JPH1017981A - High tensile strength steel material with low yield ratio for construction use, excellent in brittle crack arrest property, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel material excellent in brittle crack arrest property by dispersing martensitic phases and refining ferritic grain size in the surface layer by subjecting a carbon steel of specific composition to hot rolling and to heat treatment under respectively specified temp. conditions. SOLUTION: A steel bloom, having a composition containing, by weight, 0.01-0.20% C, 0.01-1.0% Si, 0.1-2.0% Mn, 0.001-0.1% Al, 0.001-0.01% N, <0.025% P, and <0.015% S, is heated to a temp. in the range between the Ac transformation point and 1250 deg.C and hot-rolled, and, in the course of the above process, cooling and recuperation are repeated under specific temp. conditions. By this procedure, the distribution of martensitic structure in the resultant rolled steel stock is regulated to 10-60%, and the structure in a specific range of 10-33% from the surface layer in a thickness direction in the surface layer part of the steel stock is formed into superfine-grained structure of <=3μm average ferrite grain size. By this method, the high tensile strength steel material with low yield ratio, having brittle crack arrest property required of a material for earthquake-proof building construction, can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-yield-ratio high-tensile steel mainly used for a building structure, and more particularly, to a brittle fracture before an entire structure collapses even if a brittle fracture occurs by an earthquake or the like. The present invention relates to a safe steel material capable of stopping a crack and a method for manufacturing the same.

[0002] The steel material of the present invention is used not only for earthquake-resistant building applications, but also for steel materials manufactured by this method, for example, for offshore structures, pressure vessels,
It can be used in general for welded steel structures such as shipbuilding, bridges and line pipes. Although the form of the steel material is not particularly limited, it is particularly useful for a steel plate used as a structural member and requiring low-temperature toughness, particularly a thick plate, a steel pipe material, or a shaped steel.

[0003]

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 it is intended to ensure the safety of a structure by the performance of steel without special consideration in design and construction, steel with high safety in both ductile fracture and brittle fracture is necessary 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. It has been recognized that low yield ratio characteristics are excellent in the ability to absorb energy due to earthquakes, typhoons, and the like, and are useful in suppressing local collapse of structures.

[0005] A number of means for lowering the yield ratio for improving the energy absorption capacity have been proposed. For example, a method of adjusting the chemical composition such as an increase in the amount of C, a method of coarsening crystal grains, and a method of performing an intermediate heat treatment in which a ferrite (α) + austenite (γ) two-phase region is heated between quenching and tempering heat treatment. (Hereinafter referred to as QLT treatment), there is a method of mixing α as a soft phase and bainite or martensite as a hard phase.

For example, as a production method for obtaining a mixed structure of a soft phase and a hard phase, Japanese Patent Application Laid-Open No. 53-23817 discloses a method in which a steel sheet is reheated and quenched, and then the Ac ₁ transformation point and Ac ₃
A method of reheating between the transformation points to form two phases of γ and α and then air cooling is disclosed.
No. 4 discloses a method of similarly reheating to a two-phase region and then quenching. As a method for online production without performing reheating treatment, for example, Japanese Unexamined Patent Publication No.
No. 86517 discloses a method in which after hot rolling is performed from the γ region to the two-phase region, air cooling is performed to a temperature 20 to 100 ° C. lower than the Ar ₃ transformation point to generate an α phase, and then quenching is performed. I have.

As a method for improving uniform elongation, for example, as disclosed in Japanese Patent Application Laid-Open No.
o-containing steel is reheated to the α-γ dual phase coexisting
There is a method of improving the uniform elongation in the rolling direction by adding the rolling after extremely lowering the concentration.

[0008] According to this method, a soft α-phase can be produced without relatively impairing the productivity. However, since the heating temperature is extremely low as compared with the normal billet heating temperature, the operation may be dependent on the heating furnace. It may be difficult, and it may take time for uniform heating, or the solution may be insufficient. Further, it is considered that the anisotropy of the material easily occurs.

[0009] Alternatively, a method of improving uniform elongation by aging precipitation strengthening as disclosed in Japanese Patent Application Laid-Open No. 4-74846 is disclosed in an ultra-high strength steel having a yield strength of 95 kgf / mm ² or more. It is hardly a general measure for steel having a strength of 80 kgf / mm ² or less.

[0010]

The high ductility steel typified by the low YR steel and the high uniform elongation steel, if destroyed by ductile fracture up to the ultimate collapse of the structure, will not increase the safety of the structure. It is very useful in enhancing

However, in general, since the members of a structure are joined to each other by welding, the joint always has a welded joint, and the jointed joint has an initial defect such as a welding defect and a weld metal, a weld heat affected zone (HAZ), or the like. Inevitably includes a material-deteriorated portion, and thus brittle fracture easily occurs when external stress is applied. Once brittle fracture has occurred, it does not make sense to use high-ductility steel because brittle fracture rapidly develops.

It is impossible to completely eliminate the initial failure. Further, in order to completely suppress the occurrence of brittle fracture in the presence of initial defects, it is necessary to use an expensive steel with an extremely high toughness of the HAZ, and even if such an expensive steel is used. If the initial defects are large, the possibility of brittle fracture remains. Therefore, even when an external force under extremely severe conditions is applied, such as during a large earthquake, it is considered that ensuring the safety of the structure is an important task imposed on steel materials.

[0013]

According to the present invention, a low Y
As a means for preventing the collapse of a structure due to brittle fracture while ensuring ductility by R-izing, by increasing the characteristic of stopping the propagation of brittle fracture of the base material by controlling the microstructure in the manufacturing process of steel, We thought that safety could be ensured. In other words, it is very difficult to completely suppress the occurrence of brittle fracture with respect to any initial defects and welding conditions, but after allowing the occurrence of brittle fracture, the generated brittle cracks If brittle cracks can be stopped early in the propagation stage, it will be a reliable measure that does not depend on the condition of the welded joint.

[0014] From the above viewpoint, low Y for ductile fracture
It is necessary to improve energy absorption by bridging and improve brittle crack propagation arrest characteristics by bridging.
The present invention was considered to be a necessary property as a high-strength steel material for construction with higher safety, and as a result of examining means capable of simultaneously achieving both properties, the present invention was completed.

The point is to properly disperse the hard martensite phase in the steel structure to reduce the YR, and to control the brittle crack propagation arresting property without adjusting the steel composition. In the production process, an ultrafine grain structure is formed on the surface layer portion, and a martensite phase is dispersed without impairing the ultrafine grain structure on the surface layer portion.

More specifically, the gist of the present invention is as follows. (1) By weight%, C: 0.01 to 0.20%, Si:
0.01-1.0%, Mn: 0.1-2.0%, Al:
0.001 to 0.1%, N: 0.001 to 0.010%
And the content of P and S as impurities is P: 0.
025% or less, S: 0.015% or less, a steel material comprising the balance iron and unavoidable impurities, the martensite ratio in the steel material volume is 10 to 60%, and the outer surface of the steel material A building excellent in brittle crack propagation arrestability characterized in that at least two outer surfaces have 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. For high yield steel with low yield ratio.

(2) Ti: 0.003-0.
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
The low yield ratio high tensile strength steel for architectural use having excellent brittle crack propagation arrestability according to the above (1), characterized in that it contains 0.003% or more of one or more kinds.

(3) Cr: 0.01 to 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.
0%, one or more of the following: (1) or (2), a low yield ratio high tensile strength steel material for construction having excellent brittle crack propagation arrestability.

(4) Mg: 0.0005 to 5% by weight
0.01%, Ca: 0.0005 to 0.01%, RE
M: 0.005 to 0.10%, one or more of which are contained, the brittle crack propagation arresting characteristics according to any one of the above (1) to (3), Excellent high yield strength steel with low yield ratio for construction.

(5) The steel slab having the composition described in any of (1) to (4) above is heated to a temperature of from the Ac ₃ transformation point to 1250 ° C. before starting hot rolling or hot rolling. In the middle stage, at least two outer surface regions corresponding to 10 to 33% of the thickness of the slab at that stage are cooled at a cooling rate of 2 to 40 ° C./s from a temperature not lower than the Ar ₃ transformation point. In the process of starting and stopping the cooling at the Ar ₃ transformation point or lower and reheating once or more, the cumulative rolling reduction is 20 to 90% until the reheating after the last cooling is completed. After the finish rolling is completed, the surface area of the steel material after the completion of the rolling is changed from (Ac ₁ transformation point—50 ° C.) to (Ac ₃ transformation point + 50 ° C.)
The steel material after the completion of the reheating is 0.2 ~
After cooling at a cooling rate of 2 ° C./s (transformation start temperature (Ar ₃ ) at the cooling rate−50 ° C.) to 500 ° C.,
Brittle crack propagation arresting characteristic characterized in that the steel material according to any one of (1) to (4) above is cooled at a cooling rate of 5 to 40 ° C./s to 20 to 300 ° C. Method of producing high yield strength low tensile strength steel for construction.

(6) The steel slab having the composition described in any of (1) to (4) above is heated to a temperature of not less than the Ar ₃ transformation point and not more than 1250 ° C., before starting hot rolling or hot rolling. In the middle stage, at least two outer surface regions corresponding to 10 to 33% of the thickness of the slab at that stage are cooled at a cooling rate of 2 to 40 ° C./s from a temperature not lower than the Ar ₃ transformation point. In the process of starting and stopping the cooling at the Ar ₃ transformation point or lower and reheating once or more, the cumulative rolling reduction is 20 to 90% until the reheating after the last cooling is completed. Finish rolling is completed, and the surface layer of the steel material after the completion of the rolling is (A
c ₁ transformation point -50 ° C.) ~ (by recuperation in the range of Ac ₃ transformation point + 50 ° C.), the steel post or recuperator ends to cool the steel material after recuperation ends 5 to 40 ° C. / s 20 ~
After cooling to 650 ° C., the temperature was further increased at a rate of 0.1 to 50 ° C./s from (Ac ₁ transformation point to 10 ° C.) to (Ac ₃ transformation point−).
30 ° C.), and then 1-60 s in the temperature range.
After holding, the steel material according to any one of the above (1) to (4) is subjected to a two-phase region heat treatment of cooling at 0.5 to 50 ° C./s to produce a brittle metal. A method for producing a low yield ratio, high tensile strength steel material for buildings with excellent crack propagation arrestability.

(7) The method according to (5) or (6), wherein the tempering is carried out at 450 to 650 ° C., wherein the high yield strength steel for building has a low yield ratio excellent in brittle crack propagation arrestability. In addition, the high-tensile steel material here refers to a steel material including not only a high-tensile steel plate (thick plate) but also a shaped steel and a tube material.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The first requirement of the present invention is to form an ultrafine grain structure in the surface layer in the manufacturing process without depending on the steel composition for brittle crack propagation arresting characteristics. In order to reduce the yield ratio, it is to appropriately disperse the hard martensite phase in the steel structure, and to disperse the martensite phase without damaging the ultrafine grain structure of the surface layer. .

In order to improve the characteristics of stopping the propagation of brittle cracks,
On the premise of limiting the components described later, the average ferrite grain size is 3 μm in the surface layer of at least two surfaces of the steel material.
It is necessary to have an ultrafine grain structure of m or less 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, a shear lip, which is ductile fracture, is formed in the surface layer during the development of the brittle crack, and the brittle crack propagation stopping characteristics are 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 that is growing at a high speed, it is necessary to significantly improve the brittle fracture occurrence and propagation stopping characteristics of the surface layer over the required toughness of the steel sheet. For that purpose, it is essential to remarkably reduce the ferrite grain size in the surface layer.

The finer the ferrite grain size of the surface layer is, of course, the better, but in the present invention, the average ferrite grain size of the surface layer is within a range in which the formation of the shear lip is reliable and an excessive load is not applied to the manufacturing process. The diameter is limited to 3 μm or less. It is preferable that the ferrite grain structure of the surface layer has a uniform grain size with a small variation in crystal grain size. On the other hand, if it is within 10%, it does not substantially affect the brittle fracture characteristics of the surface layer portion, so that it is acceptable.

Even if brittle fracture occurs from a defective portion or a welded portion and propagates, in order for the surface layer portion to be surely ductilely fractured to form a shear lip, it is essential to limit the ferrite grain size. However, the thickness of the surface ultrafine grain layer is also an important requirement for improving the property of stopping the propagation of brittle cracks.

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 shear lip portion. However, if the shear lip is insufficient in thickness, the shear lip may be formed. Even in some cases, brittle cracks do not stop. In order to reliably stop the propagation of a brittle crack, the shear lip needs to have a certain thickness.

Naturally, the greater the thickness of the shear lip, the greater the effect of stopping the cracks. However, if the thickness of the ultrafine grain layer is increased more than necessary, an excessive load is applied to the manufacturing process, Depending on the conditions, it may lead to deterioration of the ductility of the base material, the shape of the steel sheet, the surface properties, and the like.

In order to avoid these problems, in the present invention, the thickness of the surface ultrafine grain structure having an average ferrite grain size of 3 μm or less is determined for each of the front and back surfaces, the lower limit is 10% of the plate thickness from the surface layer, and the upper limit is the surface layer. Is limited to 33% of the maximum thickness. It is preferable that the superfine grain layer is applied to all surfaces of the steel material. However, if the above conditions are satisfied, it is effective to stop brittle cracks by applying the superfine grain layer to at least two surfaces. It is.

The ultrafine-grained layer formed on the surface layer disappears completely in a region where the temperature is reheated to 1300 ° C. or more immediately near the weld bead in the heat affected zone, but is heated to a lower temperature. In the heat-affected zone, the ultra-fine grain structure disappears, but 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 for

As described above, it is important to limit the chemical composition and improve the fineness of the surface layer of steel to improve the arrestability of brittle crack propagation. In order to achieve this, it is necessary to reduce the yield ratio of steel as a premise.

In a steel in which a superfine grain structure is formed in the surface layer to improve the property of stopping the propagation of brittle cracks, a means for obtaining a low yield ratio characteristic is to use an appropriate amount of martensite phase in the structure. Most preferably, a means for dispersing the compound in the tissue is used.
That is, various means for obtaining low yield ratio characteristics can be considered, but when a superfine grain structure is present in the surface layer, the deterioration of toughness is suppressed even when a brittle hard phase such as martensite is dispersed. It is most suitable when using a low yield ratio by dispersing martensite, which has relatively few restrictions on steel components.

Other means for lowering the yield ratio, for example, C, Cr,
An increase in the second phase due to the addition of Mo or the like causes an increase in alloy cost, and there is a concern that it may have an adverse effect on weldability and the like. In the method for reducing the yield ratio, deterioration of the internal toughness is inevitable.

The microstructure requirement for reducing the yield ratio without deteriorating the ductility characteristics is that the ratio of the martensite phase, which is a hard phase, to the steel material volume is 10 to 60%. That is, in order to lower the yield ratio, a means for dispersing a second phase having a sufficiently higher strength in the matrix than in the matrix to increase the tensile strength and reduce the yield ratio (yield stress / tensile strength). Is the most effective.

In the present invention, the martensite phase is most preferable as the hard phase based on the experimental results, and the ratio thereof is preferably in the range of 10 to 60% on average in the volume of the steel sheet. It has been found that it is most preferable in terms of compatibility with characteristics. 10% martensite phase
%, The effect of improving the tensile strength by the hard phase cannot be obtained, so that a low yield ratio cannot be achieved.

On the other hand, if the proportion of the martensite phase is more than 60%, the concentration of C in the martensite is not sufficient, so that the hardness of the martensite decreases and the difference from the hardness of the matrix decreases. As a result, the martensite phase, which is a hard phase, has an effect on the yield stress, and the yield ratio increases due to an increase in yield stress and a decrease in tensile strength.

On the other hand, if the ratio of the martensite phase is more than 60%, the martensite becomes coarse and the ductility and toughness are deteriorated. The martensitic phase here includes the MA phase partially containing the retained austenite phase.
(Martensite-Austenite Constituent).

As a means for obtaining a structure morphology partially containing a martensite phase, as disclosed in Japanese Patent Application Laid-Open No. 53-23817, etc., austenite (temporarily reheated to a two-phase region temperature by heat treatment) is used. γ) After reprecipitation of the phase,
A typical method is to allow the γ phase to be transformed into a martensite phase during cooling by cooling or rapid cooling.

However, in the present invention, it is necessary to have an ultrafine grain structure in the surface layer at the same time as lowering the yield ratio, thereby improving the property of stopping the propagation of brittle cracks. Therefore, it is necessary to devise a manufacturing method so that the ferrite grain size of the ultrafine grain structure does not become coarse or the ultrafine grain structure does not disappear.

In the present invention, based on the results of detailed research and investigation, from the viewpoint of the relationship with other material properties, the simplicity of production, and the load on the production, the ultrafine grain structure of the surface layer and the reduction of the yield ratio were reduced. It has been concluded that the following two methods are the most suitable as the production methods for achieving both the introduction of the required ratio of martensite phase.

The first method is to heat a steel slab to a temperature not lower than the Ac ₃ transformation point and not higher than 1250 ° C. in order to form an ultrafine grain layer on the surface layer portion, before starting hot rolling or hot rolling. In the middle stage, at least two outer surface regions corresponding to 10 to 33% of the thickness of the slab at that stage are cooled at a cooling rate of 2 to 40 ° C./s from a temperature not lower than the Ar ₃ transformation point. In the process of starting and stopping the cooling at the Ar ₃ transformation point or lower and reheating once or more, the cumulative rolling reduction is 20 to 90% until the reheating after the last cooling is completed. Finish rolling is completed, and the surface area of the steel material after the completion of the rolling is changed from (Ac ₁ transformation point −50 ° C.) to (Ac ₃ transformation point + 50 ° C.)
Recover in the range.

After completion of the reheating, the steel material was cooled at a cooling rate of 0.2 to 2 ° C./s, and was cooled to a temperature within a range of (transformation starting temperature (Ar ₃ ) -50 ° C. at the cooling rate) to 500 ° C. rear,
By cooling to 20 to 300 ° C. at a cooling rate of 5 to 40 ° C./s, a predetermined amount of martensite phase is formed.

That is, in order to lower the yield ratio, a martensite phase is introduced into a parent phase composed of a ferrite phase, a bainite phase, and a mixed phase of these structures. After cooling to a suitable temperature range, rapid cooling is performed to transform the austenite phase into martensite. According to such a manufacturing method, it becomes possible to introduce a predetermined amount of martensite structure without impairing the form of the ultrafine grain structure in the surface layer.

In the second method, the steel slab is made to have an Ac ₃ transformation point or more,
It is heated to a temperature of 1250 ° C. or less, and before the start of hot rolling or in the middle of hot rolling, the thickness of the slab at that stage is 1%.
At least two outer surface areas corresponding to 0 to 33% start cooling at a temperature of not less than the Ar ₃ transformation point at a cooling rate of 2 to 40 ° C./s, and stop cooling at a temperature of not more than the Ar ₃ transformation point. In the process of passing the heat recuperation one or more times, the cumulative draft is 20 to
90% finish rolling is completed, and the surface layer of the steel material after the completion of the rolling is reheated to a range of (Ac ₁ transformation point −50 ° C.) to (Ac ₃ transformation point + 50 ° C.). Or let the steel material after recuperation finish at 5-40 ° C / s
This is a method of performing the following special two-phase region heat treatment on a steel material having an ultrafine grain layer formed on the surface layer by cooling to 20 to 650 ° C at a cooling rate of:

That is, if a two-phase region heat treatment for forming martensite is performed by a normal heat treatment, the ultrafine grain structure in the surface layer portion is completely or partially impaired, and thus cannot be adopted. By increasing the rate of temperature rise until heating to the phase temperature and limiting the holding time at the heating temperature to a short time, it is possible to reduce the yield ratio in the structure without impairing the function of the surface ultrafine grain structure. It becomes possible to introduce an effective martensite phase.

In this case, at a heating rate of 0.1 to 50 ° C./s, (Ac ₁ transformation point + 10 ° C.) to (Ac ₃ transformation point−30 ° C.)
After heating to the range, the residence time in the temperature range is 1 to 6
It must be 0 s. As for cooling after heating and holding, rapid cooling is preferable for forming martensite, but 0.5 to 50 ° C./s
It is sufficient if it is within the range. Specific reasons for limiting the production conditions for introducing the martensite phase will be described later.

The above is the requirement of the low yield ratio high tensile strength steel for building use having excellent brittle crack arrestability of the present invention. However, each chemical component also needs to be limited for the following reasons.

C is contained as an effective component for improving the strength of the steel. If the content is less than 0.01%, it is difficult to secure the strength required for structural steel, but the excess content exceeding 0.20% is difficult. The deterioration of the ductile fracture characteristic causes a decrease in the fracture resistance intended by the present invention. Further, since the toughness and the resistance to welding cracking are also reduced, the content is set in the range of 0.01 to 0.20%.

Si is an element effective as a deoxidizing element and also for securing the strength of the base material. However, if the content is less than 0.01%, deoxidation becomes insufficient and disadvantageous for securing the strength. Conversely, an excessive content exceeding 1.0% forms a coarse oxide and causes deterioration in ductility and toughness. Therefore, the range of Si is 0.0
1 to 1.0%.

Mn is an element necessary for securing the strength and toughness of the base material. l% or more,
The upper limit is set to 2.0% within an allowable range for the material such as toughness and cracking property of the welded portion.

Al is an element effective for fixing solid solution N which is detrimental to ductility and toughness, and is also an element effective for deoxidation, refining of γ particle size, and the like. 0.001%
It is necessary to contain the above. On the other hand, if it is contained in excess of 0.1%, a coarse oxide is formed and the ductility is extremely deteriorated. Therefore, it is necessary to limit the content to the range of 0.001 to 0.1%.

The presence of solute N causes the deterioration of ductility and toughness, so it is necessary to optimize its content. That is, N is effectively combined with Al and Ti to reduce the size of the γ-grain.
Further, it is impossible to industrially completely remove N in steel, and it is not preferable to reduce N more than necessary because an excessive load is applied to a manufacturing process. Therefore, the lower limit is set to 0.001% as a range in which industrial control is possible and load on the manufacturing process is acceptable. If it is contained excessively, solid solution N increases, which may adversely affect ductility and toughness. Therefore, the upper limit is set to 0.010% as an allowable range.

P and S are impurity elements that deteriorate ductility and toughness, and are preferably reduced as much as possible. However, the deterioration of the material is not so large, and the upper limit of P is 0.025% as an allowable amount. Is limited to 0.015%.

The reasons for limiting the basic components of the steel material of the present invention have been described above. In the present invention, Ti, Zr, Nb, Ta, V, B,
One, two or more of Cr, Mo, Ni, Cu, and W can be contained.

Ti contributes to the improvement of the base metal strength by precipitation strengthening, and is also an element effective for the refinement of the heated γ grain size by the formation of TiN, and is also effective for the improvement of the toughness. For this purpose, a content of 0.003% or more is required. 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 forming nitride, and Ti
Has the same effect as described above, but in order to exhibit the effect, the content of 0.003% or more is required. On the other hand, 0.10
%, Coarse precipitates and inclusions are formed similarly to Ti to deteriorate toughness and ductility.
Limit to 10% range.

Nb is also an element effective for improving the strength and toughness, but if it is contained excessively, the toughness deteriorates due to precipitation embrittlement.
Therefore, the range in which the effect can be exhibited without inducing toughness is limited to the range of 0.002 to 0.05%.

Ta is also an element effective for improving the strength and toughness, but it is necessary to contain 0.005% or more in order to exhibit the effect. On the other hand, if it exceeds 0.20%, precipitation embrittlement, coarse precipitates, and toughness degradation due to inclusions occur,
The upper limit is set to 0.20%.

V is also an element which forms VN and is effective for improving the strength. However, as in the case of Nb, if it is contained excessively, the toughness deteriorates due to precipitation embrittlement. Therefore, the range in which the effect can be exhibited without causing significant deterioration in toughness is 0.005 to 0.2.
Limited to the range of 0%.

B is an element effective for improving toughness by fixing solid solution N and improving strength and toughness by improving hardenability, since it is surely linked to N in a trace amount. % Is required. On the other hand, if the BN content exceeds 0.003%, the BN becomes coarse, which adversely affects ductility and toughness. Further, the weldability is also deteriorated, so the upper limit is made 0.003%.

Although both Cr and Mo are effective elements for improving the strength of the base material, in order to produce a clear effect, the content of Cr is not limited to 0.1.
The content is required to be not less than 01%. On the other hand, if added in excess of 2.0%, the toughness and weldability tend to be deteriorated.

Ni is a very effective element that can simultaneously improve the strength and toughness of the base material, but must be contained at 0.01% or more in order to exert its effect. When the content is increased, the strength and toughness are improved, but if the content exceeds 4.0%, the effect is saturated, but the weldability is deteriorated.
The upper limit is set to 4.0%.

Although Cu has almost the same effect as Ni,
If it exceeds 2.0%, a problem occurs in hot workability.
It is limited to the range of 1 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, 2.0
%, An excessive content exceeding 2.0% results in marked deterioration in toughness, 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 can be contained as necessary. Mg, Ca, REM
Are effective in improving ductility by suppressing the elongation of sulfide during hot rolling. It also works effectively to improve the joint toughness by making the oxide finer. 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 contained excessively, sulfides and oxides are coarsened, and ductility and toughness are deteriorated. Therefore, the upper limits are respectively 0.01% for Mg and Ca and 0.10% for REM.

Next, a description will be given of the reasons for limiting the production of the high yield strength steel with a low yield ratio for buildings having excellent brittle crack propagation stopping characteristics according to the present invention. In the steel having the chemical composition limited for the above-mentioned reason, in order to improve the brittle crack propagation arresting property, in the surface layer portion of at least two surfaces of the steel material, an ultra-fine grain structure having an average ferrite grain size of 3 μm or less from the surface layer Sheet thickness 1
Must be present over a thickness of 0-33%. 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 the steel slab, an area having an appropriate thickness in the surface layer portion is once cooled to a temperature lower than the Ar ₃ transformation point by means of water cooling during hot rolling or hot rolling. After applying a temperature difference from the inside, if hot rolling is further performed from the state where the temperature difference is kept, the area once cooled to a temperature lower than the Ar ₃ transformation point is subjected to reheating and processing in the process. It becomes a ferrite-based structure.

For this reason, the surface layer having the ferrite-based structure undergoes processing while being reheated by the internal sensible heat, and the ferrite crystal grains in the surface layer are improved by optimizing the processing conditions during the reheating. Are remarkably refined.
Therefore, the proportion of the surface ultrafine grain layer in the final steel material is
When the surface layer is once cooled, the ratio almost coincides with the ratio of the region which has decreased to the Ar ₃ transformation point.

In the above 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. . Further, since the inside is also processed in the two-phase region, there is a problem that the anisotropy of the steel material increases.

On the other hand, when the temperature exceeds 1250 ° C., the grain size of the heated austenite becomes extremely coarse, so that the grain size cannot be reduced even by the subsequent rolling, and the toughness at the center 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.

Heating the steel slab after heating the steel slab according to the final plate thickness and the thickness of the slab, from the viewpoint of reducing the process load and securing the required rolling reduction after reheating to obtain a superfine grain layer of the surface layer. After the thickness of the steel slab is reduced to an appropriate thickness by flakes or rough rolling, the surface layer portion of the steel material to be an ultrafine grain layer is cooled by means of water cooling or the like, and hot rolling of the steel material before water cooling is performed. 10-3 of the plate thickness at the time
The area of each surface layer corresponding to 3% is cooled to the Ar ₃ transformation point or lower, and a temperature difference is formed between the surface layer and the inside.
At this time, the steel sheet had a thickness of 10 at the time of hot rolling before water cooling.
The cooling rate of each surface layer area corresponding to ~ 33% is 2 ° C.
/ s or more.

If the cooling rate is less than 2 ° C./s, the transformed structure after cooling becomes coarse even if the austenite is refined by hot rolling before cooling, and the uniform superstructure is obtained by rolling during subsequent reheating. This is because it becomes difficult to obtain a fine ferrite structure. A higher cooling rate is preferable from the viewpoint of refining the structure, but the effect is saturated even when quenching exceeds 40 ° C./s, and excessive quenching is not preferable for maintaining the shape of the steel sheet. Therefore, the upper limit is set to 40 ° C./s.

The above cooling is started from the Ar ₃ transformation point or higher. This is because the surface ultrafine grain layer 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.

The conditions of the rough rolling for adjusting the thickness of the steel slab, which is performed as necessary before cooling to provide the superfine grain layer on the surface layer, are not particularly specified, but are set forth in order to refine the internal structure. It is more advantageous to perform rolling in the unrecrystallized region of γ.

However, rolling in the non-recrystallized region of γ necessarily involves low-temperature rolling, and adverse effects such as a decrease in productivity and a decrease in internal sensible heat due to recuperation of the surface layer also occur. As a condition that does not cause an extreme decrease in productivity and is not disadvantageous for the formation of the surface ultrafine grain layer, the rolling reduction in the case of performing rolling in the unrecrystallized region of γ is preferably 50% or less.

For the above reasons, the area of each surface layer portion corresponding to 10 to 33% of the sheet thickness at the time of hot rolling before cooling the steel material is cooled at a cooling rate of 2 ° C./s to 40 ° C./s. When cooling to the Ar ₃ transformation point or lower, and then performing finish rolling, the area of each surface layer portion corresponding to 10 to 33% of the sheet thickness by internal sensible heat and / or using external heating is used. By performing rolling during the temperature increase, the structure in the region becomes ultra-fine, which can contribute to improvement in brittle crack propagation stopping characteristics.

As described later, the processing in the recuperation process is
May be repeated two or more times, but the recuperation temperature after rolling in the recuperation process after the last cooling is (Ac ₁ transformation point−
50 ° C.) to (Ac ₃ transformation point + 50 ° C.). That is, the final reheat temperature is (Ac ₁ transformation point −50).
If the temperature is lower than (° C.), recovery and recrystallization of the processed ferrite after processing is not sufficient, so that ultrafine graining is insufficient and brittle crack propagation stopping characteristics are not improved.

On the other hand, when the final reheat temperature is (Ac ₁ transformation point +
If the temperature is higher than 50 ° C.), part of the ferrite ultrafine-grained by the working is transformed back into austenite and disappears, and the proportion thereof becomes so large that it cannot be ignored. Impair. Therefore, in the present invention, the reheating temperature after rolling in the reheating process after the last cooling is from (Ac ₁ transformation point −50 ° C.) to (Ac
₍₃ transformation point + 50 ° C).

The processing step during cooling and recuperation below the Ar ₃ transformation point may be performed once, but the effect is superimposed by repeating a plurality of times, so that a desired microstructure can be obtained even if repeated twice or more. Is possible. In this case, even if the maximum temperature or the minimum temperature in each recuperation stage is arbitrary, ultrafine granulation is performed according to the temperature conditions of the present invention. However, it is preferable that the upper limit of the recuperation temperature in the middle is set to (Ac ₃ transformation point + 100 ° C.) or less, since the effect of the grain refining is surely superposed.

As the cumulative rolling reduction of the finish rolling as the rolling from the first cooling to the last reheating is increased, a finer 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 higher the rolling reduction, the more advantageous for ultra-fine graining.
Rolling exceeding 0% is not preferable because the effect is saturated and productivity is extremely impaired. Therefore, in the present invention, the cumulative rolling reduction of the finish rolling is limited to 20 to 90%.

Although it is possible to provide an ultrafine grain layer on the surface layer by a manufacturing method in accordance with the above-described limited conditions, in order to further reduce the yield ratio, the cooling conditions at the end of rolling or after the steel material is manufactured. Need to be limited as shown below.

In the method of introducing the martensite phase in the cooling stage after the final reheating, the steel material after the reheating is cooled at a cooling rate of 0.2 to 2 ° C./s (transformation at the cooling rate). After cooling to the range of the starting temperature (Ar ₃ ) -50 ° C.) to 500 ° C., the cooling rate is 5 to 40 ° C./s for 20 to 300 ° C.
Cool to ° C.

First, the steel material after the completion of recuperation is cooled to a temperature in the two-phase region. If the cooling rate at that time is less than 0.2 ° C./s, the cooling rate is too slow. Or, because the matrix structure, which is a mixed phase thereof, is coarsened, and the toughness is deteriorated,
It is not preferable because the crystal grain size of the ultrafine-grained layer formed in the previous stage becomes coarse and the brittle crack propagation stopping characteristics may be deteriorated.

If the cooling rate is more than 2 ° C./s, the transformation starting temperature becomes too low, so that it is difficult to form a two-phase structure of the transformation phase and the austenite phase, Is not preferred because the difference in hardness of the steels becomes so small that the yield ratio cannot be reduced.

For the above-mentioned reasons, in the present invention, the temperature at the transformation start temperature (A
r ₃₎ -50 ℃) to limit the scope of the cooling rate to to 500 ° C. in 0.2 to 2 ° C. / s.

After completion of the recuperation, the temperature is cooled to the two-phase region temperature at 0.2 to 2 ° C./s to optimize the ratio of the parent phase formed by the transformation and the untransformed austenite phase. Rapid cooling to transform to martensite phase. At this time, the temperature at which the cooling at 0.2 to 2 ° C./s is stopped is (the transformation start temperature (Ar ₃ ) −5 at the cooling rate).
0 ° C.) to 500 ° C.

In order to stably form a martensitic phase of 10 to 60% required for lowering the yield ratio in the structure, it is necessary that C be concentrated in austenite to a certain degree or more.
For that purpose, it is necessary to lower the transformation start temperature (Ar ₃ transformation point) by 50 ° C. or more at the cooling rate until the two-phase region is entered.

However, if the temperature is too low, transformation occurs before the subsequent quenching step, and the bainite phase having a small difference in hardness from the parent phase is formed instead of hard martensite having a high concentration of C. More likely to be generated. According to the experimental results, the lower limit temperature for securing the martensite ratio of 10 to 60% is 500 ° C. Therefore, the cooling stop temperature before quenching in the present invention is limited to the range of (transformation start temperature (Ar ₃ ) at the cooling rate-50 ° C) to 500 ° C.

Ar ₃ quenched from -50 to 500 ° C. to transform untransformed austenite into a martensite phase. For martensitic transformation, the higher the cooling rate, the more advantageous. Considering that the transformation is from austenite, the lower limit of the cooling rate is 5
° C / s.

Further, increasing the cooling rate is advantageous for martensitic transformation, but increases the production cost, and there is a problem that residual stress remains in the steel material and the steel material is deformed. The upper limit of the cooling rate is set to 40 ° C./s as a range that is sufficient and does not cause the above problems. After the cooling at 5 to 40 ° C./s to cause the martensitic transformation, the cooling can be stopped halfway for the purpose of reducing the residual stress and improving the material.

As the quenching stop temperature after the martensite transformation, cooling to a low temperature exceeding 20 ° C. has no effect on the properties of martensite and is meaningless. When stopped, the martensitic transformation has not yet been completed, and untransformed austenite may be transformed into a bainite phase, and a required amount of martensite may not be secured.
Limit to the range of 0 to 800 ° C.

In order to form a martensite phase by reheating and heat treatment of the rolled and cooled steel material, the steel material after the completion of the reheating is allowed to cool, or the steel material after the completion of the reheating is reduced to 5 to 40%.
After cooling to 20 to 650 ° C. at a cooling rate of 10 ° C./s, the temperature was further increased at a rate of 0.1 to 50 ° C./s (Ac ₁ transformation point +10).
° C.) was heated to a temperature in the range of ~ (Ac ₃ transformation point -30 ° C.),
After holding at this temperature range for 1 to 60 s, 0.5 to 50 ° C./s
Cool with.

After performing the hot rolling including the rolling in the recuperation step in accordance with the conditions of the present invention to form an ultrafine grain structure in the surface layer portion, the cooling rate is such that the subsequent grain growth can be suppressed. What is necessary is just to cool to the temperature at which the transformation is substantially completed. When the thickness of the thickest cross section of the steel material is 100 mm or less, cooling is sufficient.

It is naturally possible to perform rapid cooling for the purpose of adjusting the strength and shortening the production time. In this case, the cooling condition is set to 20 to 65 ° C. at a cooling rate of 5 to 40 ° C./s in the present invention.
It shall be cooled to 0 ° C. Cooling rate of 5-40 ° C / s
This is because the effect of strength adjustment is not effective at less than 5 ° C / s. At more than 40 ° C / s, the strength increasing effect and the structure control effect are saturated, but the deformation and residual stress of the steel material are large. This is because there is no point in practically increasing the cooling rate further.

The temperature at which the rapid cooling at 5 to 40 ° C./s is stopped is in the range of 20 to 650 ° C., even if the rapid cooling is performed to less than 20 ° C., there is no effect on the material and structure control. However, the rapid cooling stop temperature is set to 650 ° C.
In the case of super, the transformation near the center of the thickness is still in progress, so the desired material cannot be obtained due to the coarsening of the structure and the increase of high-temperature transformation products, and the intention of rapid cooling for the purpose of material control is completely lost. This is because

The steel material produced by the above method is reheated to a two-phase region to generate a required amount of martensite phase. The requirement for the heat treatment is (Ac) at a heating rate of 0.1 to 50 ° C./s.
After heating to the range of ( ₁ transformation point + 10 ° C.) to (Ac ₃ transformation point −30 ° C.), and after maintaining the temperature in the temperature range for 1 to 60 s,
Cooling at 0.5-50 ° C / s. A problem in performing the two-phase region heat treatment is how to preserve the ultrafine grain structure of the surface layer formed in the rolling step.

Since the ultrafine grain structure of the surface layer is a structure formed by a specially designed heat history, it can be recrystallized not only at a temperature exceeding the transformation temperature but also at a high temperature. There is a high possibility that the peculiar ultrafine grain structure is damaged by grain growth or the like. As a heat treatment for introducing martensite while retaining the ultrafine grain structure, a two-phase region heat treatment of rapid heating and holding for a short time is essential.

That is, by rapidly heating, it is possible to reach the two-phase region temperature before the ultrafine-grained structure changes with the heat as the driving force. It is possible to suppress the grain growth of the ultrafine grain structure. In that case, the heating rate needs to be in the range of 0.1 to 50 ° C / s.

If the rate of temperature rise is less than 0.1 ° C./s, there is no effect of rapid heating, and it is difficult to suppress the growth of ultrafine grains. On the other hand, if it exceeds 50 ° C./s, although it is effective for suppressing the grain growth of the ultrafine grain portion, the holding temperature tends to overshoot and it becomes difficult to perform industrially stable control. Limited to ° C / s.

It is preferable that the heating rate is controlled within this heating rate range from the beginning of heating to the holding temperature.
If the average rate of temperature rise from the temperature of ° C. to the holding temperature is within the range of the present invention, the two-phase heat treatment can be performed without damaging the ultrafine grain structure of the surface layer.

The heating temperature was optimized within the above-mentioned limited range of the heating rate and the holding time described later, and the ratio of the martensite phase in the steel material after heat treatment was adjusted to a value suitable for lowering the yield ratio.
Control is performed so as to be in the range of 0 to 60%. For that purpose, (Ac ₁ transformation point + 10 ° C.) to (Ac ₃ transformation point−30)
(° C.).

If the heating temperature is lower than (Ac ₁ transformation point + 10 ° C.), the ratio of the austenite phase formed during heating is small, so that the ratio of the martensite phase formed by the transformation during cooling is 10% or more. Can not.

On the contrary, when the heating temperature is (Ac ₃ transformation point−30 ° C.)
If it is excessively high, the concentration of C in the austenite phase formed during heating is not sufficient, and the hardenability of austenite irrespective of the chemical composition is not ensured. Is not reliable, and it becomes difficult to stably obtain the required amount of martensite. Also, when the heating temperature is increased, the danger of the morphology of the ultrafine grain structure in the surface layer being increased is increased.

Therefore, in the present invention, the heating rate is 0.5 to 50 ° C./s, and the holding time at the heating temperature is 1 to 60.
s, the heating temperature of the two-phase region heat treatment is (Ac) in order to stably secure the required amount of martensite and not to impair the form of the ultrafine grain structure in the surface layer. ₁ transformation point + 10 ° C) to (Ac ₃ transformation point -30 ° C)
To the range.

The reason why the holding time at the heating temperature is limited to 1 to 60 s is not to impair the morphology of the ultrafine grain structure in the surface layer, as in the case of increasing the heating rate. If the holding time is shorter than 1 s, it is difficult to control industrially. If the holding time is longer than 60 s, recrystallization and grain growth of the ultrafine grain structure in the surface layer start.

It is to be noted that increasing the heating rate and limiting the holding time at the heating temperature to a short time are effective in preserving the structure of the surface layer, and at the same time, reducing the martensite phase during the two-phase region heat treatment. It has an additional effect on miniaturization and is also effective on improving toughness.

The cooling condition after maintaining the temperature at (Ac ₁ transformation point + 10 ° C.) to (Ac ₃ transformation point−30 ° C.) for 1 to 60 s is within a range in which a required amount of martensite phase is formed during cooling transformation. I just need. In the present invention, the cooling rate is 0.5
If the cooling rate is lower than 50 ° C./s, the formation of martensite phase is not reliable, and the higher the cooling rate, the more advantageous.
If the cooling rate exceeds 0.5 ° C / s, the effect is saturated with respect to the formation of the martensite phase during the heat treatment in the two-phase region, but there are disadvantages in the shape and cost of the steel material.
To the range.

The above-described manufacturing method in which the ultrafine grain layer is formed and then rapidly cooled from the temperature in the two-phase region or the rapid heating and short-time holding described in claim 6 are characterized. The steel material manufactured by the manufacturing method using the two-phase region heat treatment described above may be further subjected to a tempering treatment for the purpose of adjusting strength, improving toughness, and improving shape. In that case, it is an essential condition that the ultrafine grain structure formed in the surface layer portion is not damaged.

In the present invention, the tempering temperature is 450 to 650 ° C.
However, the effect of tempering is not clear below 450 ° C., and if it exceeds 650 ° C., the morphology of the ultrafine grain structure in the surface layer may be impaired. Note that the heating and holding time for the tempering is arbitrary within the tempering temperature range, but from the viewpoint of preserving the ultrafine grain structure of the surface layer, the holding time is preferably within 5 hours.

[0111]

EXAMPLES Using test steels having the chemical compositions shown in Table 1, Table 2
For a 50 mm or 70 mm thick steel plate manufactured under the manufacturing conditions shown in Table 1, the tensile properties of the base metal and the toughness (fracture transition temperature vTrs) by Charpy test, and the brittle crack propagation arrest property (Kca value of 400 kgf by ESSO test) - a mm ^-3/2 and comprising temperature) shown in Table 3.

[0112]

[Table 1]

[0113]

[Table 2]

[0114]

[Table 3]

[0115]

[Table 4]

[0116]

[Table 5]

[0117]

[Table 6]

The tensile properties of the base material were measured using a round bar test piece having a parallel part diameter of 6 mm and a distance between marks of 25 mm taken from the t / 4 part of the plate thickness so that the test direction was perpendicular to the rolling direction. did. The Charpy impact characteristics of the base material were also sampled at the same position and in the same direction as the tensile test piece, and the fracture surface transition temperature (vTrs) was determined.

As shown in Table 1, since the steel sheets of steel numbers A1 to A16 have a chemical composition, a superfine grain structure of the surface layer and a martensite volume fraction within the range of the present invention, the yield ratio is 8%.
Propagation of brittle cracks showing good low YR characteristics of less than 0%, good fracture toughness of Charpy impact characteristics having a surface transition of -100 ° C or less at the surface layer and -50 ° C or less at the center. all temperatures Kca value determined by ESSO test is an indication of the stopping characteristic is 400 kgf · mm ^-3/2 is -
70 ° C. or less, which is good, and it is clear that good low Y property, toughness, and brittle crack propagation arresting property can be simultaneously achieved.

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,
Steel No. B1 has an excessive amount of N, and thus has insufficient Charpy properties and brittle crack propagation arresting properties even when a superfine grain layer is formed. Since steel number B2 has an excessive P content, the Charpy property and the brittle crack propagation arrest property are similarly inferior to the steel of the present invention.

Since the steel number B3 has an excessive S content, the Charpy property and the brittle crack propagation stopping property are inferior to the steel of the present invention. Steel No. B4 has too much C, so it has Charpy properties,
Both of the brittle crack propagation arresting characteristics are significantly deteriorated. Steel number B5 is within the scope of the steel of the present invention as a chemical component,
Since it is manufactured by a normal TMCP process, it does not have an ultrafine grain structure in the surface layer portion, and the brittle crack propagation arresting property is significantly deteriorated.

In steel No. B6, although the fine grain portion was formed on the surface layer, the average particle size of the surface layer portion was more than 3 μm because the recuperation temperature in the final recuperation process was too high. In comparison, it is difficult to say that sufficient surface Charpy properties and brittle crack propagation arrest properties have been obtained.

Steel No. B7 is used for cooling Ar ₃ when cooling the surface layer.
Since the thickness of the part cooled below the transformation point is insufficient, the thickness of the final superfine grain layer on the surface has not reached the required amount, and good surface Charpy characteristics and brittle crack propagation stopping characteristics have been obtained. Not been.

Steel No. B8 has no accelerated cooling from the temperature in the two-phase region after rolling and has not been subjected to the rapid heating and tempering treatment in the two-phase region, so that the martensite ratio in the structure is too small, and the yield ratio is low. However, it is insufficient as a low yield ratio steel for construction.
Conversely, in steel number B9, since the heating temperature of the two-phase region tempering is too high and the martensite ratio is excessive, both Charpy properties and brittle crack propagation arresting properties are significantly deteriorated.

Steel No. B10 has a low temperature rise rate in the two-phase tempering and an excessive holding time, so that the form of the ultrafine grain layer once formed on the surface layer has been distorted, and the average grain size has been coarsened. No improvement in crack propagation arrestability is observed. From the above examples, it is clear that according to the present invention, it is possible to produce a steel material having both low YR characteristics, toughness and brittle crack propagation stopping characteristics.

[0126]

The present invention relates to a high-yield-ratio high-strength steel material mainly used for a building structure, which has a low yield ratio characteristic and a high brittleness capable of stopping a brittle crack even if a fracture should occur. Extremely safe structural steel with crack propagation arrestability combined with the use of special alloying components without the need for special alloying components. large.

Claims (7)

[Claims] C: 0.01 to 0.20% Si: 0.01 to 1.0% Mn: 0.1 to 2.0% Al: 0.001 to 0.1% N by weight% : A steel material containing 0.001 to 0.010%, the content of P and S as impurities is P: 0.025% or less, S: 0.015% or less, and the balance is iron and unavoidable impurities. The ratio of martensite to the volume of the steel material is 10 to 60%, and the average ferrite grains in the range of 10 to 33% of the total thickness from the surface layer with respect to at least two of the outer surfaces constituting the steel material. A low-yield-ratio high-strength steel for architectural use excellent in brittle crack propagation arresting characteristics, characterized by having an ultrafine grain structure having a diameter of 3 µm or less. 2. In% by weight, 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% of one or more of the following: for architectural materials excellent in brittle crack propagation arresting characteristics according to claim 1. High yield strength steel with low yield ratio. 3. In% by weight, Cr: 0.01 to 2.0% Mo: 0.01 to 2.0% Ni: 0.01 to 4.0% Cu: 0.01 to 2.0% W The low-yield-ratio high-strength steel for architectural use excellent in brittle crack propagation arresting characteristics according to claim 1 or 2, which contains one or more of 0.01 to 2.0%. 4. The composition contains one or more of Mg: 0.0005 to 0.01% Ca: 0.0005 to 0.01% REM: 0.005 to 0.10% by weight% The low-yield-ratio high-strength steel material for building according to any one of claims 1 to 3, which is excellent in brittle crack propagation arrestability. 5. A steel slab having a composition according to any one of claims 1 to 4, which is heated to a temperature of from the Ac ₃ transformation point to 1250 ° C., before starting hot rolling or in the middle of hot rolling. ,
At this stage, at least two surface layer regions on the outer surface corresponding to 10 to 33% of the thickness of the slab are started to be cooled at a cooling rate of 2 to 40 ° C./s from a temperature of the Ar ₃ transformation point or higher, and Ar ₃
In the process of stopping cooling at the transformation point or lower and reheating once or more, finish rolling with a cumulative rolling reduction of 20 to 90% is completed until reheating after the last cooling is completed. After the rolling, the surface layer of the steel material after the completion of the rolling is reheated to a range of (Ac ₁ transformation point-50 ° C) to (Ac ₃ transformation point + 50 ° C), and the steel material after the completion of the reheating is 0.2 At a cooling rate of ２2 ° C./s (transformation start temperature (Ar ₃ ) at the cooling rate)
After cooling to the range of -50 ° C) to 500 ° C, 5 to 40 ° C
cooling at a cooling rate of 20 to 300 ° C at a cooling rate of 1 / s.
A method for producing a high-strength low-yield-ratio steel material for buildings having excellent brittle crack arrestability, characterized by producing the steel material according to any one of (1) to (4). 6. A steel slab having the composition according to claim 1 is heated to a temperature not lower than the Ar ₃ transformation point and not higher than 1250 ° C., before starting hot rolling or in the middle of hot rolling. ,
At this stage, at least two surface layer regions on the outer surface corresponding to 10 to 33% of the thickness of the slab are started to be cooled at a cooling rate of 2 to 40 ° C./s from a temperature of the Ar ₃ transformation point or higher, and Ar ₃
In the process of stopping cooling at the transformation point or lower and reheating once or more, finish rolling with a cumulative rolling reduction of 20 to 90% is completed until reheating after the last cooling is completed. Reheating the surface layer of the steel material after the completion of the rolling to a range of (Ac ₁ transformation point −50 ° C.) to (Ac ₃ transformation point + 50 ° C.), or allowing the steel material after the completion of the reheating to cool down, Alternatively, the steel material after the completion of reheating is cooled to 20 to 650 ° C. at a cooling rate of 5 to 40 ° C./s.
After cooling to a temperature of 0.1 to 50 ° C./s, (Ac ₁ transformation point -10 ° C.) to (Ac ₃ transformation point −30 ° C.)
After heating to the range described above, after holding for 1 to 60 s in the temperature range, a two-phase region heat treatment of cooling at 0.5 to 50 ° C./s is performed, and the heat treatment according to any one of claims 1 to 4. A method for producing a low-yield-ratio high-strength steel for architectural use having excellent brittle crack propagation arresting characteristics, characterized by producing a steel. 7. The method according to claim 5, wherein tempering is performed at a temperature of 450 to 650 ° C.