JP2000144309A - Steel excellent in corrosion resistance for structural purpose and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the corrosion resistance of steel for structural purpose, particularly the corrosion resistance in a water environment contg. chlorine or chloride by controlling the structure of steel. SOLUTION: As to the steel for structural purpose excellent in corrosion resistance and a method for producing it, the structure in which the ferrite crystal grain boundaries and/or crystal sub-grain boundaries in the surface layer part of steel or the surface and back layer parts of a steel sheet are provided with cementite and/or the carbonitrides of Nb, Ti and Ta of <=0.5 μm, and in which the pearlite fractional ratio is controlled to <=10% and the balance is essentially composed of ferrite or bainite with <=3 μm average grain size is obtained.

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 or a welding structural steel excellent in corrosion resistance and applied to a welding steel 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 an alloying element as described above, but this increases the cost as a structural steel, and contains a large amount of alloying elements, which makes it necessary as a structural steel. There was a problem that weldability and workability deteriorated.

[0004]

SUMMARY OF THE INVENTION In view of the above background, an object of the present invention is to control a steel structure by
It is intended to improve the corrosion resistance of structural steel, particularly in a water environment containing chlorine or chloride. That is, it is an object of the present invention to improve the corrosion resistance of conventional structural steels without adding an alloying element effective for the corrosion resistance as described above, while suppressing the cost increase and securing the weldability. Further, it is another object of the present invention to reduce the alloying elements and improve the weldability of the conventional corrosion-resistant structural steel, or to further greatly improve the conventional corrosion resistance by using the same component system.

[0005]

In order to achieve the above-mentioned object, the present invention provides a ferrite grain boundary and / or a sub-grain boundary of 0.5 μm or less in the surface layer of steel or the front and back layers of steel sheet. Precipitate cementite and / or Nb-Ti-Ta carbonitride to modify the structure mainly composed of ferrite or bainite to ultrafine particles having an average particle size of 3 µm or less, and reduce the pearlite fraction to 10% or less. Accordingly, the present invention provides a structural steel (including a structural steel for welding) having excellent corrosion resistance and a method for producing the same.

The gist of the present invention is as follows.

(1) C: 0.04 to 0.2% by weight
5%, Si: 0.01-1.0%, Mn: 0.3-2.
0%, S: 0.01% or less, the balance consisting of iron and unavoidable impurities, and crystal grains in the region of 5% or more of the diameter or thickness of the steel from the surface layer of the steel or the front and back layers of the steel sheet, respectively. It has a cementite phase of 0.5 μm or less at the grain boundaries and / or subgrain boundaries, and has a pearlite fraction of 10% or less and an average crystal grain size of 3 μm or less, and is mainly composed of ferrite or bainite. Structural steel with excellent corrosion resistance.

(2) C: 0.04 to 0.2% by weight
5%, Si: 0.01-1.0%, Mn: 0.3-2.
0%, S: 0.01% or less, Al: 0.005 to 0.6
%, Nb: 0.005 to 0.1%, T
i: 0.005 to 0.05%, Ta: 0.005 to 0.
Grain boundaries in the region containing 5% or more of the steel diameter or thickness from the surface layer of steel or the front and back layers of steel sheet, respectively, containing one or more kinds of nickel of 0.05%, the balance being iron and unavoidable impurities. And / or a cementite phase of 0.5 μm or less and / or a carbonitride phase of Nb · Ti · Ta at a subgrain boundary and a pearlite fraction of 10% or less and an average crystal grain size of 3 μm or less Structural steel with excellent corrosion resistance, characterized in that the structural steel is mainly composed of ferrite or bainite.

(3) Further, 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:
The structural steel excellent in corrosion resistance according to any one of the above (1) or (2), which contains one or more kinds of 0.15% or less.

(4) Further, Ca: 0.00% by weight.
01-0.02%, Mg: 0.0001-0.02%,
REM: any one of the above (1) to (3), characterized by containing one or more of 0.001% to 0.2%.
Structural steel excellent in corrosion resistance described in (1).

(5) C: 0.04 to 0.2% by weight
5%, Si: 0.01-1.0%, Mn: 0.3-2.
0%, S: 0.01% or less, steel or steel material consisting of iron and unavoidable impurities is heated to more than ₃ points of Ac to form a solid solution of C, before or during hot working At
After quenching a region of 5% or more of the diameter or thickness of the steel from the surface layer of the steel or the front and back layers of the steel plate 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 reheating the surface layer or the front / back layer region, (A
The hot working is started or restarted at a temperature of c ₁ point-150) ° C. or more, and the hot working is completed in a temperature range of (Ac ₁ point−50) ° C. to (Ac ₃ points) ° C. Before the surface layer or the front / back layer region is reheated to Ac ₃ or more points, the surface layer or the front / back layer of the steel sheet is cooled to 5 mm in diameter or thickness of the steel, respectively.
% Ferrite or bainite having a cementite phase of 0.5 μm or less at a grain boundary and / or a sub-grain boundary in a region of at least 10% and a pearlite fraction of 10% or less and an average crystal grain size of 3 μm or less. A method for producing a structural steel having excellent corrosion resistance, characterized by comprising a structure having the following structure.

(6) C: 0.04-0.2% by weight
5%, Si: 0.01-1.0%, Mn: 0.3-2.
0%, S: 0.01% or less, Al: 0.005 to 0.6
%, Nb: 0.005 to 0.1%, T
i: 0.005 to 0.05%, Ta: 0.005 to 0.
A steel or steel material containing 0.05% or more of one or more kinds and the balance consisting of iron and unavoidable impurities is heated to _three or more Ac to solidify one or more of C and Nb.Ti.Ta. After melting, before or during hot working, the area of 5% or more of the diameter or thickness of the steel from the surface layer of steel or the front and back layers of steel sheet, respectively, is cooled at a cooling rate of 3 ° C / sec or more. Rate 5
After rapidly cooling to a temperature of 0% or more, in the process of reheating the surface layer or the front / back layer region, (Ac ₁ point-1
50) Start or restart hot working at a temperature of not less than ℃,
The hot working is completed in the temperature range of (Ac ₁ point -50) ° C. to Ac ₃ points, and then the surface layer or the front / back layer region is Ac ₃
Cooling is performed before the heat is recovered to a point or more, and the temperature is reduced from the surface layer of steel or the front and back layers of steel sheet to grain boundaries and / or sub-grain boundaries in a region of 5% or more of the diameter or thickness of steel, respectively. A structure mainly composed of ferrite or bainite having a cementite phase of 5 μm or less and / or a carbonitride phase of Nb · Ti · Ta and having a pearlite fraction of 10% or less and an average crystal grain size of 3 μm or less. A method for producing a structural steel having excellent corrosion resistance, characterized by being constituted.

(7) After the completion of the hot working, before the surface layer or the front and back layer regions are reheated to _three or more Ac points, accelerated cooling or direct quenching at a cooling rate of 5 ° C./sec or more. The method for producing a structural steel having excellent corrosion resistance according to any one of the above (5) and (6), characterized in that:

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

(9) Further, 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:
The method for producing a structural steel excellent in corrosion resistance according to any one of the above (5) to (8), wherein one or more kinds of the steels are contained in an amount of 0.15% or less.

(10) Further, by weight%, Ca: 0.0
001-0.02%, Mg: 0.0001-0.02
%, REM: 0.001% to 0.2% of one or more of the above (5) to (9) for a structure having excellent corrosion resistance. Steel production method.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.

The present inventor has proposed a water environment containing chlorine of various steels,
As a result of detailed examination of the corrosion resistance in a wet environment and a dry / wet repeated environment, it was found that ferrite or bainite was extremely fine in a steel structure, and that cementite and / or Nb.
It has been found that the corrosion resistance of steel is significantly improved by precipitating one or two or more types of carbonitride phases of Ti and Ta at a grain boundary and / or a sub-grain boundary thereof at 0.5 μm or less.

Further, cementite and / or Nb-Ti-Ta may be present at ferrite grain boundaries and / or sub-grain boundaries.
In order to precipitate one or more kinds of carbonitride phases to 0.5 μm or less, a steel material or steel containing one or more kinds of C and / or Nb · Ti · Ta is required to have _three points of Ac. In the state where one or two or more of C and / or Nb / Ti / Ta are solid-dissolved by heating as described above, the ferrite fraction becomes 50% or more before or during hot working such as controlled rolling. After rapidly cooling to a temperature and dissolving one or more of C and / or Nb.Ti.Ta in a supersaturated state, hot working is started or restarted in the process of reheating the steel, and Ac ₃ It is indispensable to end the hot working below the point and cool down without reheating to more than the Ac point ₃ in order to effectively secure a structure mainly composed of ferrite or bainite having an average grain size of 3 μm or less. The inventor invented a certain technology.

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 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, the low-temperature toughness is impaired and the pearlite fraction exceeds 10% even in the surface layer portion of the steel or the front and back layer portions of the steel plate according to the present invention,
If the content is less than 0.04%, the amount of cementite necessary for pinning becomes insufficient, so the content is limited to 0.04 to 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 it to 0.04 to 0.2%.

Si is effective as a strength improving element and 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 a useful element for improving the strength.
Ar ₃ with the addition of ultra inhibit matrix toughness, weldability
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.

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. From that viewpoint, it is preferable that S is as low as possible.

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 was found that the thermal stability of the ultrafine precipitated cementite and the effect of suppressing the growth of ferrite grains were significantly increased. Therefore, if it is less than 0.005%, the amount of NbC or Nb (C, N) precipitated from the supersaturated solid solution state to 0.5 μm or less at the ferrite crystal grain boundary or crystal sub-grain boundary becomes insufficient, and the precipitation becomes 0.5 μm or less. In addition, the thermal stability of cementite 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 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. The effects of precipitation strengthening during expansion / rolling and HAZ toughness improvement during large heat input welding are generally known. Further, from the detailed examination by the present inventors, it has been found that, similarly to Nb, the thermal stability and the effect of suppressing the growth of ferrite grains of the ultrafine precipitated cementite are improved. Therefore, 0.0
If it is less than 0.05%, the amount of TiC or TiCN precipitated from the supersaturated solid solution state to 0.5 μm or less at the ferrite grain boundary or sub-grain boundary is insufficient, and the thermal stability of the cementite precipitated to 0.5 μm or less is also reduced. Shortage, 0.0
If it is more than 5%, the weldability is impaired.
Limited to 5%.

Ta is known as TaC or Ta (C, N), which has the effect of suppressing the growth of γ grains during reheating of steel and improving the HAZ toughness at the time of large heat input. Not used for However, from the in-depth study of the inventor, N
As in the case of b.Ti, it has been found that the thermal stability and the effect of suppressing the growth of ferrite grains of the ultrafine precipitated cementite are improved. Therefore, if the content is less than 0.005%, the amount of TaC or TaCN precipitated from the supersaturated solid solution state to 0.5 μm or less at the ferrite crystal grain boundary or sub-grain boundary is insufficient, and the thermal behavior of cementite precipitated to 0.5 μm or less is 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%.

Al is an element necessary for deoxidation similarly to 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, excessive addition of 0.6% or more impairs the HAZ toughness, so that 0.005 to 0.6%
Limited to.

The above are the basic components of the steel to which the present invention is directed. Furthermore, in order to improve the strength of the base material and the low carbon equivalent for the purpose of improving the low temperature toughness and weldability, the quality required is reduced. Cu, Ni, Cr, Mo, from the viewpoint of improving the strength, low-temperature toughness, and weldability according to the characteristics or the size of the steel material and the thickness of the steel sheet.
V, B: Cu: 0.05-1.0%, Ni: 0.1-
2.0%, Cr: 0.03 to 3.0%, Mo: 0.05
1.0%, V: 0.01-0.4%, B: 0.000
The effect of the present invention is not impaired even if one or more kinds are added in the range of 2 to 0.002%. Also, C
Although u, 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, in the present invention, by including these elements in steel, it is possible to further improve the corrosion resistance. Improved corrosion resistance is obtained.

Further, the addition of P is also effective for the corrosion resistance. In the present invention, the addition of P alone or the above Cu, Ni, Cr,
Mo, V, and B can be added together with the elements. However, if added in excess of 0.15%, the toughness and weldability are significantly reduced, so that the P content is 0.15% or less. Limited.

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, Ca, Mg, and REM are changed from Ca: 0.0001 to
0.02%, Mg: 0.0001-0.02%, RE
M: Addition of one or more kinds in the range of 0.001% to 0.2% is effective in superimposition with the effects of the present invention.

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

According to a detailed investigation by the present inventors, in ferrite-pearlite steel containing bainite, even if the ferrite grain size is 5 μm or less, the corrosion resistance is not necessarily improved. When it contained, it was found that the frequency of occurrence of corrosion pits in a water environment containing chlorine or chloride was high and the corrosion depth was large. Further, it was also found that the corrosion resistance was improved along with the refinement of the ferrite grain size only when the pearlite fraction was reduced to 10% or less, including fine cementite, and that the effect was particularly significant at 3 μm or less.

On the other hand, the structure composed mainly of fine cementite or carbonitride phase cannot stably achieve the average grain size of the 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 Nb-Ti-Ta carbonitride of 0.5 μm or less is precipitated at the ferrite crystal grain boundary or crystal sub-grain boundary, the same pinning effect as cementite is recognized, and further, the ferrite crystal grain boundary or crystal sub-grain is observed. It has also been found that the thermal stability of cementite itself, which is ultrafinely precipitated in the field, increases.

On the other hand, when the ratio of the ultrafine grain structure is less than 5% of the diameter or thickness of the steel in each of the surface layer portion of the steel and the front and back layer portions of the steel plate, the corrosion resistance on the long-time side is remarkably varied. 5% or more in order to avoid improvement. The higher the proportion of the ultrafine grain structure is, the better the corrosion resistance is, and the upper limit is not preferably defined. However, an excessive increase leads to an increase in manufacturing cost.

For the above-mentioned reasons, the crystal structure of the present invention is obtained by forming a crystal grain boundary and / or a crystal sub-grain boundary in a region of not less than 5% of the diameter or thickness of the steel from the surface layer of the steel or the front and back layers of the steel sheet, respectively. 0.5 μm or less cementite phase and / or Nb
-It must have a structure mainly composed of ferrite or bainite having a Ti-Ta carbonitride phase, a pearlite fraction of 10% or less, and an average crystal grain size of 3 µm or less. is there.

Next, in the present invention, the surface layer of steel or the surface of steel sheet
The reason for defining the manufacturing method for realizing the ultrafine grain structure in the back layer will be described.

The heating temperature at the time of reheating the steel material or steel of the present invention is limited to _three or more Ac in order to dissolve one or more of C and / or Nb.Ti.Ta.

Further, in order to sufficiently dissolve one or more of Nb, Ti, and Ta, a heating temperature of 1000 ° C.
The heating temperature is preferably set to 1200 ° C. or lower in order to prevent the γ grains from becoming coarse during heating.

In the region of 5% or more of the diameter or thickness of the steel from the surface layer of the steel of the present invention or the front and back layers of the steel sheet, respectively, a ferrite grain boundary and / or a crystal sub-grain boundary has a superfineness of 0.5 μm or less. Fine cementite and / or Nb-Ti-Ta
In order to precipitate one or two or more carbonitrides, one or two or more of C and / or Nb.Ti.Ta are dissolved in steel in a solid solution, and 3 ° C /
By cooling at a cooling rate of at least 2 seconds, the component is dissolved in the steel in a supersaturated manner, and thereafter, the process is performed by using the sensible heat of the central portion of the steel, which has a small temperature drop even by this cooling, to recover the temperature. Things.

In the region of 5% or more of the diameter or thickness of the steel from the surface layer of the steel of the present invention or the front and back layers of the steel plate, respectively, the average grain size of the structure mainly composed of ferrite or bainite is 3 μm or less. After heating a steel or a steel material to an Ac point of ₃ or more, before or during hot working, the surface layer or the front and back layers are cooled at a rate of 3 ° C./sec or more, and the ferrite fraction is 50% or more. Quenched to a temperature at which the surface layer or the front / back layer region is re-heated by utilizing the sensible heat of the central portion of the steel, the temperature of which is small even by this cooling.
Recovery / recrystallization of ferrite by starting or restarting hot working from a temperature of c ₁ point-150 ° C. or higher and ending hot working in the range of (Ac ₁ point-50 ° C.) to Ac ₃ points. To make the crystal structure ultra-fine, further cool the surface layer or the front / back layer region without reheating to _three or more Ac points, and precipitate at the ferrite crystal grain boundary and / or crystal sub-grain boundary. This is achieved by effectively utilizing the pinning of one or more carbonitrides of ultrafine cementite of 0.5 μm or less and / or Nb · Ti · Ta to prevent the growth of the ultrafine grain structure. Things.

Further, before or during the hot working of the present invention, the above-mentioned surface layer or front / back layer portion is cooled to a point of Ar ₃ or less, and then, in the recuperation process by sensible heat inside the steel, hot working is performed. Is carried out in the non-recrystallization temperature range at the center of the steel, and the low-temperature toughness of the steel is significantly improved.

The hot working of the present invention is intended for general hot working such as rolling, extrusion and drawing. When the size of the steel material is large and the heating temperature is as high as 1170 ° C. or when the low temperature toughness of the product is strict, the addition of Nb, Ti, and Ta and the controlled rolling after heating are performed. Therefore, it is preferable to reduce the initial γ grain size before cooling the surface portion of the steel or the front and back layers of the steel plate. Further, in the case where the steel is cooled without being subjected to hot working after heating, the initial γ grains of the steel may be reduced by heating at a low temperature and adding Nb, Ti, and Ta, or the initial γ grains may be reduced in advance. It is preferable to use a hot-worked semi-finished product.

After the surface layer portion of the steel or the front and back layers of the steel sheet is ultrafine-grained by hot rolling, when the steel or the steel sheet is returned to _three or more Ac points by sensible heat at the center, the surface layer becomes superfine. Not only does the effect of fine graining be impaired, but also the cementite finely precipitated at the ferrite crystal grain boundary or crystal sub-grain boundary re-dissolves in γ to lose the pinning effect. Therefore, in the present invention, after hot rolling, so that the surface layer or the front and back layer portion does not reheat to more than Ac ₃ points, if the diameter of the steel or the steel plate thickness is less than 18 mm, air cooling is performed When the diameter or the thickness of the steel sheet is larger than that, it is preferable to perform accelerated cooling at a cooling rate of 2 ° C./sec or more.

[0045] To further increase the strength of the steel or steel sheet, the strength required level adjustment additive components in accordance with, and / or after completion of the hot working without causing recuperation than Ac ₃ point, 5 ° C.
Accelerated cooling or direct quenching may be performed at a cooling rate of at least / sec.

In the present invention, subsequent to the accelerated cooling or the direct quenching after the hot rolling, the steel or the steel sheet may be further tempered using ordinary heat treatment equipment. In the case of accelerated cooling using a TMCP facility or direct quenching using a DQ facility, an auto-temper that stops water cooling during accelerated cooling or direct quenching may be used instead.

[0047]

Embodiments of the present invention will be described below.

First, a steel slab obtained by melting and casting steel having the chemical components shown in Table 1 was used. In Table 1, steels A to E are examples of the present invention satisfying the components and contents of the present invention, and steel F is a comparative example in which C and S are out of the range of the present invention.

Next, steel slabs having the components shown in Table 1 were manufactured under the manufacturing conditions shown in Table 2. Table 3 shows the α grain size (grain size of ferrite and / or bainite), precipitate size, and corrosion resistance evaluation results of the steel sheets produced and obtained.

In Table 3, A-1, B-1, C-1,
D-1 and E-1 are examples of the present invention. Meanwhile, in Table 3, when A-2 is to cool the surface layer region in the middle hot-rolled steel plate, the because the cooling rate was high temperature inside slow steel sheet, the surface layer region Ac ₃ point or more after the end of rolling This is a comparative example that has regained its temperature. B-2 had a sufficient cooling rate at the time of cooling during hot rolling, but the cooling time was short and the thickness of the surface layer region where the α fraction was 50% or more was 5%.
%. C-2 and D-2
Is a comparative example of a steel sheet having no fine grain layer formed on the surface layer because cooling was not performed during hot rolling, and E-2 was insufficiently cooled during hot rolling. It is a comparative example of the steel plate whose end temperature was high. Finally, F-1 is a comparative example in which the production conditions are almost the same as C-1 of the present invention, but the main components C and S are out of the scope of the present invention.

Table 3 shows the results of evaluating the corrosion resistance of each steel sheet obtained under the manufacturing conditions shown in Table 2. As an evaluation method, a salt water spray exposure test and a seawater immersion test were performed. The salt water spray exposure test was 150 mm long x 50 m collected from the surface layer of the steel plate.
A test piece having a width of 5 m and a thickness of 5 mm was exposed outdoors to 5% NaCl.
The aqueous solution is sprayed on the test surface once a day using a sprayer, and the thickness loss and weight loss due to the corrosion of the test surface are measured. The exposure period was 3 months and 6 months. Each period, each steel type, and 3 test pieces were tested. In addition, the seawater immersion test was conducted by adding 150% aqueous solution of 3.5% NaCl equivalent to seawater at 50 ° C.
A test piece having a length of 50 mm, a width of 5 mm and a thickness of 5 mm is immersed in the test piece to measure a decrease in thickness and a decrease in weight due to occurrence of corrosion. The immersion period was one month and three months, and each period, each steel type, and three test pieces were tested. The results in Table 3 are the average values of three test pieces in each test.

From the evaluation results shown in Table 3, the steels A to E
A-1 to E of the present invention examples
In the case of -1, the surface structure satisfies the requirements of the present invention, and as a result, the exposure test and the immersion test are clearly superior in corrosion resistance as compared with the comparative example. For example, in A-1 of the present invention, both α grains and precipitates in the surface layer are smaller than half the size of A-2 of the comparative example, and accordingly, the corrosion loss is improved to less than half. In the comparative example A-2, the α grains refined by reheating to the Ac3 point or more are inversely transformed into γ, and the cementite precipitated ultra-finely re-dissolves in γ. As a result, the α grains and cementite in the surface layer are also reduced. The pearlite fraction became 10% or more while coarsening. Correspondingly, corrosion pits generated on the steel sheet surface
A-2 is larger and deeper than 1.

Also, B- to which Nb.Ti.Ta is added
In the case of C-1 and C-1, cementite or carbonitride precipitates extremely finely at the ferrite crystal grain boundary and the crystal sub-grain boundary to effectively suppress the growth of ferrite or partially bainite. It is extremely stable as compared with A-1 as an example, and as a result, corrosion pits are further miniaturized, and the corrosion loss is further improved. On the other hand, in Comparative Example B-2, the cooling conditions during the rolling before the finish rolling were insufficient, and the thickness of the fine grain layer was less than 5%, which was insufficient for the present invention.
The particle size and precipitate size do not satisfy the present invention, and the corrosion resistance is greatly inferior to the examples of the present invention. Steel sheet C-2, which is a comparative example in which cooling was not performed during hot rolling, is naturally inferior in characteristics to the example of the present invention. A similar trend is D
-1 and D-2, and between E-1 and E-2.

Further, A- which satisfies the requirements of the present invention example
1 and B-1 to E-1 show that the steel component is Cu, N
B-1 to which i, Cr, and Ca, REM, and Mg are added
E-1 is superior in corrosion resistance on an absolute level. Therefore, by applying the microstructure control of the present invention, Cu, Ni, Cr, and Ca, REM,
It can be seen that the corrosion resistance of the corrosion-resistant structural steel to which an alloying element, such as Mg, which has been conventionally effective in improving the corrosion resistance can be significantly improved.

Finally, while the production conditions were almost the same as those of the steel A-2 of the present invention, the comparative example F-1 in which C and S deviated higher than those of the present invention exhibited fine grain layer thickness and cementite. The dimensions also satisfy the conditions of the present invention, but the pearlite fraction is high, and the high S results in inferior corrosion resistance to the examples of the present invention.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

[0059]

According to the present invention, a ferrite grain boundary and / or a region of 5% or more of the surface layer of steel or the front and back layers of steel sheet are obtained.
Alternatively, cementite or N of 0.5 μm or less
By depositing a carbonitride phase of b-Ti-Ta and forming a structure mainly composed of ferrite or bainite having a stable average particle size of 3 µm or less in the region, water containing chloride such as seawater can be obtained. The corrosion resistance of structural steel (including structural steel for welding) in the environment can be improved. This makes it possible to improve the corrosion resistance of mechanical parts or steel structures from the viewpoint of not only the chemical composition of the steel material but also the structure of the steel material. 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 present invention,
The present invention is a base of a steel material having high resistance to corrosion fatigue and SCC caused by corrosion.

Continuing from the front page (72) Inventor Masaki Minagawa 1 Oshino-shi, Oita-shi Nippon Steel Corporation In-house Oita Works (72) Inventor Tadashi Ishikawa 1-larger Nishinoshu Oita-shi Nippon Steel Corporation Oita Works F-term (reference)

Claims (10)

[Claims] (1) C: 0.04 to 0.25% by weight,
Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%,
S: 0.01% or less, the balance consisting of iron and unavoidable impurities, and from the surface layer of steel or the front and back layers of steel sheet, the crystal grain boundaries and / or in the region of 5% or more of the diameter or thickness of steel, respectively. Alternatively, it is characterized by having a cementite phase of 0.5 μm or less at the crystal subgrain boundaries, a structure of mainly ferrite or bainite having a pearlite fraction of 10% or less and an average crystal grain size of 3 μm or less. Structural steel with excellent corrosion resistance. 2. C: 0.04 to 0.25% by weight,
Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%,
S: 0.01% or less, Al: 0.005 to 0.6%, Nb: 0.005 to 0.1%, Ti: 0.
005-0.05%, Ta: 0.005-0.05%, containing one or more kinds, the balance consisting of iron and unavoidable impurities. Grain boundaries in a region of 5% or more of the diameter or thickness of
Alternatively, a ferrite having a cementite phase of 0.5 μm or less and / or a carbonitride phase of Nb · Ti · Ta at a subgrain boundary and having a pearlite fraction of 10% or less and an average grain size of 3 μm or less. Alternatively, a structural steel having excellent corrosion resistance, characterized by being composed of a structure mainly composed of bainite. 3. The composition according to claim 1, further comprising: Cu: 0.05-1.
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
The structural steel having excellent corrosion resistance according to claim 1, wherein the structural steel contains 1% or less of one or more kinds. 4. Further, in terms of% by weight, Ca: 0.0001 to
0.02%, Mg: 0.0001-0.02%, RE
The structural steel excellent in corrosion resistance according to any one of claims 1 to 3, wherein one or more kinds of M: 0.001% to 0.2% are contained. 5. C: 0.04 to 0.25% by weight,
Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%,
S: A steel containing 0.01% or less, the balance consisting of iron and unavoidable impurities, or a steel material is heated to _three or more Ac to form a solid solution with C, and then before or during hot working, Of the diameter or thickness of the steel from the surface layer of
% Or more at a cooling rate of 3 ° C./sec or more to a temperature at which the ferrite fraction becomes 50% or more, and then in the process of reheating the surface layer or the front and back layer regions, (Ac ₁ point −150 ) The hot working is started or restarted at a temperature of ≧ ° C., the hot working is completed in a temperature range of (Ac ₁ point −50) ° C. to (Ac ₃ point) ° C., and subsequently the surface layer or the front and back surfaces The layer region is cooled before being reheated to _three or more points of Ac, and from the surface layer of steel or the front and back layers of steel plate, the grain boundaries and / or subcrystals in the region of 5% or more of the diameter or thickness of steel, respectively. 0 at grain boundaries.
Structural material with excellent corrosion resistance, characterized by having a cementite phase of 5 μm or less, a pearlite fraction of 10% or less, and a structure mainly composed of ferrite or bainite having an average crystal grain size of 3 μm or less. Steel production method. 6. C: 0.04 to 0.25% by weight,
Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%,
S: 0.01% or less, Al: 0.005 to 0.6%, Nb: 0.005 to 0.1%, Ti: 0.
A steel or a steel material containing one or more of 005-0.05% and Ta: 0.005-0.05%, and the balance consisting of iron and unavoidable impurities is heated to _three or more Ac and C And one or more of Nb, Ti, and Ta are dissolved,
Before or during hot working, the area of 5% or more of the diameter or thickness of the steel from the surface layer of steel or the front and back layers of steel sheet, respectively, has a ferrite fraction of 50% or more at a cooling rate of 3 ° C / sec or more. After rapidly cooling to a temperature at which the surface layer or the front / back layer region is reheated, (Ac ₁ point-150) ° C.
The hot working is started or restarted at the above temperature, and the hot working is completed in a temperature range of (Ac ₁ point -50) ° C. to Ac ₃ points,
Subsequently, the surface layer or the front / back layer region is cooled before being reheated to _three or more points of Ac, and from the surface layer of the steel or the front / back layer of the steel plate, in the region of 5% or more of the diameter or thickness of the steel, respectively. Having a cementite phase of 0.5 μm or less and / or a carbonitride phase of Nb · Ti · Ta at a crystal grain boundary and / or a sub-grain boundary and having a pearlite fraction of 10% or less and an average crystal grain size Characterized by having a structure mainly composed of ferrite or bainite of 3 μm or less. 7. After completion of hot working, before the surface layer or the front and back layer regions are reheated to _three or more Ac points, accelerated cooling or direct quenching at a cooling rate of 5 ° C./sec or more. The method for producing a structural steel having excellent corrosion resistance according to any one of claims 5 and 6, characterized in that: 8. The method for producing a structural steel having excellent corrosion resistance according to claim 7, wherein tempering is performed after accelerated cooling or direct quenching is completed. 9. Further, in weight%, Cu: 0.05-1.
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
The method for producing a structural steel having excellent corrosion resistance according to any one of claims 5 to 8, wherein the steel contains at least one kind or two or more kinds. 10. The composition according to claim 1, further comprising:
-0.02%, Mg: 0.0001-0.02%, RE
The production of structural steel having excellent corrosion resistance according to any one of claims 5 to 9, wherein one or more of M: 0.001% to 0.2% is contained. Method.