JP2001164334A - Steel for structure purpose excellent in corrosion resistance and corrosion fatigue resistance and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the corrosion resistance of the steel while securing its weldability without depending on alloy elements effective for corrosion resistance and to remarkably improve the corrosion resistance in the case the above alloy elements are added. SOLUTION: As to this steel for structural purpose excellent in corrosion resistance and corrosion fatigue resistance, the crystal boundaries and crystal sub-boundaries are provided with a cementitic phase of <=0.5 μm, moreover, ferrite occupies by >=95%, also, the hardness H of the ferritic structure satisfies a prescribed equation, and the intensity ratio of the (100) plate in the texture parallel to the rolling face is >=1.5, and in the method for producing the same, a steel stock having specified components is heated to the Ac3 point or more to enter C into solid solution, in this state, before or in the process of hot working, the surface layer region of the steel sheet is rapidly cooled at a cooling rate of >=3 deg.C/set to a temperature by which the ferritic fractional ratio reaches >=50%, and after that, the surface layer region is recuperated, hot rolling is stated or restarted from the temperature of (the Ac1 point-150 deg.C) or more and is finished in the temperature range of (the Ac1 point+50 deg.C) to the Ac3 Point, and successively, the surface layer region is cooled without being recuperated to the Ac3 point or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a high-strength steel bar, wire rod,
Steel for machine structural use, or shipbuilding, construction, bridges / piers, tanks,
The present invention relates to a structural steel excellent in corrosion resistance and corrosion fatigue resistance applied to a welded structural steel and the like for a large steel structure such as a pressure vessel, an marine / port structure, and a chemical plant, and a method for producing the same.

[0002]

BACKGROUND OF THE INVENTION Corrosion, alone or as a starting point for fatigue, unstable and brittle fractures, causes severe damage to steel structures. Corrosion and damage cases originating from corrosion account for a large proportion of the damage cases of the entire steel structure, and therefore its improvement is extremely important.

[0003] The use environment of steel structures is wide, but corrosion and corrosion fatigue are particularly problematic in seawater environments and other water environments containing chlorine or chloride. On the other hand, for example, the 159th Nishiyama Memorial Lecture by the Iron and Steel Institute of Japan (19
96) p. As summarized in No. 123, heretofore, steel materials having improved seawater resistance by adding and increasing alloy components such as Cu, Ni, Cr, and P, including Mariner steel, have been developed. Further, it has been found that the corrosion resistance of steel is determined by the alloy composition in the steel and does not depend on the structure of the steel. Therefore, in order to impart corrosion resistance to steel, it is necessary to add alloying elements as described above, but this increases the cost as a structural steel for welding (hereinafter simply referred to as structural steel) and increases the amount of alloy. There was a problem that the weldability and workability required for structural steel were reduced due to the inclusion of the element.

[0004]

From such a background,
An object of the present invention is to improve the corrosion resistance of structural steel, particularly in a water environment containing chlorine or chloride, by controlling the structure of the steel material. That is, it is possible to improve the corrosion resistance while securing the weldability without relying on the alloy element effective for the corrosion resistance as in the related art, and to greatly improve the corrosion resistance when the alloy element is added. Make it an issue.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and the gist thereof is as follows.

(1) C: 0.04 to 0.2% by weight
5%, Si: 0.01-1.0%, Mn: 0.3-2.
0%, S: a steel or steel sheet having a component of 0.01% or less, the balance being iron and unavoidable impurities, and the diameter or steel sheet of the steel from the surface layer of the steel or from the front and back layers of the steel sheet, respectively. Has a cementite phase of 0.5 μm or less at the grain boundaries and / or sub-grain boundaries in a surface layer region of 5% or more of the thickness of ferrite, and 95% or more of ferrite, and has a hardness H of the ferrite structure. Is characterized by satisfying the following formula (1), and further having a (100) plane strength ratio of a texture parallel to the rolled surface of 1.5 or more, for a structure excellent in corrosion resistance and corrosion fatigue resistance. steel. H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d) (1) where [Ceq%] = C% + Si% / 24 + Mn% / 6
Where C%, Si% and Mn% are C and S, respectively.
i is the weight% of Mn, and d is the average equivalent circle diameter of ferrite.

(2) Al: 0.00% by weight
Contains 5 to 0.6% of component, and Nb: 0.005
0.1%, Ti: 0.005 to 0.05%, Ta:
Structural steel excellent in corrosion resistance and corrosion fatigue resistance according to the above (1), characterized by containing one or more kinds of 0.005 to 0.05%.

(3) Cu: 0.05% by weight
1.0%, Ni: 0.1 to 2.0%, Cr: 0.03
3.0%, Mo: 0.05-1.0%, V: 0.01
~ 0.4%, B: 0.0002-0.002%, P:
0.15% or less, Ca: 0.0001 to 0.02%, M
g: 0.0001-0.02%, REM: 0.001%
(1) or (2), wherein the ferrite structure has a hardness H satisfying the following formula (2): Structural steel with excellent corrosion resistance and corrosion fatigue resistance. H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d) (2) where [Ceq%] = C% + Si% / 24 + Mn% / 6
+ (Cu% + Ni%) / 15, where C%, Si
%, Mn%, Cu%, and Ni% are C, Si, and M, respectively.
The weight percent of n, Cu, and Ni, and d is the average equivalent circle diameter of ferrite.

(4) C: 0.04 to 0.2% by weight
5%, Si: 0.01-1.0%, Mn: 0.3-2.
0%, S: A steel or a steel plate material having a component of 0.01% or less, the balance being iron and unavoidable impurities, heated to _three or more Ac to form a solid solution with C, and Before or during processing, the surface layer area of 5% or more of the steel diameter or steel sheet thickness from the surface layer at that time is rapidly cooled at a cooling rate of 3 ° C / sec or more to a temperature at which the ferrite fraction becomes 50% or more. Then, in the process of reheating the surface layer region, (Ac
By hot working start or resume from _one point -150 ° C.) or higher temperatures, (Ac ₁ point + 50 ° C.) hot working ends in the temperature range of to Ac ₃ point, Ac ₃ the surface layer region subsequently Cooling is performed without reheating to a temperature of more than a point. In the surface layer region, a cementite phase having a grain size of 0.5 μm or less is present at a grain boundary and / or a sub-grain boundary, and ferrite occupies 95% or more. The hardness H of the structure satisfies the following expression (3), and further, the (100) plane strength ratio of the texture parallel to the rolled surface has a strength of 1.5 or more. Method for producing structural steel with excellent properties. H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d) (3) where [Ceq%] = C% + Si% / 24 + Mn% / 6
Where C%, Si% and Mn% are C and S, respectively.
i is the weight% of Mn, and d is the average equivalent circle diameter of ferrite.

(5) Al: 0.00% by weight
Contains 5 to 0.6% of component, and Nb: 0.005
0.1%, Ti: 0.005 to 0.05%, Ta:
The method for producing a structural steel having excellent corrosion resistance and corrosion fatigue resistance according to the above item (4), comprising one or more of 0.005 to 0.05%.

(6) After the completion of hot working, accelerated cooling or direct quenching is performed at a cooling rate of 5 ° C./sec or more without successively reheating the surface layer to _three or more Ac points. The method for producing a structural steel having excellent corrosion resistance and corrosion fatigue resistance described in the above item (4) or (5).

(7) The method for producing a structural steel having excellent corrosion resistance according to the above (6), wherein tempering is performed subsequently after the completion of the accelerated cooling or direct quenching.

(8) By weight%, Cu: 0.05
1.0%, Ni: 0.1 to 2.0%, Cr: 0.03
3.0%, Mo: 0.05-1.0%, V: 0.01
~ 0.4%, B: 0.0002-0.002%, P:
0.15% or less, Ca: 0.0001 to 0.02%, M
g: 0.0001-0.02%, REM: 0.001%
The ferrite structure according to any one of the above (4) to (7), wherein the ferrite structure contains at least one kind or two or more kinds and the hardness H of the ferrite structure satisfies the following expression (4). A method for producing a structural steel excellent in corrosion resistance and corrosion fatigue resistance according to any of the above. H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d) (4) where [Ceq%] = C% + Si% / 24 + Mn% / 6
+ (Cu% + Ni%) / 15.
%, Mn%, Cu%, and Ni% are C, Si, and M, respectively.
n is the weight percent of Cu and Ni, and d is the average equivalent circle diameter of ferrite.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of detailed examination of the corrosion resistance of various steels in a water environment containing chlorine, in a wet environment, and in a wet / wet repeated environment, the present inventors have found that the steel structure is mainly composed of ferrite having a low dislocation density. A cementite phase or a cementite phase having a (100) plane strength ratio of not less than 1.5 and a ferrite grain boundary and / or sub-grain boundary of 0.5 μm or less and one of Nb, Ti, and Ta or It has been found that the corrosion resistance of steel is greatly improved by precipitating two or more carbonitride phases.

The mechanism by which the ferrite has a low dislocation density to improve the corrosion resistance is not clear, but a structure having a high dislocation density has a higher strain energy due to the dislocation, and is therefore activated even during a corrosion reaction. It is presumed that there is not.

Ferrite has a structure with a relatively low dislocation density, but since there is also a structure with a high dislocation density, such as a processed ferrite, it is necessary to have a ferrite structure and a low dislocation density.

For this reason, first, it is necessary to evaluate the dislocation density inside the ferrite. It is extremely difficult to quantitatively measure the dislocation density.

Therefore, there is a method of evaluating the relationship between the dislocation densities inside ferrite by utilizing the fact that the relationship between the grain size and the yield stress changes depending on the characteristics of the grain boundaries. This means that in a normal ferrite structure formed by austenite / ferrite transformation, there is a certain relationship between the grain size defined by the ferrite grain boundaries and hardness, and can be related to the Hall-Petch relationship. However, as the dislocation density inside the ferrite increases, the relationship between grain size and hardness changes. Focusing on this experimental fact, the dislocation density inside ferrite was evaluated. For this, first, it is necessary to measure the crystal grain size. In the present invention, not only the grain boundaries due to the normal austenite / ferrite transformation but also the grain boundaries due to the work recrystallization are targeted, so that the appearance of the grain boundaries due to the normal nital corrosion is insufficient. Therefore, in order to make clear grain boundaries appear even in the processed structure, we found that Marshall's reagent, which is a corrosive solution mainly composed of oxalic acid aqueous solution, hydrogen peroxide aqueous solution, and sulfuric acid aqueous solution, was suitable. Then, the crystal grain size that appeared was measured.

By using such an evaluation method,
It has been found that the relationship represented by the following formula is established between the hardness H of ferrite and the respective components and the crystal grain size d in a structure having a low dislocation density.

H ≦ 200 [Ceq%] + 20+ (9 [C
eq%] + 3.7) / √ (d) where [Ceq%] = C% + Si% / 24 + Mn% / 6
+ (Cu% + Ni%) / 15.
%, Mn%, C%, and Ni% are C, Si, Mn,
It is the weight percentage of Cu and Ni.

This equation is an index indicating the state of the dislocation density, and is an index determined by the chemical component (Ceq) and the crystal grain size d. The measurement of the particle size d may be performed by the appearance of a structure due to normal nital corrosion, but the appearance of the structure is not clear in a structure having a high dislocation density, so that the appearance of a grain boundary by a Marshall reagent is more appropriate. The appearance method is described below.

The Marshall reagent is a corrosive solution mainly composed of an aqueous solution of oxalic acid, aqueous hydrogen peroxide and 7 ml of sulfuric acid. Usually, 50 ml of an 8% aqueous oxalic acid solution, 50 ml of aqueous hydrogen peroxide,
It consists of 7 ml of 50% sulfuric acid solution, but 50 ml of 8% oxalic acid aqueous solution and 5 ml of hydrogen peroxide solution
0ml, 50% sulfuric acid solution 14ml, ethyl alcohol 1m
An improved Marshall reagent consisting of 1 was used. The sample was first immersed in a 5% hydrochloric acid solution for 3 to 4 seconds, washed with water, dried, and corroded at room temperature for 3 to 5 seconds using the improved Marshall reagent.
The grain boundaries are revealed by washing with water and drying. This corrosion method is a typical example, and although it is difficult to observe the grain boundary even if the composition of the etchant is slightly changed, the grain boundary to be observed appears to corrode, and thus falls within the scope of the present invention. Things.

Further, in order to precipitate cementite of 0.5 μm or less or one or more carbonitride phases of Nb, Ti and Ta at the ferrite grain boundary and / or sub-grain boundary, , C or C and Nb, Ti, T
A material (eg, billet, slab) containing steel or steel containing one or more of a is heated to _three or more Ac points,
Alternatively, in a state where one or two or more of C and Nb, Ti, and Ta are dissolved, before or during hot working such as controlled rolling, the temperature is rapidly cooled to a temperature at which the ferrite fraction becomes 50% or more. One or two or more of C or C and Nb, Ti, or Ta are dissolved in supersaturation, and hot working is started or restarted in the subsequent recuperation process, and hot working is completed at an Ac point of ₃ or less. And
Subsequently, cooling without reheating to _three or more Ac points is mainly made of elite, and a texture having a (100) plane strength ratio of a texture parallel to the rolling surface of 1.5 or more is effectively secured. Turned out to be essential.

Hereinafter, the reasons for defining the components in 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 ultra-fine grain ferrite by cementite precipitated at the ferrite grain boundary or sub-grain boundary below μm and is the most effective element to improve strength at low cost, but 0.25% If it exceeds, low temperature toughness is impaired. Further, even in the surface layer region of steel or steel plate, the pearlite fraction exceeds 10%, and if it is less than 0.04%, the amount of cementite necessary for pinning becomes insufficient.
Limited to 4-0.25%. In the case of structural steel for welding, if it exceeds 0.2%, the weldability (weld part toughness) deteriorates, so it is preferable to set the content to 0.04 to 0.2%.

Si is effective as a strength improving element and is also useful as an inexpensive deoxidizing element for molten steel. However, if it exceeds 1.0%, the weldability deteriorates, and if it is less than 0.01%, the deoxidizing effect is not sufficient. 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 an element necessary for improving the strength. From the necessity, Mn is set to 0.3% or more, and if added over 2.0%, the base material toughness and weldability are inhibited and the Ar ₃ transformation point is reduced. As a result of reduction, the content is limited to 0.3 to 2.0% in order to make hot rolling such as two-phase rolling difficult.

S is limited to 0.01% or less from the viewpoint of corrosion resistance and toughness. It is well known that MnS dissolves in an aqueous environment containing chlorine or chloride and becomes a selective corrosion starting point, and from that viewpoint, the lower the S, the better.

Nb is the most useful element together with Ti in thermomechanical processing (TMCP) steel, and NbC or Nb
As (C, N) (Carbo-nitride), suppression of γ-grain growth during reheating of steel material, expansion of non-recrystallization temperature range during controlled rolling, precipitation strengthening in deformation zone during rolling, large heat input welding The effect of preventing HAZ softening in the heat affected zone (HAZ) is generally known. Further, from the detailed studies by the present inventors, it has been found that the thermal stability of the ultrafine precipitated cementite and the effect of suppressing the growth of ferrite grains are remarkably increased. Therefore, if the content is less than 0.005%, NbC or Nb (C, N) precipitated from a supersaturated solid solution state to a ferrite grain boundary or a sub-grain boundary to 0.5 μm or less.
The amount is insufficient and the thermal stability of cementite precipitated to 0.5 μm or less is also insufficient. If it 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 the TMCP steel. As TiC or Ti (C, N), the suppression of γ-grain growth during reheating of the steel material and the temperature in the unrecrystallized region during controlled rolling are controlled. It is generally known that precipitation strengthening during expansion / rolling and HAZ toughness during large heat input welding can be improved. Furthermore, from the detailed examination of the present inventor, it has been found that, similarly to Nb, the thermal stability of the ultrafine precipitated cementite and the effect of suppressing the growth of ferrite grains are improved. Therefore, if the content is less than 0.005%, the supersaturated solid solution state becomes 0.5 μm.
m or less, the amount of TiC or TiCN precipitated at the ferrite grain boundary or sub-grain boundary is insufficient, and the thermal stability of cementite precipitated at 0.5 μm or less is insufficient. 0.0 in order to impair
Limited to 05-0.05%.

Ta is known as TaC or Ta (C, N) to suppress the growth of γ grains during reheating of steel and to improve the HAZ toughness during large heat input. Not used for However, the inventor's detailed studies have found that, similarly to Nb.Ti, the thermal stability and the effect of suppressing the growth of ferrite grains of ultrafine precipitated cementite are improved. Therefore, if the content is less than 0.005%, the amount of TaC or TaCN 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 0.05% or more, the weldability is impaired, so the content is limited to 0.005 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. Must be added in an amount of 0.005% or more.
Furthermore, the present inventors have found that the addition of Al is also effective for the corrosion resistance of the steel of the present invention. However, an excessive addition of 0.6% or more impairs HAZ toughness, so that 0.005 to 0.5% is added.
Limited to 6%.

The above are the basic components of the steel targeted by the present invention. Furthermore, the quality required for the low carbon equivalent for the purpose of improving the base material strength and the low temperature toughness and weldability is further improved. 0.05% Cu depending on properties or steel material size / steel plate thickness
~ 1.0%, Ni 0.1 ~ 2.0%, Cr 0.03%
To 3.0%, Mo to 0.05 to 1.0%, V to 0.01
It is preferable to add B in the range of 0.0002 to 0.002% from the viewpoint of improving the strength, low-temperature toughness, and weldability, and the effect of the present invention is impaired by this addition. Never. In addition, Cu, Ni, and Cr are conventionally known as elements that improve the corrosion resistance of steel in an aqueous environment containing chlorine or chloride such as seawater, but by applying the present invention to steel containing these elements. ,
Further improvement in corrosion resistance is achieved. Further, addition of P together with these elements is also effective for corrosion resistance, and it is preferable to add P also in the present invention. However, if the addition exceeds 0.15%, the toughness and weldability are remarkably reduced.
15% or less.

Further, as described above, in an aqueous environment containing chlorine or chloride, MnS is harmful as a starting point of corrosion,
In order to reduce this, from the viewpoint of controlling the morphology and dispersion of sulfide in steel, 0.0001 to 0.02% of Ca and 0.1% of Mg are used.
0001-0.02%, REM 0.001% -0.2
% Is effective in overlapping with the effect of the present invention.

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

In the ferrite-pearlite steel containing bainite, the corrosion resistance is not always improved even if the ferrite structure is mainly contained. According to a detailed investigation by the present inventors, when the ferrite grains are processed and have a high dislocation density, the corrosion reaction is activated in a corrosive environment, and the corrosive pores in a water environment containing chlorine or chloride are formed. It was found that the frequency of occurrence was high and the corrosion depth was large. Further, it has been found that when the fraction of the ferrite structure having a fine cementite phase is 95% or more, the corrosion resistance is particularly improved.

On the other hand, only a structure composed of fine cementite or carbonitride phase cannot stably obtain a structure mainly composed of ferrite or bainite having an average grain size of 3 μm or less, and the ferrite crystal grains have a small size. They also found that controlling growth was essential. That is, ferrite or bainite can be peened for the first time by depositing cementite of 0.5 μm or less at the ferrite crystal grain boundary or crystal sub-grain boundary, and the growth thereof can be effectively suppressed.

In addition, Nb, Ti, Ta of 0.5 μm or less
When the carbonitride is precipitated at the ferrite grain boundaries or sub-grain boundaries, the same peening effect as cementite is observed, and furthermore, the cementite itself that is ultra-finely precipitated at the ferrite grain boundaries or sub-grain boundaries It has also been found that thermal stability increases.

Next, in the corrosion fatigue resistance, a fatigue crack propagates from the generated corrosion pit. To improve the corrosion fatigue resistance, it is necessary to reduce the size of the generated corrosion pit. It is. The point is that the second phase structure that is a cementite phase or a carbide phase other than the ferrite phase, which is one of the causes of the corrosion pit, is refined to 0.5 μm or less. Further, it is important to suppress the growth of fatigue cracks from the corrosion pits. In order to prevent the fatigue cracks from propagating in the thickness direction, the (100) plane set in a plane parallel to the rolling plane is required. It is effective to strengthen the tissue, and in order to obtain a remarkable effect, it is necessary to make the (100) plane intensity ratio 1.5 or more.

On the other hand, when the proportion of the ultrafine grain structure is less than 5% of the surface area of the steel or the steel sheet, that is, less than 5% of the diameter of the steel or the thickness of the steel sheet,
Since the corrosion resistance on the long-term side varied and was not significantly improved, the content was limited to 5% or more. The higher the proportion of the ultrafine grain structure, the better the corrosion resistance. The upper limit is not particularly defined. However, an excessive increase leads to an increase in the production cost, so that it is preferably up to 30%.

Next, the reason for defining the manufacturing method for realizing the above structure in the back layer region of steel or steel sheet in the present invention will be described.

When the steel material or the steel is reheated, the heating temperature at which C and one or more of Nb, Ti and Ta are dissolved in C and C is limited to _three or more Ac. Nb, Ti, T
The heating temperature at which one or more of a is sufficiently dissolved is preferably 1000 ° C. or higher, and the heating temperature is preferably 1200 ° C. or lower in order to prevent coarsening of γ grains during heating. In the surface area of steel or steel sheet,
0.5 μm at ferrite grain boundaries and / or sub-grain boundaries
The following cementite and cementite are Nb, Ti,
In order to deposit one or more carbonitrides of Ta,
C, Nb, Ti, and Ta are cooled by cooling the surface layer region at a cooling rate of 3 ° C./second or more in a state where one or two or more of C or C and Nb, Ti, and Ta are dissolved in steel. Is super-saturated in steel, and is super-finely precipitated in the process of recovering heat by utilizing the sensible heat of the central portion in the steel, which has a small decrease in temperature even by cooling.

In order to form a structure mainly composed of ferrite having a low dislocation density (structure mainly composed of ferrite or bainite having an average grain size of 3 μm or less) in the back layer region of steel or steel sheet, the steel or steel material is made of Ac. _After heating to ₃ points or more, before or during hot working, the surface layer is rapidly cooled at a cooling rate of 3 ° C./sec or more to a temperature at which the ferrite fraction becomes 50% or more. In the process of recovering heat using the sensible heat in the central part of the steel, (Ac ₁ point-
The hot working is started or restarted from a temperature of 150 ° C. or higher, and the hot working is completed in the range of (Ac ₁ point + 50 ° C.) to Ac ₃ points, thereby causing the recovery and recrystallization of ferrite to be performed. One of the cementite of 0.5 μm or less and / or Nb, Ti, Ta precipitated at the ferrite crystal grain boundary and / or the crystal sub-grain boundary while cooling without refining into fine grains and reheating to _three or more Ac points. The purpose is to effectively utilize the pinning effect of one or more carbonitrides to prevent the growth of an ultrafine grain structure.

Further, after the primary processing, the surface layer is made of Ar ₃
After cooling to below the point, when secondary processing is performed during sensible heat recovery inside the steel, hot working in the non-recrystallization temperature range up to the center of the steel significantly improves the low temperature toughness of the steel I do.

The hot working is intended for general processes such as rolling, extrusion, and drawing. Further, when the material size of the steel is large and the heating temperature is 1170 ° C. or higher, or when the requirement of low-temperature toughness is severe, in order to make the initial γ grains before cooling the surface region of the steel or the steel sheet fine, Nb, T
It is preferable to perform hot working such as addition of i and Ta and controlled rolling. Further, when hot working is not performed before cooling subsequent to heating the steel, low temperature heating and addition of Nb, Ti, Ta or fine hot γ The use of semi-finished products is preferred.

After the surface region of the steel or steel plate is ultra-fine, if the temperature of the central portion of the steel or steel plate is restored to _three or more Ac by sensible heat, not only the effect of the ultra-fine surface region but also the effect of ferrite is lost. Cementite finely precipitated at the crystal grain boundaries or sub-grain boundaries re-dissolves in γ to lose the pinning effect. Therefore, it is necessary to prevent the surface layer from reheating to more than the Ac ₃ point, and to monitor the temperature during rolling, and to reduce the temperature to the Ac ₃ point or more, if necessary, while monitoring the temperature during rolling. Later, it can be cooled.

Further, in order to increase the strength of the steel or steel sheet, request with intensity level adjusting additive component according to, 5 ° C. without recuperation to the Ac ₃ point or more subsequent after completion of hot working Accelerated cooling by the TMCP facility or direct quenching by the DQ facility may be performed at a cooling rate of / sec or more.

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.

[0049]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to Tables 1 to 3.

First, Table 1 shows the components of steel.
Steels A to E are examples of the present invention, and steel F is a comparative example in which C and S fall outside the scope of the present invention.

[0051]

[Table 1]

Next, Table 2 shows the manufacturing conditions of the steel sheet, and Table 3 shows the structure and characteristics of the surface layer region of the manufactured steel sheet. Steel A-1, Steel B-1, Steel C-1, and Steel D in Table 3
1, steel E-1 is an example of the present invention, steel A-2, steel A-3,
Steel B-2, steel C-2, steel D-2, steel E-2, and steel F-1 are comparative examples.

[0053]

[Table 2]

Further, the evaluation method of the corrosion resistance evaluation results of the steel sheet shown in Table 3 is based on a salt water spray exposure test and a seawater immersion test. In this salt water spray exposure test, a 150 mm long × 50 mm wide × 5 mm thick test piece collected from the surface of a steel sheet was exposed outdoors, and a 5% NaCl aqueous solution was sprayed once a day on a test surface with a sprayer, and the test surface was sprayed. It measures the thickness loss and weight loss associated with the occurrence of corrosion. The exposure period was 3 months and 6 months, and three specimens were provided for each steel type in each period. The seawater immersion test was performed using 3.5% NaC equivalent to seawater.
l 50mm aqueous solution 150mm length x 50mm width x 5m
A test piece having a thickness of m is immersed in the test piece to measure the thickness loss and weight loss associated with the occurrence of corrosion. The immersion period was one month and three months, and three specimens were provided for each steel type in each period. And the result of Table 3 is an average value of three test pieces in each test.

Further, the evaluation method of the corrosion fatigue resistance evaluation results shown in Table 3 is based on a tensile test piece of a flat plate (smooth, stress concentration coefficient Kt = 1.1, a plate thickness portion sealed with a polymer, and a steel plate surface (Assess fatigue crack initiation from steel) at 25 ° C
A repetitive stress is applied at 0.1 Hz by pulsating tension in artificial seawater specified by TM. Then, tests were performed in various stress ranges, and stress rupture diagrams (S-Nf curves) were measured. And, as an index of the corrosion fatigue strength, Nf = 5 ×
The fatigue strength at 10 ⁵ divided by the tensile strength.

The ferrite hardness Hv in Table 3 is 10 k
g was measured using a Vickers hardness tester, and the crystal grain size was measured using a microscope and an image analyzer by exposing the tissue with the aforementioned Marshall reagent.

[0057]

[Table 3]

[0058] Steel A-2 of the comparative example, in the steel A-3 the middle cooling of the steel sheet surface layer region, since the cooling rate was slower than the scope of the present invention, the steel sheet surface layer region is not recuperation in the Ac ₃ point or more Nevertheless, this is an example in which the size of the precipitated cementite is coarsened. The steels B-2 and E-2 had a cooling rate within the range of the present invention due to intermediate cooling, however, the cooling time was short, and the thickness of the surface region where the ferrite fraction was 50% or more was 5% of the steel sheet. It is an example that was smaller than less. Since steel C-2 and steel D-2 did not carry out cooling on the way, respectively.
This is an example in which a ferrite layer was not formed in the surface layer region, and steel F-1 has almost the same manufacturing conditions as steel C-1 of the present invention, but its main components C and S are within the scope of the present invention. This is an example that deviates from the above.

From these results, it can be seen that in any of the steel sheets A, B, C, D and E, the present invention example satisfies the conditions of the present invention in the structure of the surface layer region. In comparison, both the exposure test and the immersion test clearly have excellent corrosion resistance,
Furthermore, corrosion fatigue resistance is also excellent.

For example, in the A-1 steel of the present invention,
Ferrite fraction in the surface layer region as compared with the A-2 steel of the comparative example,
Both the precipitates satisfy the conditions of the present invention, and accordingly the corrosion loss is reduced to less than half, the corrosion fatigue strength is about 1.5 times in absolute value, and even when divided by the tensile strength, about 1. 68
The times have been greatly improved. Further, in the steel A-2 of the comparative example, the α grains refined by reheating to the Ac ₃ point or more are reverse transformed into γ, and the cementite precipitated ultra-finely is also γ.
As a result, the α grains and cementite in the surface layer became coarse, and the ferrite fraction became 90% or less. Correspondingly, the corrosion pits generated on the steel sheet surface were larger and deeper in steel A-2 than in steel A-1,
It turns out that corrosion fatigue resistance is also inferior.

Further, steel B- containing Nb, Ti, and Ta
In C-1, cementite or carbonitride precipitates extremely finely at ferrite grain boundaries and crystal sub-grain boundaries to effectively suppress the growth of ferrite or part of bainite. Compared with -1, the ferrite fraction is ensured in spite of the fact that the carbon equivalent is large, the corrosion pits are further refined, and the corrosion weight loss is reduced.
It is much better in terms of corrosion fatigue strength. On the other hand, the steel B-2 of the comparative example had insufficient cooling conditions before finish rolling and the thickness of the fine grain layer was less than 5%, which did not satisfy the conditions of the present invention. Although sufficiently secured, the ferrite hardness, precipitate size, and (100) plane strength cannot satisfy the conditions of the present invention, and the corrosion resistance and corrosion fatigue strength are significantly inferior to those of the examples of the present invention. Since steel C-2 was not cooled in the middle, corrosion resistance and corrosion fatigue strength were inferior to those of the examples of the present invention. A similar tendency is found in steels D-1 and D-2, steel E-
Also observed between 1 and E-2.

Further, when the steels of the present invention are compared with each other, for example, steel A-1 and steel B-1 to steel E-1, Cu, N
Addition of i, Cr and Ca, REM, and Mg is superior in corrosion resistance in absolute level comparison. This indicates that the effect of these additional elements on corrosion resistance (conventional findings) can be superimposed on the present invention. According to the present invention, not only can the corrosion resistance of ordinary structural steel be improved, but also the corrosion resistance of conventional corrosion-resistant structural steel can be significantly improved.

Lastly, the steel EF-1 of the comparative example in which C and S are deviated to the higher side of the example of the present invention, while the production conditions are substantially the same as the steel EA-2 of the example of the present invention, the ferrite fraction and The ferrite hardness satisfies the conditions of the present invention, but as a result of a high perlate fraction and a high S, the corrosion resistance is inferior to the examples of the present invention.

[0064]

According to the present invention, the surface region of steel or steel sheet is constituted mainly by a ferrite structure having a low dislocation density, and the (100) plane strength ratio is set to 1.5 or more, so that only the surface of the chemical component is obtained. In addition, from the viewpoint of steel structure, it is possible to improve the corrosion resistance and corrosion fatigue strength characteristics of structural steel or structural steel for welding in a water environment containing chlorides such as seawater, and to improve mechanical parts or steel structures. Can prolong the life of the device. Furthermore, the present invention can improve the corrosion resistance without adding a large amount of expensive elements such as Cu and Ni, and it is considered that there is a great economic benefit that can be enjoyed by the industry. Furthermore, coupled with the excellent mechanical properties of the steel of the invention, the invention provides a corrosion-based corrosion fatigue, S
It is a base for steel materials with high resistance to CC.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between (ferrite hardness / value of equation (1)) and the amount of corrosion.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Tomohiko Hata Oita-shi, Nishi-no-state 1 Nippon Steel Corporation Oita Works F-term (reference) 4K032 AA00 AA01 AA02 AA04 AA05 AA08 AA11 AA12 AA14 AA16 AA19 AA22 AA23 AA24 AA27 AA29 AA31 AA33 AA35 AA36 AA40 BA01 BA02 CA02 CB02 CC03 CC04 CD02 CD03

Claims (8)

[Claims] (1) C: 0.04 to 0.25% by weight,
Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%,
S: Steel or steel sheet having a content of 0.01% or less, the balance being iron and unavoidable impurities. The diameter of the steel or the thickness of the steel sheet from the surface layer portion of the steel or from the front and back layers of the steel plate, respectively. 5% of
In the above-mentioned surface layer region, a cementite phase having a grain size of 0.5 μm or less is present at a grain boundary and / or a sub-grain boundary, ferrite accounts for 95% or more, and the hardness H of the ferrite structure is expressed by the following (1) A structural steel excellent in corrosion resistance and corrosion fatigue resistance, which satisfies the formula and further has a (100) plane strength ratio of a texture parallel to the rolled surface of 1.5 or more. H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d) (1) where [Ceq%] = C% + Si% / 24 + Mn% / 6
Where C%, Si% and Mn% are C and S, respectively.
i is the weight% of Mn, and d is the average equivalent circle diameter of ferrite. 2. In% by weight, further, Al: 0.005 to
Contains 0.6% of component, and Nb: 0.005 to
0.1%, Ti: 0.005 to 0.05%, Ta: 0.
The structural steel according to claim 1, which contains one or more of 005 to 0.05%. 3. The composition according to claim 1, wherein the content of Cu is 0.05 to 1.
0%, Ni: 0.1 to 2.0%, Cr: 0.03 to 3.
0%, Mo: 0.05-1.0%, V: 0.01-0.
4%, B: 0.0002-0.002%, P: 0.15
% Or less, Ca: 0.0001 to 0.02%, Mg: 0.1%.
0001-0.02%, REM: 0.001% -0.2
% Or more, and the hardness H of the ferrite structure satisfies the following formula (2): excellent in corrosion resistance and corrosion fatigue resistance characteristics according to claim 1 or 2. Structural steel. H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d) (2) where [Ceq%] = C% + Si% / 24 + Mn% / 6
+ (Cu% + Ni%) / 15, where C%, Si
%, Mn%, Cu%, and Ni% are C, Si, and M, respectively.
The weight percent of n, Cu, and Ni, and d is the average equivalent circle diameter of ferrite. 4. C: 0.04 to 0.25% by weight,
Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%,
S: A steel or steel plate material having a content of 0.01% or less, the balance being iron and unavoidable impurities, heated to _three or more Ac and solid-dissolved C before hot working. Or, after quenching the surface layer region of 5% or more of the diameter of the steel or the thickness of the steel plate from the surface layer at that time to a temperature at which the ferrite fraction becomes 50% or more at a cooling rate of 3 ° C / sec or more, In the process of reheating the surface layer, (Ac ₁ point-
0.99 ° C.) or higher starting hot working from the temperature of the or resumed, (Ac ₁ point + 50 ° C.) hot working ends in the temperature range of to Ac ₃ point, the surface area following the Ac ₃ point or more By cooling without recuperation, in the surface layer region, a cementite phase having a grain size of 0.5 μm or less is present at a grain boundary and / or a sub-grain boundary, ferrite accounts for 95% or more, and the hardening of the ferrite structure. H is the following (3)
Which satisfies the formula, and the texture (10
0) A method for producing a structural steel excellent in corrosion resistance and corrosion fatigue resistance, wherein the surface strength ratio is 1.5 or more. H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d) (3) where [Ceq%] = C% + Si% / 24 + Mn% / 6
Where C%, Si% and Mn% are C and S, respectively.
i is the weight% of Mn, and d is the average equivalent circle diameter of ferrite. 5. The method according to claim 1, further comprising:
Contains 0.6% of component, and Nb: 0.005 to
0.1%, Ti: 0.005 to 0.05%, Ta: 0.
The method for producing a structural steel having excellent corrosion resistance and corrosion fatigue resistance according to claim 4, characterized in that the steel contains one or more of 005 to 0.05%. 6. The method according to claim 6, wherein after the hot working is completed, accelerated cooling or direct quenching is performed at a cooling rate of 5 ° C./sec or more without successively reheating the surface layer to _three or more Ac points. A method for producing a structural steel having excellent corrosion resistance and corrosion fatigue resistance according to claim 4 or 5. 7. The method for producing a structural steel having excellent corrosion resistance according to claim 6, wherein tempering is performed subsequently after the completion of the accelerated cooling or the direct quenching. 8. The composition according to claim 1, wherein the content of Cu is 0.05 to 1.
0%, Ni: 0.1 to 2.0%, Cr: 0.03 to 3.
0%, Mo: 0.05-1.0%, V: 0.01-0.
4%, B: 0.0002-0.002%, P: 0.15
% Or less, Ca: 0.0001 to 0.02%, Mg: 0.1%.
0001-0.02%, REM: 0.001% -0.2
%, And the hardness H of the ferrite structure satisfies the following formula (4): Corrosion resistance and corrosion resistance according to any one of claims 4 to 7. A method for producing structural steel with excellent fatigue properties. H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d) (4) where [Ceq%] = C% + Si% / 24 + Mn% / 6
+ (Cu% + Ni%) / 15.
%, Mn%, Cu%, and Ni% are C, Si, and M, respectively.
n is the weight percent of Cu and Ni, and d is the average equivalent circle diameter of ferrite.