JPH11181546A - Structural steel for welding, excellent in toughness at low temperature, fracture toughness, and fatigue characteristic, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel product in which toughness at low temp. and fracture toughness of steel are remarkably improved by modification of the structure in the range from the surface layer part of steel plate to the central part into superfine grains and also fatigue characteristic is improved by the increase in the density of texture of steel, and its production. SOLUTION: This steel product has a composition consisting of, by weight, 0.04-0.2% C, 0.01-1.0% Si, 0.3-2.0% Mn, <=0.025% P, <=0.025% S, and the balance iron with inevitable impurities and also has a structure having, in the whole or >=80% of the steel, <=0.5 μm cementite phases in the grain boundaries and/or subgrain boundaries and also having <=5% pearlite fraction and composed essentially of ferrite or bainite of <=3 μm average grain size.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to shipbuilding, architecture, bridges,
Low temperature toughness for large steel structures such as tanks and pressure vessels
The present invention relates to a structural steel for welding having excellent fracture toughness and fatigue properties and a method for producing the same. Further, the present invention can be applied to a cut / processed part using the structural steel for welding obtained in the present invention or a secondary processed product such as a steel pipe or a column.

[0002]

2. Description of the Related Art In recent years, the demand for steel used with the weight reduction of steel structures or the enlargement of steel structures has become more severe. Therefore, improvement in low-temperature toughness, fracture toughness, and fatigue properties has been desired, and various conventional techniques have been proposed but have not always met expectations.

[0003] That is, as the prior art, the Iron and Steel Institute, Materials and Processes, No. 6 (1990); 1796 or JP-B-6-4903 (Yoshie patent), JP-B 7-59
No. 67 (Nomiyama patent) and Japanese Patent Application Laid-Open No. 5-271861 (Ishikawa patent).

The Iron and Steel Institute of Japan, Materials and Processes, No. 6 (1
990), p. The technique of 1796 obtains ultrafine polygonal ferrite grains of 3 μm or less only at the cooling stop temperature of 500 ° C. by repeating ordinary thermomechanical treatment, but the effect of a small amount of pearlite and coarse grain boundary cementite No improvement in low-temperature toughness was observed at all due to the presence of.

[0005] The thick steel plate described in Japanese Patent Publication No. 6-4903 and the method of manufacturing the same are disclosed in US Pat.
This is a thick steel plate excellent in brittle crack propagation arrestability (arrestability) characterized in that 50% or more of an area ratio of ferrite crystal grains of μm or less is present, and a ferrite crystal grain is formed at the center of the steel sheet. Has not been made at all. Furthermore, since no consideration is given to the effect of carbides on the suppression of ferrite grain growth (pinning), even fine ferrite grains on both surfaces of a thick steel plate cannot always be ensured. On the other hand, in order to technical idea that for recuperator to a temperature in excess of Ac ₃ during or after completion of rolling the rolling at recuperated when both surfaces of the steel plate, the fine ferrite by obtained by reverse transformed again γ granulation The effect of was lost. Furthermore, not only is there no description of the effect of pearlite, but the cementite finely precipitated at the time of reheating is re-dissolved at the time of γ formation, and as a result, the pearlite area ratio is remarkably increased by γ ferrite + pearlite transformation. Is done. On the other hand, defines and N is heated to Ac ₃ or more in the reheating of the slab containing Nb · Ti
Although there is a general description that b.Ti is effective in refining austenite crystal grains, the existence state of Nb.Ti.Ta and the ferrite of their carbonitrides at the time of refining both surfaces of a thick steel plate There is no disclosure, not only of the technical idea on the effect on the grain pinning.

[0006] In the manufacturing method described in Japanese Patent Publication No. 7-5967, after the slab is cooled with water from a temperature of _three or more points of Ac to cool more than one- _third t of the front and back surfaces of the slab to Ar ₃ or less, reheating is completed. The steel plate having excellent brittle crack propagation characteristics (arrestability), characterized in that the finish rolling is started before finishing and the finish rolling is completed at Ac ₃ points or less, and then the front and back surfaces are further reheated to Ac ₃ points or more. It is a manufacturing method. However, the invention has no technical idea to refine the ferrite crystal grains in the central part of the steel sheet, and lacks the idea of rough rolling at all, and lacks the reality of manufacturing a steel material. Further, in order to that for recuperator to a temperature in excess of Ac ₃ after the end of the final rolling at recuperated on the front and back surfaces of the slab and technical idea, the effect of grain refinement of the ferrite by obtained by reverse transformed again into γ Not only is spoiled,
The pearlite area ratio also increased significantly. Not only the description of the state of the presence of C or Nb.Ti.Ta at the time of fine graining of the front and back surfaces of the steel sheet and the technical idea about the effect of cementite or their carbonitride on the fine grain ferrite pinning, There is no description.

The steel described in Japanese Patent Application Laid-Open No. Hei 5-271861 and a method for producing the same are mainly composed of a ferrite or bainite structure having an average equivalent circle diameter of 3 μm or less and a spherical carbide having a diameter of 0.6 μm or less. This is a welding structural steel with good brittle fracture arrestability (arrestability) characterized by a structure composed of phases and a method of manufacturing the same. However, there is no technical idea to ultra-fine ferrite grains in the center of the steel sheet. Did not. In addition, since the method of forming fine spherical carbides and its influence on pinning are unclear, the stability of the ultrafine graining effect in the surface layer structure and even the variation of crystal grains due to the subsequent heat history can always be satisfied. It was not something. Furthermore, there is no disclosure of the effect of pearlite, and the existence of Nb, Ti, and Ta during the fine-graining of the front and back layers of the steel sheet and their effects on the pinning of ultrafine-grain ferrite by carbonitride. There is no description of it, not only the technical thought of the company.

[0008] In the above-mentioned prior art, it was not possible at all to ultrafine-grain a structure mainly composed of ferrite or bainite at the center of the steel sheet. Further, even in the front and back layers of the steel sheet, the ultrafine graining of the structure cannot be stably achieved, resulting in insufficient improvement in low-temperature toughness and insufficient improvement in brittle crack fracture characteristics (arrestability). Significant improvement in fatigue properties (fatigue strength / corrosion fatigue strength) was not possible.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to drastically improve low temperature toughness and fracture toughness of a steel material as a whole, and at the same time, to greatly improve fatigue characteristics, an old and new problem. It is a further object of the present invention to stably achieve ultra-fine graining of the entire steel material including the central portion as well as the surface layer portion of the steel or most (80% or more) of the steel material.

That is, the present invention effectively improves the disadvantages of the prior art by modifying the structure from the surface layer to the center of the steel sheet into ultrafine grains, thereby improving the low-temperature toughness and fracture toughness of the steel. It is an object of the present invention to provide a steel material excellent in fatigue characteristics (fatigue strength and corrosion fatigue strength) by increasing the texture density of the steel material, while significantly improving brittle fracture crack initiation characteristics and arrestability) and a method of manufacturing the same. I do.

Another object of the present invention is to provide a steel excellent in low-temperature toughness and fracture toughness and a method for producing the same without using a lot of expensive elements such as Ni which greatly increase the production cost.

[0012]

According to the present invention, a steel structure in the entire steel material or most of the steel material (80% or more) is formed at a ferrite grain boundary and / or a sub-grain boundary with cementite and / or 0.5 μm or less. By precipitating carbonitrides of Nb, Ti, and Ta to modify the structure mainly composed of ferrite or bainite to ultrafine particles having an average particle size of 3 µm or less and to reduce the pearlite fraction to 5% or less, Provided is a structural steel or a structural steel for welding having excellent low-temperature toughness, fracture toughness and fatigue properties, and a method for producing the same.

The gist of the present invention is as follows.

(1) In weight%, C: 0.04 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, P: 0.025% or less S: 0.025% or less, the balance being iron and unavoidable impurities, and having a cementite phase of 0.5 μm or less at the crystal grain boundary and / or sub-grain boundary in the whole steel or 80% or more. And
A welding structural steel having excellent low-temperature toughness, fracture toughness, and fatigue properties, characterized by having a structure mainly composed of ferrite or bainite having a pearlite fraction of 5% or less and an average grain size of 3 μm or less.

(2) By weight%, C: 0.04 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, P: 0.025% or less , S: 0.025% or less, Sol. Al: 0.005 to 0.2% as a basic component, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.05%, Ta: 0.005 to 0.05% One or two or more kinds, the balance being iron and unavoidable impurities, and in the whole or 80% or more of the steel material, at least 0.5 μm of cementite and Nb · Ti · Ta at grain boundaries and / or sub-grain boundaries. Or low-temperature toughness / fracture characterized by having a structure mainly composed of ferrite or bainite having two or more carbonitride phases, a pearlite fraction of 5% or less, and an average grain size of 3 μm or less. Structural steel for welding with excellent toughness and fatigue properties.

(3) Cu: 0.05 to 1.0%, Ni: 0.1 to 10.0%, Cr: 0.03 to 1.0%, Mo: 0.05 to 1% by weight 1.0%, V: 0.01 to 0.4%, B: 0.0002 to 0.002% one or more of the above (1) or (2). Structural steel for welding with excellent low-temperature toughness, fracture toughness and fatigue properties.

(4) C: 0.04 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, P: 0.025% or less by weight% S: A steel or a steel material having a composition of 0.025% or less and consisting of iron and unavoidable impurities is heated to _three or more Ac to form a solid solution with C before hot working or On the way, a region of 70% or more of the thickness of the steel sheet from the surface layer at that time is cooled at a cooling rate of 3 ° C./second or more to a temperature at which the ferrite fraction becomes 50% or more.
After the solid solution is supersaturated, hot working is started or restarted from a temperature of (Ac ₁ point-100 ° C.) or higher in the process of reheating the surface layer region, and (Ac ₃ point-100
° C.) to exit the hot working range of to Ac ₃ point, subsequently the surface layer region is cooled without causing recuperation in the Ac ₃ point or more, the grain boundaries and / or in the entire steel or 80% It has a cementite phase precipitated to 0.5 μm or less at the crystal sub-grain boundaries, has a pearlite fraction of 5% or less, and has a structure mainly composed of ferrite or bainite with an average grain size of 3 μm or less. For producing welding structural steel having excellent low-temperature toughness, fracture toughness and fatigue properties.

(5) By weight%, C: 0.04 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, P: 0.025% or less , S: 0.025% or less, Sol. Al: 0.005 to 0.2% as a basic component, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.05%, Ta: 0.005 to 0.05% In a state where one or two or more kinds of C and Nb, Ti, and Ta are solid-dissolved by heating a steel or a steel material containing one or more kinds and a balance of iron and unavoidable impurities to _three or more points of Ac. Before or during hot working, a region of 70% or more of the steel sheet thickness from the surface layer at that time is cooled at a cooling rate of 3 ° C./second or more to a temperature at which the ferrite fraction becomes 50% or more, and C and Nb · After one or two or more of Ti and Ta are dissolved in supersaturation, hot working is started or restarted from a temperature of (Ac ₁ point-100 ° C.) or more in the process of reheating the surface layer region, Ac ₃ points-100 ° C)-Ac ₃
The hot working is completed within the range of the point, and subsequently, the surface layer is cooled without reheating to the Ac ₃ point or more, and the whole or 80% or more of the steel material becomes a crystal grain boundary and / or a crystal sub-grain boundary. Ferrite or bainite having one or two or more carbonitride phases of cementite and Nb.Ti.Ta precipitated to 0.5 μm or less, having a pearlite fraction of 5% or less, and an average particle size of 3 μm or less. A method for producing a welding structural steel having excellent low-temperature toughness, fracture toughness, and fatigue characteristics, characterized by being composed of a main structure.

(6) After the completion of the hot working, the surface region is accelerated cooling or directly quenched at a cooling rate of 5 ° C./sec or more without reheating the surface region to ₃ or more Ac points. The method for producing a structural steel for welding according to the above (4) or (5), which is excellent in low-temperature toughness, fracture toughness and fatigue properties.

(7) The method for producing a structural steel for welding having excellent low-temperature toughness / fracture toughness and fatigue characteristics according to the above (6), wherein tempering is performed after accelerated cooling or direct quenching is completed.

(8) By weight%, Cu: 0.05 to 1.0%, Ni: 0.1 to 10.0%, Cr: 0.03 to 1.0%, Mo: 0.05 to 1 0.04%, V: 0.01 to 0.4%, B: 0.0002 to 0.002%, any one of the above (4) to (7). A method for producing a welding structural steel having excellent low-temperature toughness, fracture toughness, and fatigue properties according to the above-described item.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventor has conducted detailed studies on the low-temperature toughness, fracture toughness and fatigue properties of a structural steel for welding, and found that reduction of the pearlite fraction and ultra-fine grain refinement are necessary conditions. , And found that it was not a necessary and sufficient condition. That is, in order to configure a structure mainly composed of ferrite or bainite having an average particle size of 3 μm or less, as shown in FIG. 1, one or more carbonitride phases of cementite and / or Nb · Ti · Ta (arrows) ) At the crystal grain boundaries and / or crystal sub-grain boundaries of 0.5 μm or less was found to be necessary and indispensable.

In order to precipitate one or two or more carbonitride phases of cementite and / or Nb.Ti.Ta at a ferrite grain boundary and / or a sub-grain boundary at a grain size of 0.5 μm or less, C and And / or heating a steel material or steel containing one or more of Nb.Ti.Ta to more than _three points of Ac to form a solid solution of one or more of C and / or Nb.Ti.Ta. In this state, before or during hot working such as controlled rolling, the steel is rapidly cooled to a temperature at which the ferrite fraction becomes 50% or more, and one or more of C and / or Nb, Ti, and Ta are dissolved in a supersaturated state. After the heat treatment, it is essential to reheat the steel, and in the reheating process, hot working is started or restarted, and the hot working is completed at an Ac point of ₃ or less. cold is not recuperation in more than Ac ₃ point in order not to re-solid solution It has been led to invent technologies and the average particle size is essential for effective securing structure mainly the following ferrite or bainite 3μm to.

Hereinafter, the present invention will be described in detail.

C is 0.5% from the supersaturated solid solution state in the present invention.
It is an essential element for pinning ultrafine grain ferrite by cementite precipitated at ferrite grain boundaries or sub-grain boundaries below μm, and is the most effective element to improve strength at low cost. If it exceeds, the weldability (weld part toughness) is impaired, and if it is less than 0.04%, the amount of cementite necessary for pinning becomes insufficient, so
%.

Si is effective as a strength improving element and is also useful as an inexpensive deoxidizing element for molten steel. Since it is necessary to use a large amount of expensive deoxidizing elements such as Ti and Al, the content is limited to 0.01 to 1.0%.

Mn is a useful element for improving the strength.
Excessive addition impairs the base metal toughness and weldability, and also causes Ar ₃
As a result of lowering the transformation point, it is limited to 0.3 to 2.0% in order to make hot rolling such as two-phase rolling difficult.

P is limited to 0.025% or less from the viewpoint of base material toughness. Although P as an impurity is preferably as low as possible, it is preferably 0.015 or less from the viewpoint of weldability in consideration of economy.

S is limited to 0.025% or less from the viewpoint of base metal toughness. It should be noted that S as an impurity is preferably as low as possible, but from the viewpoint of weldability and workability, 0.008% or less is preferable in consideration of economy. Furthermore, from the viewpoint of large heat input weldability, 0.0008 to 0.004% is required.

Nb is the most useful element together with Ti in thermomechanical processing (TMCP) steel, and NbC or NbC
Suppression of γ-grain growth during reheating of steel material as N (Carbo-Nitride), expansion of non-recrystallization region temperature range during controlled rolling, precipitation strengthening in deformation zone during rolling, welding heat effect during large heat input welding The effect of preventing HAZ softening in the portion (HAZ) is generally known. Further, the present inventor has found from detailed studies that the effect of suppressing the growth of ferrite grains and the thermal stability of ultrafine precipitated cementite are significantly increased. Therefore, if the content is less than 0.005%, the amount of NbC or NbCN precipitated at the ferrite crystal grain boundary or sub-grain boundary from the supersaturated solid solution state to 0.5 μm or less becomes insufficient, and the thermal behavior of cementite precipitated to 0.5 μm or less becomes insufficient. The stability is also insufficient. If the content is 0.1% or more, the weldability is impaired, so that the content is limited to 0.005 to 0.1%.

Ti is also the most useful element together with Nb in TMCP steel, and TiC or TiCN (Carb
o-Nitride) is generally the effect of suppressing the growth of γ grains during reheating of steel materials, expanding the temperature range of the non-recrystallization region during controlled rolling, strengthening precipitation during rolling, and improving HAZ toughness during large heat input welding. Is known to. Furthermore, from the in-depth study of the present inventors, it has been found that, like Nb, the effect of suppressing the growth of ferrite grains and the thermal stability of ultrafine precipitated cementite are improved. Therefore, if the content is less than 0.005%, the amount of TiC or TiCN precipitated at the ferrite crystal grain boundary or sub-grain boundary from the supersaturated solid solution state to 0.5 μm or less becomes insufficient, and the thermal behavior of cementite precipitated to 0.5 μm or less becomes insufficient. The stability is insufficient, and if the content is 0.05% or more, the weldability is impaired. Therefore, the content is limited to 0.005 to 0.05%.

Ta is TaC or TaCN (Carbo-
Nitride) is known to have the effect of suppressing the growth of γ grains at the time of reheating the steel material and to improve the HAZ toughness at the time of large heat input. However, it is not widely used because it is expensive. However, from the detailed examination of the present inventors, it has been found that, similarly to Nb.Ti, the effect of suppressing the growth of ferrite grains and the thermal stability of ultrafine precipitated cementite are improved. Therefore,
When the content is less than 0.005%, T precipitates from a supersaturated solid solution state to a ferrite grain boundary or a sub-grain boundary to 0.5 μm or less.
Insufficient aC or TaCN amount and insufficient thermal stability of cementite precipitated to 0.5 μm or less,
If it exceeds 0.05%, the weldability is impaired.
Limited to 0.05%.

Al is an element necessary for deoxidation like Si, and from the technical idea of the present invention, when a small amount of Ti.Ta or Nb is added, deoxidation of Si alone is insufficient to prevent its oxidation. To 0.005% or more, excessive addition of 0.2% or more impairs HAZ toughness.
Limited to 0.2%.

The above are the basic components of the steel targeted by the present invention. However, in order to improve the strength of the base material and to reduce the carbon equivalent for the purpose of improving the low temperature toughness and the weldability, the size of the steel material and the steel plate Alloy elements (Cu, Ni, Cr,
Even if one or more of Mo, V and B) are added, the effect of the present invention is not impaired at all. However, in order to increase the deformation resistance of the two-phase region rolling and not to make the hot working difficult, the addition amount is preferably as small as possible and preferably 5% or less in total, but a rolling mill having a large rolling reaction force This is not the case in cases such as Further, other elements (Ca, RE
M, Mg, etc.) have a strong affinity with O and S, and should be added in order to improve low-temperature toughness, weldability, and workability by controlling the morphology.
Even when added in the range of 0005 to 0.01%, the effect of the present invention is not impaired.

Next, the reason for defining the crystal structure which is the technical idea of the present invention will be described.

In a ferrite-pearlite steel containing bainite, even if the ferrite grain size is 5 μm or less, the low-temperature toughness
Fracture toughness does not always improve. Detailed investigation by the present inventors has revealed that brittle fracture / fatigue fracture in a structure having a ferrite grain size of 5 μm or less originates from a fine pearlite colony. Furthermore, low-temperature toughness, fracture toughness, and fatigue strength are improved with the reduction in ferrite grain size only when the pearlite fraction is 5% or less, including coarse cementite. I also learned the thing.

On the other hand, a structure composed mainly of fine cementite or carbonitride phase cannot stably achieve an average grain size of a structure mainly composed of ferrite or bainite of 3 μm or less, and the growth of ferrite crystal grains is not achieved. They also found that suppression was essential. That is, fermentation can be effectively suppressed by pinning ferrite or bainite for the first time by depositing cementite of 0.5 μm or less at the ferrite crystal grain boundary or crystal sub-grain boundary.
Further, when a carbonitride of Nb.Ti.Ta of 0.5 μm or less is precipitated at a ferrite crystal grain boundary or a crystal sub-grain boundary, the same pinning effect as cementite is recognized,
Furthermore, it was also found that the thermal stability of cementite itself precipitated ultrafinely at the ferrite crystal grain boundaries or crystal sub-grain boundaries increases.

Next, in the present invention, the entire steel material or 80%
The reason for defining the manufacturing method for realizing the above ultrafine grain structure will be described.

When the steel material or steel is reheated, the heating temperature at which one or more of C and / or Nb.Ti.Ta is dissolved is limited to _three or more Ac points. Nb ・ Ti ・
The heating temperature at which one or two or more kinds of Ta are sufficiently dissolved is preferably 1100 ° C. or higher, and the heating temperature is preferably 1200 ° C. or lower in order to prevent coarsening of γ grains during heating.

In order to precipitate cementite and / or one or more carbon nitrides of Nb.Ti.Ta of 0.5 .mu.m or less at grain boundaries and / or sub-grain boundaries in the whole or 80% or more of the steel material. Is C and / or Nb.Ti
-In a state where one or two or more kinds of Ta are dissolved in steel,
After cooling the area of 70% or more of the steel sheet thickness from the surface layer at a cooling rate of 3 ° C./sec or more to dissolve the components into a supersaturated solid solution in the steel, use a heating function by an external heat source such as induction heating. In this process, ultra-fine precipitation is performed in the process of reheating.

In all or more than 80% of the steel material,
In order to reduce the average grain size of the structure mainly composed of ferrite or bainite to 3 μm or less, 70% of the steel sheet thickness from the surface layer before or during hot working after heating the steel sheet or the steel material to _three or more Ac points. After rapidly cooling the above region to a temperature at which the ferrite fraction becomes 50% or more at a cooling rate of 3 ° C./second or more, the temperature is reduced from the temperature of Ar1 point or more in the process of reheating using an external heat source such as induction heating. The hot working is started or restarted, and the central part of the steel material is also cooled in the reheating process to satisfy the above conditions, and the whole or 80% or more of the steel material is (A)
By finishing the hot working in the range of c ₃ points-100 ° C.) to Ac ₃ points, the recovery and recrystallization of ferrite is caused to form ultrafine grains, and the ferrite is cooled without being heated to ₃ or more points. In addition, by effectively utilizing pinning by one or two or more carbonitrides of 0.5 μm or less of cementite and / or Nb / Ti / Ta precipitated at ferrite grain boundaries and / or sub-grain boundaries. The purpose is to prevent the growth of the ultrafine grain structure.

The hot working is intended for general hot working such as rolling. In the case where the material size of the steel is large and the heating temperature is higher than 1200 ° C. or when the requirement of the low temperature toughness is more severe, the initial γ grains before cooling the steel material to a temperature at which the ferrite fraction becomes 50% or more are used. To keep it fine,
It is preferable to perform hot working such as addition of Nb / Ti / Ta and controlled rolling. Further, when hot working is not performed before cooling subsequent to heating of the steel, low-temperature heating of 1150 ° C. or less and Nb · Ti · Ta are performed in order to reduce the initial γ grains of the steel.
Or the use of a hot-worked semi-finished product such as a steel sheet having an initial γ grain.

When the entire or 80% or more of the steel material is ultra-fine-grained and then reheated to an Ac point of ₃ or more by the temperature raising function of the external heat source, the effect of the ultra-fine-graining is not only impaired, but also the ferrite grain size is reduced. Cementite finely precipitated in the boundaries or sub-grain boundaries re-dissolves in γ to lose the pinning effect and increase the pearlite fraction.
Therefore, in order to cool the fine grain structure without reheating to the Ac ₃ point or more, air cooling is sufficient when the thickness of the steel sheet is less than 25 mm, and cooling at 1 ° C./sec or more is performed when the thickness is more than 25 mm. It is preferable to perform accelerated cooling so as to achieve a speed.

[0044] with in order to increase the strength of the steel sheet to adjust the additive component on demand intensity levels, hot working after the end of the subsequent 5 without causing recuperation in the Ac ₃ point or more by ° C. /
Accelerated cooling by TMCP equipment at a cooling rate of
What is necessary is just to implement direct quenching by Q equipment.

Following accelerated cooling or direct quenching,
In order to temper the steel or the steel sheet, tempering is performed by a usual heat treatment equipment. In the case of accelerated cooling by the TMCP facility or direct quenching by the DQ facility, an auto-temper which stops the cooling at the time of accelerated cooling or direct quenching may be used.

[0046]

EXAMPLES Examples of the present invention are shown in Table 1. Steels A to C are examples of the present invention, and steels D and E are comparative examples in which C is out of the scope of the present invention.

[0047]

[Table 1] Table 2 shows the manufacturing conditions in the example of the present invention. In all embodiments of the present invention, an induction heating device was used as an external heat source. Steel A-1, A-2, Steel B-1, B-2, Steel C
-1, is an example of the present invention of a steel sheet for welding in the present invention, and especially steel B-2 is obtained by reheating a 100 mm steel sheet to 25%.
1 is an example of the present invention that has been finish-rolled to a steel sheet having a thickness of 1 mm.

On the other hand, in the case of steel A-3, the heating time by the external heat source was too long although the area where the α fraction was 50% or more during cooling of the steel sheet was 90% of the steel sheet thickness. This is a comparative example in which the temperature of the steel material was restored to _three or more Ac points. Further, steel B-3 has a short cooling time on the way and an α fraction of 5%.
This is a comparative example in which the area of 0% or more was as small as less than 70% of the steel sheet thickness. Further, steel C-2 is a comparative example of a steel sheet in which cooling was not performed halfway. Finally, steel D-1 and steel E-1
Is a comparative example in which the production conditions are almost the same as those of the steel B-1 and the steel A-2 of the present invention, but C, which is a main component thereof, is higher or lower than the range of the present invention.

[0049]

[Table 2] Table 3 summarizes the mechanical properties and low-temperature toughness, arrest properties, fatigue strength, and corrosion fatigue strength of the examples of the present invention. A 410 MPa grade steel A for welding structure which is an example of the present invention.
1, A-2, 490MPA grade steel B-1, B-2, 570
As a result of the state of the structure of the steel sheet satisfying the requirements of the present invention, the mechanical properties of the MPa-grade steel C-1 are superior to those of the comparative example.
In addition, the low-temperature toughness and brittle crack propagation arrest characteristics at the center of the sheet thickness, the fatigue strength and the corrosion fatigue strength of the turning welded part are much better than those of the comparative example, and the low-temperature toughness at the center of the steel sheet It is a level comparable to that of. Further, steels B-1 and B- added with Nb.Ti.Ta
2. In steel C-1, cementite or carbonitride precipitates extremely finely at the ferrite crystal grain boundaries and crystal sub-grain boundaries, effectively suppressing the growth of ferrite or partial bainite. Inventive steels A-1 and A-
It is extremely stable compared to No. 2 and has excellent mechanical properties and other properties.

On the other hand, in the steel A-3 of the comparative example, the α grains refined by reheating to the Ac ₃ point or more are inversely transformed into γ, and the cementite precipitated ultra-finely again forms a solid solution with γ. The α grains and cementite in the part are coarsened and the pearlite fraction is 5% or more, and their low-temperature toughness, arrest properties, fatigue strength, and corrosion fatigue strength are inferior to those of the present invention. The steel B-3 of the comparative example had insufficient cooling conditions before finish rolling and insufficient ultrafine graining at the center of the steel sheet, and the microstructure of the steel sheet was only 50% and did not satisfy the requirements of the present invention. Other properties (low temperature toughness, arrest properties, fatigue strength) as a result of the diameter and precipitate size being larger than those of the present invention
Are inferior to the examples of the present invention. As a matter of course, the steel sheet C-2, which is a comparative example in which cooling was not performed on the way, is inferior in the structure state, mechanical properties, and other characteristics as compared with the examples of the present invention.

On the other hand, the steel D-1 of the comparative example, in which the production conditions are almost the same as the steel B-1 of the present invention, but the C is deviated from the present invention to the higher side, also has an average α grain size and a cementite size. Although it satisfies the invention, the mechanical properties and other properties are inferior to the examples of the present invention as a result of the pearlite fraction being increased to 5% or more. The steel E-1 of the comparative example, in which the production conditions were substantially the same as the steel A-2 of the present invention but the C deviated on the lower side from the present invention, also satisfied the present invention in the pearlite fraction and the cementite size. However, the amount of cementite is insufficient to suppress the growth of ultra-fine α grains, so that the average α grain size becomes large and the mechanical properties and other properties are inferior to those of the examples of the present invention.

[0052]

[Table 3]

[0053]

According to the present invention, a carbonitride phase of 0.5 μm or less of cementite or Nb · Ti · Ta is precipitated at ferrite crystal grain boundaries and / or crystal sub-grain boundaries in 80% or more of the steel material. By forming a structure mainly composed of ferrite or bainite having a mean particle size of 3 μm or less in the region concerned, low-temperature toughness, arrest characteristics, fatigue characteristics (fatigue strength, corrosion fatigue strength) of structural steel for welding.
Could be greatly improved. This enables the safety design of large steel structures not only from the construction side but also from the steel material side. Further, it is considered that the present invention makes it possible to achieve both safety design and economic design, and that there is a great economic benefit that can be enjoyed by the industry. Further, the present invention also provides an idea for developing a new steel material.

[Brief description of the drawings]

FIG. 1 is a photomicrograph of steel B-1 according to the present invention, showing 0.5 μm or less cementite precipitated at crystal grain boundaries and crystal sub-grain boundaries.

Claims (8)

[Claims] 1. In weight%, C: 0.04 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, P: 0.025% or less, S: 0.025% or less, the balance consisting of iron and unavoidable impurities, and having a cementite phase of 0.5 μm or less at the crystal grain boundaries and / or sub-grain boundaries in the entire steel material or in 80% or more. ,
A welding structural steel having excellent low-temperature toughness, fracture toughness, and fatigue properties, characterized by having a structure mainly composed of ferrite or bainite having a pearlite fraction of 5% or less and an average grain size of 3 μm or less. 2. In% by weight, C: 0.04 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, P: 0.025% or less, S: 0.025% or less, Sol. Al: 0.005 to 0.2% as a basic component, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.05%, Ta: 0.005 to 0.05% One or two or more kinds, the balance being iron and unavoidable impurities, and in the whole or 80% or more of the steel material, at least 0.5 μm of cementite and Nb · Ti · Ta at grain boundaries and / or sub-grain boundaries. Or low-temperature toughness / fracture characterized by having a structure mainly composed of ferrite or bainite having two or more carbonitride phases, a pearlite fraction of 5% or less, and an average grain size of 3 μm or less. Structural steel for welding with excellent toughness and fatigue properties. 3. Cu: 0.05-1.0%, Ni: 0.1-10.0%, Cr: 0.03-1.0%, Mo: 0.05-1. The low-temperature toughness according to claim 1 or 2, wherein one or more of 0%, V: 0.01 to 0.4%, and B: 0.0002 to 0.002% are contained.・ Structural steel for welding with excellent fracture toughness and fatigue properties. 4. In% by weight, C: 0.04 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, P: 0.025% or less, S: Before or during hot working in a state where steel having a composition of 0.025% or less and consisting of iron and unavoidable impurities, or a steel material, is heated to _three or more Ac to form a solid solution of C, Then, a region of 70% or more of the steel sheet thickness from the surface layer at that time is cooled at a cooling rate of 3 ° C./sec or more to a temperature at which the ferrite fraction becomes 50% or more.
After the solid solution is supersaturated, hot working is started or restarted from a temperature of (Ac ₁ point-100 ° C.) or higher in the process of reheating the surface layer region, and (Ac ₃ point-100
° C.) to exit the hot working range of to Ac ₃ point, subsequently the surface layer region is cooled without causing recuperation in the Ac ₃ point or more, the grain boundaries and / or in the entire steel or 80% It has a cementite phase precipitated to 0.5 μm or less at the crystal sub-grain boundaries, has a pearlite fraction of 5% or less, and has a structure mainly composed of ferrite or bainite with an average grain size of 3 μm or less. For producing welding structural steel having excellent low-temperature toughness, fracture toughness and fatigue properties. 5. In% by weight, C: 0.04 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, P: 0.025% or less, S: 0.025% or less, Sol. Al: 0.005 to 0.2% as a basic component, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.05%, Ta: 0.005 to 0.05% In a state where one or two or more kinds of C and Nb, Ti, and Ta are solid-dissolved by heating a steel or a steel material containing one or more kinds and a balance of iron and unavoidable impurities to _three or more points of Ac. Before or during hot working, a region of 70% or more of the steel sheet thickness from the surface layer at that time is cooled at a cooling rate of 3 ° C./second or more to a temperature at which the ferrite fraction becomes 50% or more, and C and Nb · After one or two or more of Ti and Ta are dissolved in supersaturation, hot working is started or restarted from a temperature of (Ac ₁ point-100 ° C.) or more in the process of reheating the surface layer region, Ac ₃ points-100 ° C)-Ac ₃
The hot working is completed within the range of the point, and subsequently, the surface layer is cooled without reheating to the Ac ₃ point or more, and the whole or 80% or more of the steel material becomes a crystal grain boundary and / or a crystal sub-grain boundary. Ferrite or bainite having one or two or more carbonitride phases of cementite and Nb.Ti.Ta precipitated to 0.5 μm or less, having a pearlite fraction of 5% or less, and an average particle size of 3 μm or less. A method for producing a welding structural steel having excellent low-temperature toughness, fracture toughness, and fatigue characteristics, characterized by being composed of a main structure. After 6. hot working ends, without thereby recuperator the surface layer region above Ac ₃ point subsequently cooling rate is 5
The method for producing a structural steel for welding having excellent low-temperature toughness / fracture toughness and fatigue properties according to claim 4 or 5, wherein the steel is subjected to accelerated cooling or direct quenching at a temperature of at least ° C / sec. 7. The method for producing a structural steel for welding having excellent low-temperature toughness, fracture toughness and fatigue properties according to claim 6, wherein tempering is performed after the completion of accelerated cooling or direct quenching. 8. Cu: 0.05-1.0%, Ni: 0.1-10.0%, Cr: 0.03-1.0%, Mo: 0.05-1. 0%, V: 0.01 to 0.4%, B: 0.0002 to 0.002%. One or more of the following: A method for producing a structural steel for welding having excellent low-temperature toughness, fracture toughness and fatigue properties as described above.