JP2001049338A - Production of high weldability weather resistant steel excellent in fire resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing steel having corrosion resistance to salt damage, low in a yield ratio and having excellent weldability, fire resistance, characteristics in the sheet thickness direction and toughness after cold working. SOLUTION: A slab contg., by weight, 0.01 to 0.15% C, 0.05 to 0.55% Si, 0.3 to 2.0% Mn, 0.30 to 1.00% Cu, 1.0 to 5.5% Ni, 0.1 to 1.5% Mo, 0.005 to 0.090% Al and 0.0010 to 0.0070% N and moreover inevitably contg. 0.030% P, 0.010% S and <=0.1% Cr, and the balance Fe with inevitable impurities is reheated in the temp. range of 1,050 to 1,250 deg.C, subsequently, hot rolling is finished in the temp. range of 750 to 1,000 deg.C, and after the hot rolling, it is allowed to cool or is cooled at the average cooling rate of >=1 deg.C/sec.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to various constructions in the fields of architecture, civil engineering and marine structures in the air environment where salt damage is likely to occur in a high flying sea salt particle environment, such as a beach area or an area where snow melting salt is sprayed. The present invention relates to a method for producing a steel material which is excellent in fire resistance and weldability used for articles and has weather resistance in accordance with the amount of flying sea salt particles in the environment.

[0002]

2. Description of the Related Art Conventionally, as corrosion protection of steel structural members used in places where salt damage occurs, such as coastal areas, painting of ordinary steel materials, use of plated steel sheets, use of surface coating such as thermal spraying and mortar lining, use of stainless steel, and the like. Use of high alloy and high corrosion resistant materials such as titanium and titanium. Furthermore, as a technology for reducing maintenance costs of steel structures, weather-resistant steel (JIS)
G3141 weather resistant steel for welded structures).

[0003] In the case of painting, there has been a problem that maintenance and maintenance costs are required because repainting is inevitably required. Also,
In the case of plating, the hot-dip plating of the structure has problems such as deformation due to thermal stress and peeling of the plating. Also in the case of thermal spraying and mortar lining, peeling and deterioration of the anticorrosion film were problems. In the case of a high-alloy corrosion-resistant material, there is a problem that the material cost is high and cannot be widely used as a main structural member.

[0004] In the case of a weather-resistant steel material, when it is used without painting, a fine stable rust having excellent corrosion resistance is formed on the surface of the steel material within several years to ten years after use, and this stable rust causes the progress of corrosion of the steel material thereafter. It is a steel material that prevents it. For steel structures such as bridges, weather-resistant welded structural steel in consideration of weldability has been widely used mainly for bridges and buildings. However, "Design and construction guidelines for unpainted weatherproof bridges (revised plan): Ministry of Construction, Public Works Research Institute, Steel Club, Japan Bridge Construction Association, March 1993
As shown in the `` Month '', in areas where the amount of incoming sea salt particles is high, such as beach areas and areas where snowmelt is sprayed, the formation of stable rust with excellent protection is impeded by the salt attached to the steel surface, There was a problem that it was not suitable for unpainted use.

[0005] With regard to the improvement of the weather resistance of the weather-resistant steel in the seaside area, for example, in the invention of Japanese Patent Publication No. 56-9356, P (0.03-0.20%) is included, and the weldability is excellent.
Further, a steel material having excellent weather resistance in a corrosive environment involving seawater or a general atmospheric environment is disclosed. In addition, Japanese Unexamined Patent Publication
In the invention described in Japanese Patent No. 125839, the low Si-PC
It is stated that the addition of a composite oxide of Ca and Al is effective for the composite addition of u-Ni. In the invention of JP-A-3-51668, it is effective to finely disperse an oxide in a steel material to suppress a decrease in pH of the steel material surface. Further, in the invention described in Japanese Patent Application Laid-Open No. 7-242993, it is stated that the composite addition of Ni-Cr-Al is effective.

As described above, steel materials having excellent weather resistance in the beach area, which is a drawback of conventional weather resistant steel, have been developed.
The problem is that the weldability, which is essential as a welded structural steel material, is not always sufficient. Also, Japanese Patent Application Laid-Open No. 2-1
No. 25839, and Japanese Patent Laid-Open No. 5-516.
The inventions described in Japanese Patent Publication No. 68 are all inventions containing several percent of Ni-added steel as a basic component. These steel materials have excellent weather resistance in a beach area, but are applicable to the amount of flying sea salt particles. Since the limit was unknown, there was a problem that it was difficult to determine whether or not the application to an actual structure was possible.

Further, as a result of research and investigation by the inventors, for example, in the invention of Japanese Patent Publication No.
(0.03 to 0.20%), there is a problem that when a certain level of mechanical properties are required for the mechanical properties of the welded joint, heat input restriction and preheating during welding are indispensable.

On the other hand, although weather resistance is not mentioned, as a steel material having improved weldability, for example, Japanese Patent Publication No. Sho 60
In the invention of Japanese Patent No. 24576, notch in the weld heat-affected zone is obtained by adding a Ti oxide having a specified particle size and a composite of Ti oxide and Ti nitride to steel containing Mn or the like as a basic component. It is said that high toughness steel for welding with improved toughness can be obtained. In addition, Japanese Patent Publication No. 61-11724
In the invention of Japanese Patent Publication No. 5 (1994), a specific amount of C, Si, Mn,
Excellent in low-temperature notch toughness of weld heat affected zone by containing N, Al, S, B, and simultaneously containing both Ti oxide and specific composite of Ti nitride and MnS with specific particle size and specific number of particles. Steel can be obtained.

[0009] However, even if Cu, Ni and Ti are added to these steel materials, no mention is made of the weather resistance performance of the steel materials in the beach area and applications based on the performance. Further, there is no description that the preferable range of the added amount of Ni is different depending on the amount of flying sea salt particles in the environment in which it is used. Therefore, in order to apply these steel sheets excellent in weldability to non-painting use in the beach area, it was necessary to separately examine the influence of chemical components on the weather resistance in the beach area.

Further, in the case of a structure such as a steel frame structure, a steel material is coated with a refractory material such as rock wool in order to ensure sufficient strength even in the event of a fire so that the temperature of the steel material does not rise to 350 ° C. or more. Measures were required. In recent years, it has been allowed to simplify or omit the refractory coating according to the strength of steel at high temperatures. That is, when a steel material has a sufficient strength at a high temperature such as 600 ° C. (2/3 or more of the normal yield strength at room temperature), it is said that the refractory coating is omitted and bare use becomes possible.

In addition, it has excellent properties in the thickness direction for buildings, low yield ratio for earthquake-resistant buildings, and excellent toughness after cold working when manufactured by bending. Is required for the steel used.

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems. The present invention provides a stable rust in an environment where salt damage is a concern, exhibits excellent weather resistance, and is a steel for welded structures. The present invention provides a method for producing a steel material having sufficient base material strength, toughness, weldability and fire resistance.

[0012]

Means for Solving the Problems The present inventors have paid attention to the fact that it is difficult for a weather-resistant steel to form a protective rust film having excellent corrosion resistance in an air environment with a lot of flying sea salt particles. We have been conducting research based on the composition of low alloy steels. As a result, Cr, which is effective in improving the weather resistance of steel in inland areas where the amount of flying sea salt particles is small, has a remarkable effect on the weather resistance in high flying sea salt particle environments such as beach areas and areas where snowmelt is sprayed. Has been found to have a significant adverse effect.

Further, as a result of examining the improvement of weather resistance with respect to various alloy elements, it was found that the composite addition of Ni of 1% or more of Cu significantly affects the formation of stable rust in a beach area. Furthermore, the application limit of the Cu-Ni system (the upper limit of the annual average amount of sea salt particles that can be sufficiently formed for stable rust)
Has been found to be substantially controlled by the amount of Ni added. Ni
Since is an expensive additive element, this finding has made it possible to provide the most economical steel material according to the severity of the corrosive environment.

Next, the characteristics of the Cu-Ni base material were examined. Generally, the knowledge that a steel material excellent in strength and toughness of a base material can be obtained by reducing C, P, and S and performing deoxidation with Si, Mn, and Al. Further, even in the case of the steel of the present invention to which 1% or more of Ni is added, it is preferable to add Ti for the purpose of improving weather resistance. In this case, by controlling the N content relative to the added Ti, TiN can be finely dispersed and precipitated in the ferrite phase,
It has been found that it is possible to reduce the toughness deterioration of the weld heat affected zone caused by the welding condition with a large heat input and to further improve the beach weather resistance.

Next, as a result of various studies on the influence of chemical composition and manufacturing conditions on the strength at 600 ° C. of the structural steel material used as-rolled, the end-of-rolling temperature of the Mo-added steel was markedly higher than the high-temperature strength. Found to affect.

Furthermore, it has been found that the steel material completed by the present invention has a far superior coating durability even in the state of being coated with corrosion protection such as painting or thermal spraying, as compared with ordinary steel or conventional weathering steel. did. This means that even if corrosion of the base steel sheet progresses from a local defect in the film, the generated rust is dense and has excellent protection, so that further peeling of the anticorrosion film and the progress of under-film corrosion are suppressed. Inferred.

The present invention is based on the above findings,
The summary is as follows. (1) By weight%, C: 0.01 to 0.15%, Si: 0.05 to 0.55%, Mn: 0.3 to 2.0%, Cu: 0.30 to 1.00% , Ni: 1.0 to 5.5%, Mo: 0.1 to 1.5%, Al: 0.005 to 0.090%, N: 0.0010 to 0.0070%, further inevitable Specifically, a steel slab containing P: 0.030% or less, S: 0.010% or less, and Cr: 0.1% or less, with the balance being Fe and unavoidable impurities, was produced in a temperature range of 1050 to 1250 ° C. After reheating, hot rolling is completed in a temperature range of 750 to 1000 ° C., and the steel is left to cool after rolling or cooled at an average cooling rate of 1 ° C./sec or more. Method for producing resistant steel.

(2) The steel slab further contains Ti:
The composition according to (1), comprising 0.005 to 0.02%, wherein the ratio of Ti to N is 2.0 ≦ Ti / N ≦ 3.5. Method for producing weldable weatherable steel.

(3) The steel slab further contains Ca:
0.0005-0.0100%, Mg: 0.0005-
0.0100%, REM: 0.0005 to 0.0100
% Of one or more of the above-mentioned (1) or (2), the method for producing a highly weldable and weather-resistant steel excellent in fire resistance.

(4) The steel slab further contains W:
The method for producing a highly weldable and weatherable steel excellent in fire resistance according to any one of the above (1) to (3), wherein the steel contains 0.1 to 1.0%.

(5) The steel slab is one type of Nb: 0.002 to 0.04%, V: 0.01 to 0.1%, and B: 0.0003 to 0.0050% in weight%. Alternatively, the method for producing a high weldability weatherable steel excellent in fire resistance according to any one of the above (1) to (4), comprising two or more types.

(6) Any one of the above (1) to (5), wherein normalizing or tempering is performed after cooling after rolling or cooling at an average cooling rate of 1 ° C./sec or more. The method for producing a highly weldable and weatherable steel having excellent fire resistance according to the above item.

(7) The above-mentioned (1) to (6), wherein the surface is anticorrosion-coated with an organic resin, metal or inorganic substance.
The method for producing a highly weldable weatherable steel excellent in fire resistance according to any one of the above.

[0024]

Embodiments of the present invention will be described below. C: C is necessary to secure the strength as a structural material, and is added in an amount of 0.01% or more. However, if it is contained in excess of 0.15%, the toughness of the matrix of the welded joint part is reduced. Since the weldability is impaired, the upper limit is 0.1
5%.

Si: Si is an essential element for deoxidation,
Although 0.05% or more is added, if it exceeds 0.55%, high-carbon island-like martensite is formed in the welded portion and the weldability is impaired, so the upper limit was made 0.55%.

Mn: Mn is used for deoxidation, strength adjustment, and fixing S as an impurity as MnS, to prevent hot embrittlement due to S, and to form MnS which is a finely dispersed precipitation site of TiN described later. 3% or more
%, The weldability is impaired, so the range was made 0.3 to 2.0%.

Cu: Cu elutes together with Fe in steel, suppresses crystallization and coarsening of the rust particles during formation of the rust layer, and keeps the rust dense. Therefore, weather resistance in an environment with a lot of flying sea salt particles. Element is essential to improve the properties, 0.30%
The above addition is effective. The more the effect, the better,
If the content exceeds 1.0%, deterioration in weldability and cracking during hot working pose problems, so the range was set to 0.3 to 1.0%. 0.30 to 0.50% if weldability is considered with priority
Is preferred.

Ni: Ni is an element important for improving the weather resistance of the seashore. If Ni is contained in the rust layer in an amount of 0.5% or more, it is eluted together with Fe in the steel, and is substantially uniformly contained in the rust layer. By suppressing the penetration of Cl ions adhering to the surface of the layer as flying sea salt particles into the rust layer / ground iron interface, and suppressing the crystallization and coarse growth of the rust particles by setting the inside of the rust layer to a low Cl environment. Has the effect of maintaining the fineness of
Further, as the amount of Ni added to the steel increases, the corrosion resistance of the steel material is improved in a dry and wet corrosion environment in an aqueous solution containing Cl ions.

According to the study of the inventors, it was found that 0.1% was contained in the rust layer.
In order to contain 5% or more of Ni, 1.0% or more of Ni
The addition proved to be effective. Also, 5.5
%, The cost increases, so 1.0% to 5.5%
And Preferably, if the addition amount of Ni is further limited according to the amount of flying sea salt particles in the use environment, it is clear that both economic efficiency and weldability are favorable. When the annual average amount of incoming sea salt particles in the use environment is 0.05 to 0.2 mg / 100 cm ² / day, it is preferable to add 1.0 to 2.5% Ni. 0.2 ~
In the case of 0.5 mg / 100 cm ² / day, it is preferable to add 2.5 to 3.5% of Ni. 0.5~0.8mg / 100cm ² / d
In the case of ay, addition of 2.5 to 5.5% Ni is preferable, and
1.0% to 3.5 when exceeding 8 mg / 100 cm ² / day
The use of anticorrosive coatings such as painting on steels with Ni-% is preferred.

Mo: Mo is an important element for improving fire resistance.
When the value of Mo is 0.1% or more, the improvement of the fire resistance is remarkable, and when it exceeds 1.5%, the effect of improving the fire resistance is saturated. Further, if added 0.1% or more, molybdic acid is generated after being eluted from the base iron, and the voids generated between the rust particles which are adsorbed and agglomerated on the rust particle surface are made to have a negative charge excess, such as Cl ions and sulfate ions. It has the effect of suppressing the penetration of anions to the ground iron interface and further improving the amount of critical sea salt particles that form weather resistance or stable rust, but the effect is 1.5%.
Saturated with In addition, Mo has an effect of increasing the strength of the base material, and is added in an amount of 0.1% or more to a thick material, a high-strength material, and the like. Generate island martensite etc.
Significantly reduces joint toughness.

Al: Al is 0.005% as a deoxidizing element
Although the above is necessary, if the amount of addition is large, inclusions increase, so the upper limit is made 0.090%.

Ti, N: If Ti is added to steel in a certain amount or more and in a certain ratio with N as needed, TiN
As finely dispersed and precipitated in the ferrite phase, and promotes a reaction in which Cu and Ni in steel are eluted uniformly with Fe,
It promotes the formation of a dense rust layer that suppresses the penetration of Cl ions. To obtain this effect, 0.0010% or more of N and 0.0050 or more of Ti must be added. Also, 0.
Addition of 02% or more of Ti or 0.0070% or more of N causes precipitation of coarse precipitates and deteriorates toughness.
02% and 0.0010 to 0.0070%.

If Ti / N is less than 2.0, Ti
If the N particles do not sufficiently precipitate in the ferrite phase and exceed 3.5, coarse agglomeration of the TiN particles occurs and a fine and dispersed state cannot be obtained, and improvement in weather resistance and weldability cannot be obtained. / N was set to 2.0 to 3.5.

P: P is an element effective for improving the weather resistance, but if it is contained more than 0.030%, the weldability is deteriorated. Therefore, the range is set to 0.030% or less. In particular, when sufficiently securing large heat input welding characteristics, the content is preferably 0.005% or less.

S: S is an unavoidable impurity that deteriorates the toughness and weather resistance of the steel material, and the smaller the content, the better. Especially 0.
If the content exceeds 0.01%, inclusions increase and the toughness of the ferrite phase in the joint portion is remarkably deteriorated. Therefore, the range is set to 0.01% or less. In order to secure sufficient weather resistance, 0.005% or less is preferable.

Cr: Since Cr is a metal lower than Fe,
Addition of a few% impairs the weather resistance in an environment with a lot of sea salt particles and also impairs the weldability, so the smaller the better, the better. If it is 0.1% or less, the inhibitory effect on weather resistance and weldability can be almost ignored, so the range is set to 0.1% or less.

Ca, Mg: When Ca and Mg are added as required, they are present as oxides or sulfides in the steel and eluted from the base iron to form Ca (OH) ₂ or M
Since g (OH) _{2 and the} like are formed to suppress the growth of rust colloid particles at the initial stage of formation, fine precipitation and aggregation of rust particles are promoted. The effect is effective when added at 0.0005% or more and saturates at 0.0100%. Therefore, the range of those elements is set to 0.0005% to 0.0100%.

REM: REM is also added as needed, and it is effective in controlling the form of inclusions to improve the tensile properties in the sheet thickness direction, reducing lamellar tears and improving low-temperature toughness. For this reason, the content is made 0.0005% or more, but if the added amount is too large, the inclusion increases, so the upper limit was made 0.0100%.

W: If W is added in an amount of 0.1% or more as required, it elutes from the base iron, and forms tungstic acid.
The void formed between the rust particles adsorbed and agglomerated on the rust particle surface is made an excess of negative charge to suppress the penetration of anions such as Cl ions and sulfate ions to the ground iron interface, thereby forming weather resistance or stable rust. There is an action to further increase the amount of the sea salt particles at the limit, but the effect is saturated at 1.0%. Therefore, when priority is given to improving the weather resistance, 0.1 to 1.
0% is preferred.

Nb, V, B: Nb, V, B have the effect of improving the strength of the base material.
Nb is added in an amount of 0.002% or more, V is added in an amount of 0.01% or more, and B is added in an amount of 0.0003% or more. However, if any element is added beyond the following range, the toughness deteriorates.
04% or less, V: 0.1% or less and B: 0.0050
% Or less.

Anticorrosion coating: When the steel of the present invention is coated with an organic resin, sprayed with metal, and plated to be used in an atmospheric environment where there is a concern about salt damage, a similar anticorrosion coating is applied to ordinary steel and conventional weather-resistant steel. It shows much better weather resistance and durability than when it is applied. As the organic resin, epoxy resin type, phthalic acid type, urethane resin type, vinyl butyral resin type and other resin types all improve coating durability. In addition, the metal coating exhibits excellent weather resistance by Zn, Zn-Al, Al plating, thermal spraying, and the like. In any case, when corrosion of the ground iron progresses from microscopic or macroscopic defects of the coating layer, a dense rust layer containing Ni, Cu, etc. is formed, and the progress of corrosion thereafter is suppressed. . Further, a rust stabilizing treatment film as a technique for preventing rust juice from flowing out of weather-resistant steel, which is disclosed in JP-A-53-65232, JP-A-55-97477, and JP-A-55-97478, is used. When applied to the steel of the present invention, stable rust is formed while preventing initial rust juice even in a high sea salt particle environment.

Next, the manufacturing conditions will be described. Steel having the above chemical composition is melted in a converter and an electric furnace,
It is obtained by performing ladle refining or vacuum degassing if necessary, and is usually formed into a slab by lumping with a mold or a one-way solidification mold. Further, the slab may be directly manufactured by a continuous casting method. The soaking and reduction in the mass may be any. That is, the slab may be cooled and then soaked, or may be charged into a soaking furnace with a hot piece as a lump. After soaking at 1000 to 1320 ° C., a slab is formed by rolling or forging. Slab thickness is 1.3 ~ of product thickness
About 2.5 times is preferable. Heating temperature before final rolling is 10
50 ° C. or higher. However, if the temperature exceeds 1250 ° C., the austenite grains become too coarse, and it becomes difficult to reduce the size by rolling, so that the temperature is preferably 1250 ° C. or less. The rolling end temperature is 1000 as described later.
℃ or lower is required.

On the other hand, the lower limit is set to 750 ° C. or more. However, at a rolling end temperature of less than 750 ° C., the transformation progresses remarkably, the effect of improving the fire resistance is small, and conversely, the deformation resistance in rolling becomes large. This is because it is not preferable from the viewpoint of the rolling mill load. The cooling after the rolling is natural cooling or accelerated cooling by water cooling. Thus, accelerated cooling requires cooling at a cooling rate of 1 ° C./sec or more. This is 1 ° C / s
If the cooling rate is less than ec, the amount of ferrite becomes too large, or lattice defects such as dislocations introduced by rolling are greatly recovered, which hinders improvement in room temperature strength and fire resistance. After the rolling, heat treatment such as normalizing, tempering, quenching and tempering is performed as necessary from the viewpoint of the required strength and toughness of the steel material. Although the applicable product is mainly a thick plate, the present invention is also applicable to an H shape, a continuous hot rolled (hot) material, a thick plate material for a steel pipe or a column, and a hot material for a column. Further, the hot material undergoes a cooling step after cooling by hot rolling, or accelerated cooling and coil winding.

The present invention relates to the improvement of the fire resistance which is the object of the present invention, and relates to 0.08% C-1.3% Mn-3.0% Ni-0.
FIG. 1 shows the effect of the end-of-rolling temperature on the fire resistance at 600 ° C. of the as-rolled 5% Mo steel. After heating and holding at 600 ° C. for 15 minutes, the sample was deformed at a tensile rate of 0.15% / min., And the strength at a plastic strain of 0.2% was determined as the fire resistance. As shown in FIG. 1, at 1000 ° C. or lower, the lower the rolling finishing temperature, the higher the fire resistance. At a rolling finishing temperature exceeding 1000 ° C., no change in fire resistance is observed.

The improvement in the fire resistance by the low-temperature finishing is related to the value of% Mo. As shown in FIG.
In the range of 00 to 750 ° C, the improvement of the fire resistance is remarkable when the value of% Mo is 0.1% or more, and when it exceeds 1.5%, the effect of improving the fire resistance is saturated. Thus, the reason why the refractory strength is improved by the low-temperature rolling finish temperature in the steel containing Mo is that lattice defects such as dislocations introduced into the steel in the low-temperature finish also exist in the structure after cooling, and the refractory strength is reduced. This is because during the temperature rise during the test, carbide mainly composed of Mo nucleates and precipitates on the lattice defect to hinder the movement of dislocation.
It is necessary both to contain and to finish rolling at a low temperature.

When a steel material is provided with an anticorrosion coating,
The material is limited with sufficient consideration of the fire resistance temperature, etc., and the method of applying it is a conventional method such as a roll coater or curtain coater for organic resin coating, electroplating or hot-dip plating for metal coating, and thermal spraying for inorganic substances. Are used alone or in combination.

[0047]

EXAMPLE A steel having the chemical composition shown in Table 1 was melted, and 1150
C., hot rolling was completed at 900.degree. C., and after rolling, the steel plate was allowed to cool to produce a steel plate having a thickness of 25 mm. Medium heat input welding (40
(kJ / cm) and large heat input welding (150 kJ / cm) to produce an actual joint, and the toughness of the welded joint was evaluated by the absorbed energy of the Charpy impact test at -40 ° C. In addition, the weather resistance of the prototype steel in a high sea salt particle environment was subjected to exposure tests at four locations in the seaside area of Futtsu City, Chiba Prefecture for 1, 3 and 7 years. Exposure points were at point V: 5 m (1.
3mdd), point W: 50m (0.8mdd), point X: 20
0 m (0.5 mdd), point Y: 800 m (0.2 mdd).

The weather resistance of the steel and the amount of sea salt particles in the application limit were determined by the following evaluation. That is, rust appearance rating evaluation,
The three items are the rust ion permeation resistance measurement and the average sheet thickness reduction obtained from the corrosion amount.

The condition of rust stabilization was evaluated in terms of the appearance score of the rust layer, where the score was 1 to 4, 4 was the best and stable rust was formed, and 1 was a layered exfoliated rust. The score was evaluated using an index indicating the state where prevention of progress could not be expected. In rust ion permeation resistance measurement,
The ion transmission resistance value of the rust was measured by the AC impedance method, and it was determined that a stable rust was formed at 3 kΩ or more.
The average thickness reduction is 5 from the corrosion amount-time curve at four locations.
Extrapolation of the estimated thickness reduction after 0 years on the log-log plot of the average thickness reduction and time was calculated. As a guideline, the amount of sea salt particles that could be applied was determined from the results of the exposure tests at four locations.

Table 2 shows the characteristic values of the steel. The rust score and the ion permeation resistance are evaluation results at the point Y where the amount of incoming sea salt particles is the smallest among the four places. Comparative Examples A1 to A1
1 is C, Si, Mn, P, Cu, Ni, A
Since l, Ti, and N are excessive, the joint toughness value is low, and the large heat input weldability is poor.

Further, Comparative Examples C1 to C7 are C, S, C
Since u, Ni, and Cr are out of the range of the present invention, the weather resistance is insufficient as compared with the examples of the present invention. C8 is a conventional weather-resistant steel (JIS G3141 weather-resistant steel plate for welded structure SM
A490), it can be seen that no stable rust is formed even at the point Y where the amount of flying sea salt particles is the smallest among the four places of the points V to Y, which is not suitable for unpainted use.

D1 to D40 are the results of the steel of the present invention. In the environment of the point Y, the rust score, the ion permeation resistance and the estimated amount of corrosion after 50 years were excellent in all the test steels. It turns out that it has the property. Also, 4
The critical sea salt particle amount estimated from the exposure test results at
It can be seen that they can be roughly classified by the amount of i added. That is, the annual average amount of incoming sea salt particles in the environment is (1) 0.05 to 0.2.
In the case of mg / 100 cm ² / day, it is preferable to add 1.0 to 2.5% Ni. ^{(2) 0.2~0.5mg / 100cm 2 /} day
In this case, it is preferable to add 2.5 to 3.5% Ni. (3)
In the case of 0.5 to 0.8 mg / 100 cm ² / day, 2.5 to
5.5% Ni addition is preferred.

In Table 3, the prototype steel was coated with an organic resin, and the surface of the steel was cut with a 70 mm cross-cut wound with a cutter knife and exposed to a point V (annual average flying sea salt particle amount: 1.3 mdd) for 5 years. The maximum coating blister width from the cross cut (the maximum development width of corrosion under the coating) was evaluated, and the weather resistance of the painted prototype steel in the environment was evaluated. The test materials coated in Comparative Examples C1 to C8, in which the components necessary for improving the weather resistance are out of the range of the present invention, have a maximum swelling width of at least 32 mm at the minimum. Samples should be at most 14
mm, which indicates that the steel of the present invention has excellent weather resistance even when used after being coated. In addition, the present invention is at 600 ° C.
All of the high-temperature yield strengths at 70% or more of the yield strength at room temperature indicate excellent fire resistance.

Next, steel having the chemical composition shown in Table 4 was melted,
Under the conditions of heating, rolling and cooling shown in Table 5, H-shaped (J47) and hot materials (J48) were manufactured mainly from thick steel plates. An actual joint was manufactured by medium heat input welding (40 kJ / cm) and large heat input welding (150 kJ / cm), and the toughness of the welded joint was evaluated by the absorbed energy of a Charpy impact test at -40 ° C. In addition, the weather resistance of the prototype steel in a high sea salt particle environment was subjected to exposure tests at four locations in the seaside area of Futtsu City, Chiba Prefecture for 1, 3 and 7 years. Exposure points were measured at the shore separation distance (average amount of incoming sea salt particles) at each point V: 5.
m (1.3 mdd), point W: 50 m (0.8 mdd), point X: 200 m (0.5 mdd), point Y: 800 m (0.
2mdd).

The weather resistance of the steel and the amount of sea salt particles in the application limit were determined by the following evaluation. That is, rust appearance rating evaluation,
The three items are the rust ion permeation resistance measurement and the average sheet thickness reduction obtained from the corrosion amount.

In terms of the appearance of the rust layer, the evaluation of the appearance of the rust layer was evaluated as 1 to 4, where 4 was the best and stable rust was formed, and 1 was a layered exfoliated rust. The score was evaluated using an index indicating the state where prevention of progress could not be expected. In rust ion permeation resistance measurement,
The ion transmission resistance value of the rust was measured by the AC impedance method, and it was determined that a stable rust was formed at 3 kΩ or more.
The average thickness reduction is 5 from the corrosion amount-time curve at four locations.
Extrapolation of the estimated thickness reduction after 0 years on the log-log plot of the average thickness reduction and time was calculated. As a guideline, the amount of sea salt particles that could be applied was determined from the results of the exposure tests at four locations.

Tables 5 and 6 show the characteristic values of the steel. The rust score and the ion permeation resistance are evaluation results at the point Y where the amount of incoming sea salt particles is the smallest among the four places. In Comparative Examples H11 and H21, since Mo is out of the range of the present invention, H11 has a low ratio of the yield strength at 600 ° C. to the yield strength at room temperature.
H21 has low joint toughness. HJ41 to 44 are examples in which the conditions of heating, rolling and cooling are out of the range of the present invention. HJ41 has a high heating temperature and low base material and joint toughness. HJ42 has a high rolling finish temperature and a low 600 ° C yield strength to a normal temperature yield strength. HJ43 has an undesirably low rolling finish temperature and high deformation resistance during rolling, which is undesirable from the viewpoint of rolling mill load. HJ44 has too low a cooling rate of accelerated cooling, and the ratio of the yield strength at 600 ° C. to the yield strength at room temperature is low.

J11 to 81 and J42 to 48 are the results of the steel of the present invention. In the environment of the point Y, the rust score, the ion permeation resistance, and the estimated corrosion amount after 50 years were obtained in the environment of the point Y.
It has excellent weather resistance in any of the evaluations. In addition, steel is coated with an organic resin, and the surface is cut with a cutter knife.
Exposed for 5 years to point V (average annual sea salt particle amount 1.3 mdd) with a 70 mm cross-cut wound and evaluated the maximum coating blister width from cross-cut (maximum width of under-coat corrosion). Then, the weather resistance of the painted prototype steel in the environment was evaluated. It is at most 6 mm, which indicates that the steel of the present invention has excellent weather resistance even when used after being coated. In addition, the high-temperature yield strength at 600 ° C. is all 70% or more of the yield strength at room temperature, which indicates that it has excellent fire resistance.

Further, the steel of the present invention has a yield ratio (Y
R) were all 75% or less, and as a result of performing a tensile test in the thickness direction, all the aperture values showed high values of 60% or more. Further, as a result of evaluating the toughness after 10% tensile strain was applied to all of the steels of the present invention, the base metal toughness was -40 ° C., the absorbed energy was 250 J or more, and there was almost no decrease in toughness.

As described above, the steel of the present invention has a low yield ratio and good properties in the thickness direction and good toughness after cold working. From the room temperature strength level, J71 is 60 kg steel, J81 is 40 kg steel, and all others are 50 kg steel.

[0061]

[Table 1]

[0062]

[Table 2]

[0063]

[Table 3]

[0064]

[Table 4]

[0065]

[Table 5]

[0066]

[Table 6]

[0067]

As is clear from the above embodiment, the present invention is applicable to the construction of an air environment, such as a beach area or an area where snowmelt is sprayed, where there is a possibility of salt damage in a high flying sea salt particle environment.
Low yield ratio, which is an essential property for various buildings in the fields of civil engineering and marine structures, etc., with excellent weldability and fire resistance, thickness direction properties, toughness after cold working, and stable rust even with high flying sea salt particles To provide a steel material having both excellent weather resistance and excellent weather resistance even in unpainted use and in painted use. Can be reduced. It can be said that the effect is extremely remarkable in industry.

[Brief description of the drawings]

FIG. 1: 0.08% C-1.3% Mn-3.0% Ni-
It is a figure which shows the influence of the rolling end temperature on the refractory strength at 600 degreeC of the as-rolled material of 0.5% Mo steel.

FIG. 2 is a diagram showing the dependence of fire resistance on the amount of Mo added in a low-temperature finished material.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasuji Tanabe 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Takeshi Tsuzuki 5-3 Tokai-cho, Tokai-shi, Aichi Pref. Nippon Steel Corporation Nagoya Works (72) Inventor Minoru Ito 20-1 Shintomi, Futtsu-shi, Chiba F-term in Nippon Steel Corporation Technology Development Headquarters AA19 AA20 AA21 AA22 AA24 AA27 AA29 AA31 AA35 AA36 AA37 AA40 BA01 CA02 CA03 CC02 CC03 CC04 CD02 CD05 CF02 CF03

Claims (7)

[Claims] 1. C .: 0.01-0.15%, Si: 0.05-0.55%, Mn: 0.3-2.0%, Cu: 0.30-1. Ni: 1.0 to 5.5%, Mo: 0.1 to 1.5%, Al: 0.005 to 0.090%, N: 0.0010 to 0.0070%, Inevitably, a steel slab containing P: 0.030% or less, S: 0.010% or less, and Cr: 0.1% or less, with the balance being Fe and unavoidable impurities, was heated to a temperature of 1050 to 1250 ° C. High-welding with excellent fire resistance, characterized in that hot-rolling is completed in a temperature range of 750 to 1000 ° C after reheating in a zone, and then cooled after rolling or cooled at an average cooling rate of 1 ° C / sec or more. Method for producing weatherable steel. 2. The steel slab further comprises, by weight%, Ti: 0.005 to 0.02%, and the ratio of Ti to N is 2.0 ≦ Ti / N ≦ 3.5. The method for producing a highly weldable weatherable steel having excellent fire resistance according to claim 1. 3. The steel slab further comprises, by weight, one of the following: Ca: 0.0005 to 0.0100%, Mg: 0.0005 to 0.0100%, REM: 0.0005 to 0.0100%. 2. The method according to claim 1, wherein the composition contains at least two types.
Or the method for producing a highly weldable weatherable steel excellent in fire resistance according to 2. 4. The high fire-resistant steel according to claim 1, wherein the billet further contains W: 0.1 to 1.0% by weight. Method for producing weldable weatherable steel. 5. The steel slab further comprises one or more of Nb: 0.002 to 0.04%, V: 0.01 to 0.1%, B: 0.0003 to 0.0050% by weight. 2. The method according to claim 1, wherein the composition contains at least two types.
5. The method for producing a highly weldable weatherable steel having excellent fire resistance according to any one of the above items 4 to 4. 6. The method according to claim 1, wherein after rolling, the steel sheet is cooled or cooled at an average cooling rate of 1 ° C./sec or more, followed by normalizing, tempering or quenching / tempering. The method for producing a highly weldable and weatherable steel having excellent fire resistance according to the above item. 7. The method for producing a highly weldable weatherable steel having excellent fire resistance according to claim 1, wherein the surface is coated with an organic resin, a metal or an inorganic material for corrosion protection.