction according to (5) above, wherein the high-strength steel material is excellent in fracture resistance, characterized in that it is cooled to 20 ° C to 650 ° C at a cooling rate of. (7) The method for producing a high-tensile steel material for construction according to the above (5) or (6), characterized by performing tempering at 450 ° C to 650 ° C. The high-strength steel material referred to here means not only high-tensile steel plate (thick plate) but also steel material including shaped steel and pipe material.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION The requirements for chemical components in the present invention include P as an impurity for improving ductility characteristics.
This is due to the limitation of the amounts of S and O, and the limitation of the chemical composition for fixing N for suppressing the toughness deterioration due to plastic deformation. Hereinafter, first, these requirements will be described in detail.

In order to improve the plastic deformability, suppress the initiation and growth of ductile cracks, it is necessary to increase the ductility of the ferrite matrix of the steel. To that end, the solid solution P, C and N should be reduced. Is preferred. As for C, since the solid solubility limit in ferrite is small and precipitates are easily formed, the adverse effect on ductility characteristics of practical steels can be ignored. In addition, since C is an essential element for securing the strength, it is not preferable to completely remove C.

N is effective for refining the heated austenite grain size by the nitride, and its inclusion as an impurity is unavoidable, but unlike C, practical steel also forms a solid solution in the ferrite matrix in a certain amount and becomes ductile. It adversely affects the characteristics. Furthermore, when the steel undergoes plastic deformation after plastic deformation or ductile crack growth in the presence of solute N, the toughness becomes remarkable due to interaction with dislocations caused by plastic deformation and fine precipitation on dislocation lines. Since it deteriorates, the solid solution N should be reduced to the limit.

For that purpose, it is necessary to fix N by a nitride forming element. As a nitride-forming element, Al is preferable because it has the least influence on other characteristics, and 10% is added to the ultrafine grain structure in the surface layer, which is most important for brittle crack propagation stopping characteristics.
When the Al amount necessary for the deterioration of toughness due to the addition of the plastic strain of 20 ° C. or less due to the increase of the fracture transition temperature in the Charpy test to be experimentally obtained, the relationship as shown in the equation (1) is obtained. Was obtained. N (%)-Al (%) / 3.0 ≦ 0 (1)

Therefore, in the present invention, the amount of N and Al is limited to the relationship of the formula (1) on the premise of the contents of Al and N within the limited range for the reason described later. Also,
As a nitride forming element, in addition to Al, Ti, Zr, N
One or two or more of b, Ta, V and B may be selectively used.

In that case, Al, Ti, Zr, Nb, T
The contents of a, V, and B are N necessary for suppressing the deterioration of toughness due to plastic deformation based on the same criteria as in formula (1).
Since it is necessary to adjust the content so that the relational expression with the amount (equation (2)) holds, Ti, Z
r, Nb, Ta, V, and B are limited so that the relationship of the expression (2) is established. N (%)-Al (%) / 3.0-Ti (%) / 3.4-Zr (%) / 6.5-Nb (%) / 13.2-Ta (%) / 25.8- V (%) / 10.9-B (%) / 2.0 ≦ 0 (2)

It is also important to limit the amount of P as an impurity. That is, since P deteriorates the ductility of the base material of ferrite, its content needs to be limited in order to improve plastic deformability, generation of ductile cracks, and propagation characteristics. The smaller the amount of P is, the more preferable it is. However, reducing the amount of P puts a load on the refining process, resulting in a decrease in productivity and an increase in cost. Based on this, the content is made 0.01% or less.

That is, the degree of deterioration of the ductility characteristics with the increase of the amount of P becomes remarkable when it exceeds 0.01%. When the amount of P is 0.01% or less, the degree of the adverse effect of P decreases.

Therefore, in the present invention, the amount of P as an impurity is limited to 0.01% or less. However, when it is used in a structure where local plastic deformation in the segregation part or deterioration of ductile fracture characteristics affects, if the problem of refining is neglected, the P content is 0.007% or less. It is more preferable to limit it.

Since S forms MnS, it deteriorates the ductile fracture characteristics. In particular, it deteriorates the propagation characteristics of ductile cracks. Since the characteristic of ductile fracture is deteriorated under the condition of a large amount of solid solution P and N, deterioration of the ductile crack propagation characteristic due to S has a great influence on the plastic deformability and the ductile fracture characteristic of the entire steel material. , S must be extremely reduced to about 10 ppm or less.

However, if the amount of P and N is reduced and the solid solution N is fixed by a nitride forming element as in the present invention,
Since the resistance until the occurrence of ductile fracture becomes large, the allowable amount of S widens, so in the present invention, S as an impurity is limited to 0.01% or less based on the experimental results.

Further, O is also preferably reduced as much as possible in order to form inclusions harmful to ductility, but like S, ductility of the base material is secured to some extent if the solute P and N are reduced. Therefore, the allowable amount is higher than that in the case where the solid solution P and N are not reduced, and it is 0.006 based on the experimental result.
%.

The above is the component limiting range for improving the plastic deformability and the ductile fracture property, which are the requirements of the present invention, and for suppressing the toughness deterioration after the plastic deformation, but it is another important requirement of the present invention. In order to improve the propagation stopping property of brittle cracks, the average ferrite grain size is 3 μm in the surface layer portion of at least two surfaces of the steel material on the premise that the other components will be described in addition to the above-mentioned components. It is necessary to allow the following ultrafine grain structure to exist from the surface layer to a thickness of 10 to 33% of the plate thickness.

By forming an ultrafine grain structure in the surface layer portion, a shear lip which is a ductile fracture is formed in the surface layer portion during the development of the brittle crack, and the brittle crack propagation stopping property is improved. This method is advantageous in that the brittle crack propagation stopping characteristics can be improved without adding or adjusting alloy components such as Ni addition.

In order to reliably generate a shear lip by resisting a brittle crack growing at a high speed, it is necessary to significantly improve the occurrence of brittle fracture in the surface layer and the propagation stoppage property to the required toughness of the steel sheet. Therefore, for that purpose, it is an essential condition that the ferrite grain size of the surface layer portion is remarkably reduced.

The ferrite grain size in the surface layer is preferably finer as a matter of course. However, in the present invention, the average ferrite grain in the surface layer is defined as a range in which the formation of the shear lip is sure and the manufacturing process is not overloaded. The diameter is limited to 3 μm or less.

The ferrite grain structure in the surface layer is preferably a grain size with less variation in crystal grain size.
Even if coarse particles having a size of more than twice the average particle size are present, if the existing ratio is within 10% with respect to the entire surface layer portion, there is substantially no adverse effect on the brittle fracture characteristics of the surface layer portion. Therefore, it is acceptable.

Even if brittle fracture occurs from a defective portion or a welded portion and propagates, the surface layer portion surely undergoes ductile fracture to form a shear lip, and the limitation of the ferrite grain size is an essential condition. However, in order to improve the propagation stopping property of brittle cracks, the thickness of the surface ultrafine grain layer is also an important requirement.

That is, in order to stop the brittle crack of the normal structure inside the steel sheet, it is necessary to absorb the propagation energy at the serial-up portion, but if the thickness of the shear lip is insufficient, the shear lip is formed. However, the brittle crack may not stop. The shear lip requires a certain thickness to reliably stop the propagation of the brittle crack.

Naturally, the larger the thickness of the shear lip, the greater the effect of stopping the cracks. However, if an attempt is made to secure an unnecessarily large thickness of the ultrafine grain layer, an excessive load is applied to the manufacturing process, Depending on the conditions, the ductility of the base material, the shape of the steel plate, the surface properties, etc. may be deteriorated.

In the present invention, the thickness of the superfine grain structure of the surface layer having an average ferrite grain size of 3 μm or less is defined as a range not causing these problems, the lower limit is from the surface layer to 10% of the plate thickness, and the upper limit is the surface layer. To 33% of the plate thickness.

The surface ultrafine grain layer is preferably applied to all the surfaces of the steel material, but if the above conditions are satisfied, brittle cracks can be stopped if the ultrafine grain layers are applied to at least two surfaces. Is effective for.

It should be noted that reduction of the amount of N, fixation of N by a nitride-forming element, and addition of an ultrafine grain layer to the surface layer, which are effective in securing toughness after plastic deformation, brittle crack propagation arresting property, and ductility, It was experimentally confirmed that it is effective to have an additional effect of improving the toughness and ductility of the welded joint.

That is, the solute N has a large adverse effect on the toughness of the heat-affected zone in high heat input welding, but if the N content and the solute N content are strictly controlled as in the present invention, the solid solution N is solid. The adverse effect of molten N on the toughness of the weld heat affected zone is minimized.

The ultra-fine grain layer formed on the surface layer completely disappears in the region of the heat affected zone of the welding which is reheated to 1300 ° C. or higher in the vicinity of the weld bead, but is heated to a lower temperature. Although the ultra-fine grain structure disappears in the heat-affected zone, the effect of the ultra-fine grain structure before transformation remains and has the effect of refining the structure of the weld heat-affected zone. It is also effective in improving toughness.

The above are the requirements for the high-strength steel material for construction which is excellent in fracture resistance of the present invention, but it is necessary to limit the individual chemical components for the reasons described below. That is, C is contained as an effective component for improving the strength of steel, and if it is less than 0.01%, it is difficult to secure the strength required for structural steel, but if it exceeds 0.15%, it is excessive. The deterioration of the ductile fracture characteristics leads to a reduction in the fracture resistance targeted by the present invention. Further, since toughness, weld cracking resistance and the like are also reduced, the range is set to 0.01 to 0.15%.

Next, Si is an element effective as a deoxidizing element and for securing the strength of the base material. However, if the content of Si is less than 0.01%, deoxidation is insufficient and it is disadvantageous for securing the strength. . On the contrary, if the content exceeds 1.0%, a coarse oxide is formed and ductility and toughness are deteriorated. Therefore, the range of Si is set to 0.01 to 1.0%.

Further, Mn is an element necessary for securing the strength and toughness of the base metal, and it is necessary to contain at least 0.1% or more, but within a range which is allowable in terms of material such as toughness and cracking of the welded portion. The upper limit was 2.0%.

Al is an element effective in fixing N, which is one of the requirements of the present invention, and an element effective in deoxidizing, refining the γ grain size, etc., but exerts an effect. Is 0.00
It is necessary to contain 3% or more. On the other hand, if it is contained in excess of 0.1%, a coarse oxide is formed and ductility is extremely deteriorated. Therefore, it is necessary to limit the content to 0.003% to 0.1%.

When solid solution N is present, the ductile fracture characteristics deteriorate and the toughness deteriorates after plastic deformation as described above. Therefore, according to the formula (1) or (2), Al, T
It is necessary to contain i, Zr, Nb, Ta, V, and B in appropriate amounts. However, it is necessary to limit the total content for the following reasons.

That is, N works effectively for γ grain refinement in combination with Al and Ti, so that a small amount works effectively for mechanical properties. Further, it is impossible to industrially completely remove N in steel, and it is not preferable to reduce N more than necessary because it puts an excessive load on the manufacturing process.

Therefore, the lower limit is 0.001% as a range in which industrial control is possible and the load on the manufacturing process is allowable.
And If it is contained excessively, even if the formula (1) or (2) is satisfied, the ductile fracture characteristics and the toughness after plastic deformation may be adversely affected depending on the manufacturing history. Therefore, the upper limit of the allowable range is 0. 0.005%.

As described above, since P deteriorates the ductility of the ferrite matrix, it is necessary to limit the content of P in order to improve the plastic deformability, the occurrence of ductile cracks, and the growth characteristics. The smaller the P content is, the more preferable. However, the reduction of the P content imposes a load on the refining process and lowers the productivity and raises the cost.
Is set to 0.01% or less based on experimental results.

That is, the ductility characteristic deteriorates as the amount of P increases, but when it exceeds 0.01, the degree becomes remarkable. When the amount of P is 0.01% or less, the degree of the adverse effect of P decreases. Therefore, in the present invention, the amount of P as an impurity is limited to 0.01% or less.

However, when used in a structure in which local plastic deformation in the segregation portion or deterioration of ductile fracture characteristics has an effect, the P content is 0.
It is more preferable to limit the content to 007% or less.

As for S, as described above, MnS is formed, so that the ductile fracture characteristic is deteriorated. In particular, it deteriorates the propagation characteristics of ductile cracks. Under the condition of a large amount of solute P and N, the ductile fracture initiation characteristic is deteriorated. Therefore, the deterioration of ductile crack propagation characteristic by S greatly affects the plastic deformation ability and ductile fracture characteristic of the entire steel material. , S need to be extremely reduced to about 0.001% or less.

However, if the amount of P and N is reduced and the solid solution N is fixed by a nitride forming element as in the present invention,
Since the resistance until the occurrence of ductile fracture becomes large, the allowable amount of S widens, so in the present invention, S as an impurity is limited to 0.01% or less based on the experimental results.

As for O, as described above,
Is preferably reduced as much as possible in order to form inclusions harmful to ductility. However, similar to S, if the solid solution P and N are reduced, the ductility of the base is secured to some extent. N
The permissible amount is higher than that in the case where no reduction is achieved, and based on the experimental results, it is necessary to limit the amount to 0.006% or less.

Ti, Zr, Nb, Ta, V, and B are mainly used for N-fixing, and optionally one or more of them are selectively contained, but the individual elements are also described for the following reasons. It is necessary to limit the amount of the components.

Ti is an element effective in fixing N, and contributes to the improvement of the strength of the base material by precipitation strengthening.
Although it is an element effective for the refinement of γ grains by forming TiN, it is necessary to contain 0.003% or more in order to exhibit the effect. On the other hand, if it exceeds 0.02%, coarse precipitates and inclusions are formed to deteriorate toughness and ductility, so the upper limit is made 0.02%.

Zr is also an element that forms a nitride and is effective for fixing N. However, in order to exert its effect, Zr.
It is necessary to contain 003% or more. On the other hand, if it exceeds 0.10%, as with Ti, coarse precipitates and inclusions are formed to deteriorate toughness and ductility, so 0.003 to 0.10.
%.

Nb is also an element effective in fixing N, but if contained in excess, the toughness deteriorates due to precipitation embrittlement. Therefore,
As a range where the effect can be exhibited without causing deterioration of toughness,
It is limited to the range of 0.002 to 0.05%.

Ta is also an element effective in fixing N, but in order to exert the effect, it is necessary to contain 0.005% or more. On the other hand, if it exceeds 0.02%, precipitation embrittlement, coarse precipitates and toughness deterioration due to inclusions occur, so the upper limit is made 0.20%.

V is also an element effective in forming VN and fixing N. However, like Nb, if contained in excess, the toughness deteriorates due to precipitation embrittlement. Therefore, the range in which the effect can be exhibited without causing deterioration of toughness is limited to the range of 0.005 to 0.20%.

Since B is a trace amount and is surely bound to N, it is an element effective for N fixing, and 0.0 is necessary for exerting the effect.
002% or more is required. On the other hand, if the content exceeds 0.003% and is excessive, the BN becomes coarse, and the ductility and toughness are adversely affected. In addition, the upper limit is set to 0.
003%.

In addition to the above, in order to increase the strength of the base metal and to secure the toughness in accordance with the desired strength level, C is added as necessary.
One, two or more of r, Ni, Mo, Cu, and W can be contained. First, Cr and Mo are both effective elements for improving the strength of the base material, but 0.01% or more is necessary for producing a clear effect, while if added in excess of 2.0%, Since the toughness and weldability tend to deteriorate, the range is 0.01 to 2.0%.

Ni is a very effective element because it can improve the strength and toughness of the base material at the same time, but it is necessary to contain Ni in an amount of 0.01% or more in order to exert the effect. When the content is increased, the strength and toughness are improved. However, if the content exceeds 4.0%, the effect is saturated, but the weldability is deteriorated. Therefore, the upper limit is set to 4.0%.

Next, Cu has almost the same effect as Ni, but if it exceeds 2.0%, a problem occurs in hot workability.
The range is limited to 0.01 to 2.0%. W is effective for increasing the strength of the base material by solid solution strengthening and precipitation strengthening, but is required to be 0.01% or more in order to exhibit the effect. on the other hand,
If it is contained in excess of 2.0%, the toughness is significantly deteriorated, so the upper limit is made 2.0%.

Further, in order to improve ductility and joint toughness, one or more of Mg, Ca and REM may be contained, if necessary. Mg, Ca, REM
Are effective in improving ductility by suppressing the elongation of sulfide during hot rolling. It also works effectively to improve joint toughness by refining the oxide. The lower limit contents for exhibiting the effect are 0.0005% for Mg and Ca and 0.005% for REM. On the other hand, if it is contained excessively, coarsening of sulfides and oxides occurs, leading to deterioration of ductility and toughness, so the upper limits are set to 0.01% for Mg and Ca, and 0.10% for REM.

Next, the reasons for limitation in the production of the high-strength steel sheet for construction excellent in fracture resistance of the present invention will be described. In the steel having a limited chemical composition for the above reasons, in order to improve the brittle crack propagation arresting property, in the surface layer portions of at least two surfaces of the steel material, an ultrafine grain structure having an average ferrite grain size of 3 μm or less is formed from the surface layer. It must be present over a thickness of 10-33% of the plate thickness. The surface ultrafine grain layer having the characteristics limited in the present invention can be formed by limiting the manufacturing conditions as described below.

During hot rolling of a steel slab, a region of an appropriate thickness of the surface layer portion is once cooled to a temperature lower than the Ar ₃ transformation point during hot rolling or during hot rolling by means such as water cooling. After making a temperature difference from the inside, when hot rolling is further performed from the state where the temperature difference remains, the region once cooled to a temperature lower than the Ar ₃ transformation point is reheated and processed in that process. Since it has a ferrite main structure, the surface layer part having the ferrite main structure undergoes processing while being reheated by sensible heat inside, and by optimizing the processing conditions during this reheating, the ferrite crystal of the surface layer part The grains are noticeably finer.

Therefore, the ratio of the surface ultrafine grain layer in the final steel material is almost the same as the ratio of the region where the surface layer is cooled to the Ar ₃ transformation point when the surface layer is once cooled.

In the hot rolling step, ultrafine graining is achieved by satisfying the following conditions. First, the steel slab is reheated to the austenite region. The temperature in this case is preferably from the Ac ₃ transformation point to 1250 ° C. That is, when the Ac ₃ transformation point is less than the austenite single phase, the ferrite phase remains, and when the ferrite phase remains, a uniform ultrafine grain structure cannot be formed on the surface layer regardless of the subsequent steps. .

On the other hand, if the temperature exceeds 1250 ° C., the grain size of the heated austenite becomes extremely coarse, so that the grain size cannot be refined even by the subsequent rolling, and the toughness at the central portion of the sheet thickness cannot be secured. Therefore, in the present invention, the heating temperature of the steel slab is limited to the Ac ₃ transformation point to 1250 ° C.

After the steel slab is heated, it is rolled in the austenite region at 950 ° C. or lower at a cumulative reduction of 10 to 50%. This substantially refines the austenite grain size before transformation,
This is to make the surface layer an ultrafine grain structure in a later step and to secure the toughness of the internal normal structure. Note that the substantial refinement of austenite grains refers to the refinement of recrystallized austenite as well as the expansion of austenite grains by non-recrystallization rolling.

The reduction in the austenite region at a low temperature is effective in substantially refining austenite, but the reduction above 950 ° C. is not effective in refining austenite. Therefore, in the present invention, reduction at a temperature of 950 ° C. or lower is used. Limit the rolling reduction. If the rolling reduction at 950 ° C. or less is less than 10%, the effect of processing is insufficient, and therefore there is no effect on the refinement of austenite.

The larger the rolling reduction in the austenite region of 950 ° C. or lower, the more effective the refinement of austenite is, but the effect is that if it exceeds 50%, the tendency tends to be saturated, and that the rolling reduction exceeds 50%. When it becomes large, it is effective for austenite grain refinement, but it becomes impossible to secure the reduction rate in the recuperation process, which is essential for later grain refinement of ferrite. The upper limit of the rolling reduction is 50%.

Note that the reduction at a temperature higher than 950 ° C. has a small effect on the refinement of austenite, but does not adversely affect the material as long as the required reduction rate in the subsequent reheating step can be secured. Therefore, when the initial slab thickness is large, the processing may be performed at a temperature higher than 950 ° C. as necessary.

Under the above conditions, the austenite grains were sufficiently refined and unrecrystallized region rolling was performed, and then the surface layer portion of the steel material to be an ultrafine grain layer was cooled by means of water cooling or the like, and the steel material was water-cooled. The area of each surface layer portion corresponding to 10 to 33% of the sheet thickness at the time of the previous hot rolling is cooled to the Ar ₃ transformation point or lower, and a temperature difference is made between the surface layer portion and the inside. The cooling rate in the region of each surface layer portion corresponding to 10 to 33% of the plate thickness at the time of hot rolling before water cooling of is required to be 2 ° C./s or more.

When the cooling rate is less than 2 ° C./s, hot rolling before cooling causes coarse transformation structure after cooling even if austenite is refined. This is because it becomes difficult to obtain a ferrite structure. A higher cooling rate is preferable from the viewpoint of micronization of the structure, but the effect is saturated even if it is rapidly cooled above 40 ° C / s, and unnecessary cooling is preferable for maintaining the shape of the steel sheet. Therefore, the upper limit is 40 ° C./s.

The cooling after the rolling at 950 ° C. or lower is started from the Ar ₃ transformation point or higher. this is,
This is because a surface layer of ultrafine grains is formed uniformly by cooling from single-phase austenite. That is, if the surface layer portion becomes lower than the Ar ₃ transformation point before forced cooling, ferrite is partially coarsely formed, and ultrafine graining in that portion is hindered.

For the above reasons, the area of each surface layer portion corresponding to 10 to 33% of the plate thickness at the time of hot rolling before cooling the steel material is Ar at a cooling rate of 2 ° C./s to 40 ° C./s. _When cooling to ₃ transformation points or less and then performing finish rolling, by using sensible heat inside and / or utilizing heating from outside,
By rolling the region of each surface layer portion corresponding to 10 to 33% of the plate thickness during temperature increase, the structure of the region becomes ultrafine,
It becomes possible to contribute to the improvement of brittle crack propagation stopping property.

As will be described later, the processing in the recuperation process is 1
Although it may be repeated once or twice or more, the recuperation temperature after rolling in the recuperation process after the last cooling is (Ac ₁ transformation point −5
It is necessary to be in the range of (0 ° C.) to (Ac ₃ transformation point + 50 ° C.).

That is, the final recuperation temperature is (Ac ₁ transformation point-
If it is lower than 50 ° C., the recovery and recrystallization of the worked ferrite after working is not sufficient, so that the ultrafine graining is insufficient and the brittle crack propagation arresting property is not improved.

On the other hand, the final recuperation temperature is (Ac ₁ transformation point +
If the temperature is higher than 50 ° C, a part of the ferrite that has been made into a fine grain by processing will be lost by retransforming to austenite again, and the ratio will increase to a non-negligible toughness and brittle crack propagation arresting property. Spoil. Therefore, in the present invention, the recuperation temperature after rolling in the recuperation process after the last cooling is (Ac ₁ transformation point −50 ° C.) to (Ac ₃
It is limited to the range of transformation point + 50 ° C).

The above-described processing steps during cooling to below the Ar ₃ transformation point and during the reheating may be carried out once, but the effect is superposed by repeating it a plurality of times, so the desired fine structure can be obtained even if it is repeated two or more times. Is possible.

In this case, even if the maximum temperature or the minimum temperature of each recuperation stage is arbitrary, according to the temperature condition of the present invention,
Make it ultra-fine. However, it is preferable that the upper limit of the recuperation temperature in the middle is (Ac ₃ transformation point + 100 ° C.) or less, because the effect of grain refining is surely superimposed.

If the cumulative rolling reduction of the finish rolling as the rolling from the first cooling to the final recuperation is large, an ultrafine grain structure can be uniformly and stably obtained. For that purpose, the cumulative rolling reduction of the finish rolling needs to be at least 20%. The larger the rolling reduction, the more advantageous it is for ultrafine graining, but the rolling reduction is 90
%, The effect is saturated and productivity is extremely hindered, which is not preferable. Therefore, in the present invention, the cumulative rolling reduction of the finish rolling is limited to 20 to 90%.

The above are the requirements concerning the manufacturing method of the present invention, and a desired microstructure can be obtained at the stage when rolling and reheating are completed. Cooling after the final recuperation can obtain the target brittle crack propagation stopping characteristics and toughness without any means such as cooling by cooling or forced cooling, but depending on the application such as improved strength, after the end of recuperation, 2 at a cooling rate of 5-40 ° C / s
20 to 650 ° C after cooling to 0 to 650 ° C or after allowing the steel plate after the recuperation to cool or at a cooling rate of 5 to 40 ° C / s
After cooling down to 450 ° C. to 650 ° C., tempering can be performed.

In the case of forced cooling after the final recuperation, if the cooling rate is less than 5 ° C./s, the effect of material improvement due to forced cooling is not remarkable, and if it exceeds 40 ° C./s, the effect is saturated and the shape of the steel sheet is impaired. There is a fear. Further, if the stop temperature of the forced cooling is 20 ° C. or less, the material improvement due to the cooling stop cannot be expected, and it takes time for cooling, which is not preferable, and if the cooling stop temperature exceeds 650 ° C., transformation may not be completed yet,
There is a risk of extreme reduction in strength and deterioration of toughness.

If the tempering temperature is lower than 450 ° C., tempering of the hardened phase and release of residual stress will be insufficient, and there is a high possibility of deterioration of toughness and shape. On the other hand, if the tempering temperature exceeds 650 ° C, there is a concern that ultrafine grained ferrite formed in the surface layer may grow to form a mixed grain structure, so the tempering temperature after forced cooling is limited to 450 ° C to 650 ° C.

[0096]

EXAMPLES Using test steels having the chemical compositions shown in Table 1, Table 2
For a thick steel plate having a plate thickness of 25 mm or 50 mm manufactured under the manufacturing conditions shown in 1), the strength of the base metal as manufactured and after 10% strain and the toughness by the Charpy test (fracture transition temperature vTrs), the brittleness by the ESSO test crack propagation stop characteristics (the temperature at which Kca value is 400 kgf · mm ^-3/2), limit CTOD value ductile fracture ([delta] _i), and welded joint properties (Charpy properties of the remains welded (at -20 ° C. Table 3 shows the average value of absorbed energy), as-welded and δ _i ) after applying 10% strain.

[0097]

[Table 1]

[0098]

[Table 2]

[0099]

[Table 3]

[0100]

[Table 4]

[0101]

[Table 5]

[0102]

[Table 6]

[0103]

[Table 7]

[0104]

[Table 8]

[0105]

[Table 9]

[0106]

[Table 10]

[0107]

[Table 11]

[0108]

[Table 12]

The tensile properties of the base material were measured by using a round bar test piece having a parallel part diameter of 6 mm and a distance between scores of 25 mm, which was taken from t / 4 part of the plate thickness so that the test direction was perpendicular to the rolling direction. . The Charpy impact properties of the base material were also sampled at the same position and method as in the tensile test, and the fracture surface transition temperature (vTrs) was determined. The critical CTOD value (δ _i ) for the occurrence of ductile fracture was measured using a fatigue notched three-point bending test piece that was taken from the center of the plate thickness so that the longitudinal direction of the test piece was perpendicular to the rolling direction.

Regarding the welding conditions, for a steel plate having a plate thickness of 25 mm, single-sided single layer submerged arc welding was used, and for a steel plate having a plate thickness of 50 mm, double-sided single layer welding was performed. In each case, the welding heat input was adjusted so as to fall within the range of about 190 to 200 kJ / cm and welding was performed.

The 2 mm V notch Charpy impact test piece and the test piece for measuring δ _{i of the} joint were set so that the position of 7 mm below the surface was the center of the test piece, and the weld metal and the HAZ were separated from each other (fusion part: FL). It was sampled so that the position 1 mm on the HAZ side would be the notch position. The tensile properties and the measurements of the base material and the joint δ _i were all performed at room temperature.

As shown in Table 1, since the steel sheets of steel numbers A1 to A16 have the chemical composition and the superfine grain structure of the surface layer within the scope of the present invention, they are an index of the propagation stopping property of brittle cracks, ES.
Kca value obtained by SO test is 400 kgf · mm
^-3/2 temperature is not only very good, but also 10
%, The deterioration is very small even after a large strain is applied. In addition to the elongation required in the usual round bar tensile test, the ductile fracture occurrence characteristic in the presence of cracks is also δ _i regardless of the presence or absence of strain. The welded joint has the Charpy characteristics that are required to be safely used for structures such as construction and bridges, and maintains the δ _{i of} the joint even after strain is applied, like the base metal. A sufficiently high value is obtained.

That is, the steel material produced according to the present invention has an unprecedentedly high level of safety even when used in a structure which undergoes large and repeated plastic strain due to a large earthquake during use. It is clear.

On the other hand, steel numbers B1 to B10 are comparative examples,
Any of the properties shown in Table 3 are inferior to the steels of the present invention because they do not meet the requirements of the present invention. That is,
Since steel number B1 has an excessive amount of all N, ESS before strain application
O characteristics are also inferior to those of the steel of the present invention.
SSO characteristics and δ _i are remarkably inferior.

Steel No. B2 has an excessive amount of total N and an insufficient amount of Al and the like for fixing the solid solution N. Therefore, the brittle crack propagation arresting property after straining and the value of δ _i are It is deteriorated compared to the invention steel. Steel No. B3 is within the chemical composition range of the present invention as the total amount of N, but because fixation of N is insufficient, that is,
Since the value of the equation (1) is a positive value, the material deterioration due to the strain application is large.

Steel No. B4 is Al most effective for fixing solid solution N.
Is insufficient, the N fixation is not sufficient,
Degradation of ESSO characteristics and δ _i due to strain application is remarkable. In steel number B5, since P is excessive, ductile fracture characteristics and ESSO characteristics are low even before strain is applied, and further, ductile fracture characteristics and ESSO characteristics after strain are significantly reduced.

Since the steel No. B6 has an excessive amount of S, the ductility characteristics (drawing value, δ _i ) are significantly inferior to those of the steel of the present invention both before and after applying strain. Steel No. B7 has excessive C, so
The ductile fracture characteristics and ESSO characteristics before and after applying strain are relatively low, and the toughness of the welded joint is remarkably poor.

Steel No. B8 is within the range of the steel of the present invention as a chemical composition, but since it does not have a superfine grain structure in the surface layer portion, the ESSO characteristics are remarkably deteriorated before and after strain application. ing. Steel No. B9 has a fine-grained structure in the surface layer portion as compared with the central portion, but its grain size does not satisfy the requirements of the present invention and is coarse, so that sufficient brittle crack propagation arresting characteristics are obtained. Cannot be obtained before and after applying strain.

Steel No. B10 does not have sufficient brittle crack propagation arresting properties before and after strain application because the surface ultrafine grain layer has an insufficient thickness. Steel No. B11 has an excessive amount of O, so
In particular, ductile fracture characteristics are poor. Steel No. B12 has an excessive amount of S and O, both of which are harmful to the ductility characteristics, so that the elongation and δ _i are significantly inferior even before plastic deformation.

From the above examples, according to the present invention, the Charpy characteristics are obtained not only before applying the prestrain but also after applying the prestrain of 10% assuming a large deformation such as a large earthquake. It is clear that it is possible to obtain a steel material with very good brittle crack propagation arresting properties and ductile fracture properties (drawing value, δ _i ).

[0121]

INDUSTRIAL APPLICABILITY The present invention has very little deterioration of the material due to plastic strain even when it is subjected to large and repeated plastic strain due to a large earthquake during use, and easily causes brittle cracks even after plastic deformation. Suppresses the initiation and propagation of ductile cracks
And, even if a fracture should occur, it is possible to stop the brittle crack in a very safe steel material for structures without using a special alloy component in the normal steel material manufacturing process, Its industrial effect is extremely large.

Claims (7)

[Claims] 1. By weight%, C: 0.01 to 0.15%, Si: 0.01 to 1.0%, Mn: 0.1 to 2.0%, Al: 0.003 to 0. 1%, N: 0.001 to 0.005% is contained, and the content of P, S, O as impurities is P: 0.01% or less, S: 0.01% or less, O: 0.006. % Or less and N (%)-Al (%) / 3.0 ≦ 0, which is a steel material composed of balance iron and unavoidable impurities, and at least two outer surfaces of the outer surfaces constituting the steel material A high-strength steel material for construction having excellent fracture resistance, characterized by having an ultrafine grain structure having an average ferrite grain size of 3 μm or less within a range of 10 to 33% of the total thickness from the surface layer. 2. Ti: 0.003 to 0.020%, Zr: 0.003 to 0.10%, Nb: 0.002 to 0.050%, Ta: 0.005 to 0. 20%, V: 0.005 to 0.20%, B: 0.0002 to 0.003%, one or more kinds are contained, and N (%)-Al (%) / 3.0-Ti. (%) / 3.4-
Zr (%) / 6.5-Nb (%) / 13.2-Ta
(%) / 25.8-V (%) / 10.9-B (%) /
2.0 ≦ 0, The high-strength steel material for construction having excellent fracture resistance according to claim 1. 3. By weight%, Cr: 0.01-2.0%, Mo: 0.01-2.0%, Ni: 0.01-4.0%, Cu: 0.01-2. 0%, W: 0.01-2.0% 1 type (s) or 2 or more types are contained, The high tensile steel material for construction excellent in the fracture resistance of Claim 1 or 2 characterized by the above-mentioned. 4. One or more of Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and REM: 0.005 to 0.10% by weight. The high-strength steel material for construction according to any one of claims 1 to 3, which is excellent in fracture resistance. 5. A steel slab is heated to a temperature not lower than the Ac ₃ transformation point and not higher than 1250 ° C., and rough rolling is performed at a temperature of 950 ° C. or lower so that the cumulative rolling reduction in the austenite region is 10 to 50%. Of at least two outer surface layer portions corresponding to 10% to 33% of the total thickness of the steel billet in the above step, cooling is started at a cooling rate of 2 to 40 ° C./s from the temperature of the Ar ₃ transformation point or higher, In the process of stopping the cooling at the Ar ₃ transformation point or lower and returning the heat through one or more times, finish rolling with a cumulative rolling reduction of 20 to 90% until the end of the recuperation after the last cooling. The surface layer of the steel material after completion of the rolling (A
c ₁ transformation point -50 ° C.) ~ (excellent fracture properties, characterized by producing the steel material according to any one Ac ₃ by recuperation in the range of transformation point + 50 ° C.) of claim 1 Manufacturing method of high strength steel for construction. 6. The high tensile strength for construction according to claim 5, wherein the steel material after the end of the recuperating heat is cooled to 20 ° C. to 650 ° C. at a cooling rate of 5 to 40 ° C./s. Steel material manufacturing method. 7. The method for producing a high-strength steel material for construction having excellent fracture resistance according to claim 5, wherein tempering is performed at 450 ° C. to 650 ° C.