JPH07207340A - Production of steel plate which exhibits excellent corrosion resistance and low-temperature toughness in high sea salt particle and shade environment - Google Patents

Abstract

PURPOSE:To obtain the steel plate for steel structures, such as bridges and buildings by subjecting an Ni-Al-Cr steel having a specific compsn. in which the contents of impurities of S, O, N are lowered to hot rolling at a specific condition, then to air cooling or water cooling. CONSTITUTION:A steel consisting, by weight %, 0.01 to 0.10 C, 0.03 to 0.35 Si, 0.1 to 1.5 Mn, <=0.01 S, 1.5 to 10.0 Ni, 0.1 to 10.0 Cr, 0.05 to 5.0 Al, <=0.005 O, <=0.005 N and the balance Fe is used. The steel is rolled at a heating temp. of 1000 to 1200 deg.C, finishing temp. of 900 to 750 deg.C and a draft of >=50% from 950 deg.C and is then subjected to air cooling or water cooling. The steel plate which is more inexpensive than stainless steels, has the better corrosion resistance in a high sea salt particle and shade environment than weather resistant steels and has excellent low-temp. toughness is obtd. The Al in the steel is composed of 0.03 to 4.95 sol Al. Further, specific ratios of P, Cu, Mo, W, V, Nb, Ta, Ti, B, Ca and REM may be incorporated into this steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel structure such as a bridge or a building, which is required to have corrosion resistance and low temperature toughness particularly in a shaded corrosive environment containing a large amount of chloride ions, such as in coastal areas and areas where snow melting salts are sprayed. The present invention relates to a method for producing a high weather resistance steel used in.

[0002]

2. Description of the Related Art Conventionally, for structural members used for the above-mentioned applications, various weather-resistant steels are available because they are relatively inexpensive and have sufficient corrosion resistance that is necessary and sufficient depending on the use environment. Developed and commercialized by each company. JIS G3114; weather-resistant hot-rolled steel for welded structures and JIS G3125; highly weather-resistant rolled steel according to Japanese Industrial Standard both correspond to this. Weather-resistant steel is ordinary steel containing trace amounts of Cu, Cr, P and the like, and it is known that the rust film formed on the steel surface by exposure to the atmosphere becomes stable rust having a high anticorrosion function.

[0003]

The weather-resistant steel in the above-mentioned prior art exhibits excellent corrosion resistance in an atmospheric corrosion environment, but in a shaded corrosion environment where there are many chloride ions, such as in coastal areas, for example, bridges and bridge girders are It has been pointed out that the corrosion resistance is almost the same as that of ordinary steel because it is a shaded area that is not directly exposed to rainfall. The reason for this is that in the shaded area, the steel surface is not exposed to rainfall, so sea salt particles that fly in and adhere to the surface accumulate without being washed away.Because of the shade, the wetting time is longer than in the sunshine area, and the wet-dry cycle This is because the number is small. For this reason, the coating cost for preventing corrosion is enormous.

On the other hand, stainless steel (for example, JIS stainless steel G4304-84 SUS304,410,4
Since 30) usually contains 11% or more of Cr, its corrosion resistance is far superior to that of weather resistant steel, but its price is expensive, and in a shaded corrosion environment where there are many chlorine ions, the entire surface is the same as weather resistant steel. Corrosion does not occur, and localized corrosion, which is well known as "pitting corrosion", easily occurs. This morphology of corrosion is observed as light corrosion at first glance, but the erosion rate of the corroded part is faster than that of weathering steel where general corrosion occurs, and even through holes can be formed in a relatively short time. .

On the other hand, Ni-Cr-Mo steel for pressure vessels (for example, A
STM A543) is known, but in the case of this steel, quenching and tempering treatment is performed in order to impart strength and toughness. Therefore, the manufacturing cost is higher than that of weather-resistant steel manufactured by ordinary rolling. Further, if the ordinary rolling as it is by the manufacturing method similar to that of the weather-resistant steel, the toughness is low, which is a problem. Because of these
It could not be used for the same purpose as weatherproof steel.

The present invention is a steel plate which is cheaper than stainless steel in steel structures such as bridges and buildings, has higher corrosion resistance in the shade environment with high sea salt particles than stainless steel, and has excellent low temperature toughness. It aims at providing the manufacturing method of.

[0007]

The present invention has been made to solve the above problems, and the gist of the present invention is that C: 0.01 to 0.10% by weight, Si: :
0.03-0.35%, Mn: 0.1-1.5%, S:
0.01% or less, Ni: 1.5 to 10.0%, Cr:
1.5-10.0%, Al: 0.05-5.0%, O:
Steel containing 0.005% or less and N: 0.005% or less with the balance Fe and unavoidable impurities, and sol.
Al: 0.03 to 4.95% of the steel, or P: 0.01 to 0.1%, Cu: 0.05 to 2.0
%, Mo: 0.05 to 2.0%, W: 0.01 to 3.5
%, V: 0.01 to 0.2%, Nb: 0.05 to 0.1
%, Ta: 0.05 to 0.1%, Ti: 0.005 to
0.1%, B: 0.0003 to 0.0040%, Ca:
0.001-0.01%, REM: 0.0010-0.
Steel containing 0050% of one kind or two or more kinds and the balance Fe and unavoidable impurities is heated at a heating temperature of 1000 to 1
200 ° C., finishing temperature 900 to 750 ° C., reduction ratio from 950 ° C. to 50% or more, and air cooling or water cooling after rolling, showing excellent corrosion resistance and low temperature toughness in a shade environment of high sea salt particles It is in the method of manufacturing steel sheets.

[0008]

That is, based on the findings obtained from a number of experiments, the present inventors have improved general corrosion resistance and pitting corrosion resistance in a high sea salt particle shade environment, and further made Ni for a stainless steel or a pressure vessel. The present inventors have found a method for improving low temperature toughness at low cost as compared with -Cr-Mo steel.

First, in order to improve the corrosion resistance in a shaded environment of high sea salt particles, it is necessary to add a ternary compound of Ni-Al-Cr. Corrosion resistance is dramatically improved. This, Ni-Cr-Al composite oxide was concentrated rust inner and composite oxyhydroxide layer is Cl in the environment ^- a low ion concentration ^- base steel interface Cl by inhibiting the penetration of the base steel interface It can be suppressed, the rust colloid particle size in the rust inner layer can be reduced by the ternary compound addition (about 10 nm), the gap inside the rust layer can be reduced, and the corrosion resistance of the rust layer can be improved. It suppresses the change to thick layered rust with low corrosion resistance, which is often seen in
Since the passive film with extremely high anticorrosiveness is extremely stable as compared with the composite addition of only Cr-Al, the wettability time is longer than that in the sunshine area, and even in a shaded environment with high concentration of attached Cl ^- ions, the corrosion weight loss for less active sites elution is suppressed by the combined addition, Cl to base steel interface ^- by the the Al in the steel even pitting concentrated occurs eluted into the food bore, This is due to a plurality of actions of suppressing a decrease in pH in the pit and suppressing deep growth of the pit.

That is, the present invention relates to a rust layer formed during use.
Utilizing the above actions to improve the anticorrosion function of
Utilizing the above to improve the dissolution resistance of the material itself,
Uses the action to improve one's own pitting resistance
It is constructed under the idea that Cr-Al composite additive
In addition, there is no action of ~, and the action of
Higher sea salt particles due to the lack of action
In a shadow environment, Cl at the interface of the ground iron ^-Dotted ion concentration area
However, it is said that severe pitting will occur from there.
There was a problem.

In addition, the composite addition of only Ni-Cr improves the corrosion resistance by the same action as described above, but there is no action of and the effect is inferior to the Ni-Al-Cr complex addition. It was In addition, the combined addition of only Ni-Al improves the corrosion resistance by the same action as above, but there is no action of, and the effect is Ni.
There was a problem that it was inferior to the addition of -Al-Cr composite.

On the other hand, by the addition of Ni-Al-Cr composite, the combination of Cr-Al, Ni-Cr, and Ni-Al binary addition, and the above-mentioned individual additions, which do not occur, are not exhibited.
By the action of the above, the corrosion resistance of steel can be remarkably improved in the shade environment of high sea salt particles. Next, as a manufacturing method for improving low temperature toughness, a method of performing quenching and tempering treatment after rolling is generally well known. However, this method has a disadvantage that the cost is high because reheating treatment is required.

For this reason, at present, the development of a controlled cooling method which is thorough in energy saving and resource saving (reduction of alloying elements) is being actively pursued. The steel produced by this method is characterized in that an excellent material can be obtained without adding a low alloy or an alloy, but has the following drawbacks. Since the heating temperature of the steel slab is high, the austenite grains become coarse and the structure after the cooling transformation becomes coarse, resulting in poor low temperature toughness.

Since the rolling reduction in the recrystallization region is small, the structure after cooling transformation becomes coarse and the low temperature toughness is poor. Since the two-phase region rolling is strengthened for the purpose of improving the brittle fracture propagation stopping property, the absorbed energy in the impact test becomes extremely low. Due to these drawbacks, steel produced by controlled cooling has limited applications.

In order to solve the above drawbacks, the present inventors have
As a result of diligent research on heating, rolling and cooling conditions suitable for Ni-Al-Cr ternary composite additive steel, the amounts of O and N that inevitably mix and deteriorate toughness are reduced, and the S content is limited to a low level. Or the MnS morphologically controlled steel slab is heated at a low temperature (1000 to 1200 ° C.), and a sufficient reduction ratio (950 ° C. to 50% or more) is added in the recrystallization temperature region of the austenite grains to obtain 900 to 750. By finishing the rolling at ℃ and air cooling or water cooling (controlled cooling),
The inventors have found a low-cost manufacturing method that improves the low temperature toughness of the base material.

The present invention will be described in detail below. First, the reason why the components are limited to the above range will be described. C: 0.01% or more of C is added to secure the strength, but if it exceeds 0.10%, the corrosion resistance deteriorates, so 0.10% was made the upper limit.
Si: Si requires 0.03% or more for deoxidation, but if it exceeds 0.35%, the corrosion resistance deteriorates, so 0.3
The upper limit was 5% and the range was 0.03 to 0.35%.

Mn: Mn has a deoxidation and desulfurization effect of 0.
1% or more is added, but if it exceeds 1.5%, the weldability deteriorates, so 1.5% was made the upper limit. S: S is limited to 0.01% or less from the viewpoint of low temperature toughness, but the lower the better, the more advantageous it is to add Ca and REM for controlling the morphology of MnS. Ni: Ni is an element effective in improving the corrosion resistance of steel by a mechanism of suppressing the invasion of Cl ⁻ ions in the environment to the base iron interface by concentrating in the rust layer. This effect is doubled when Cr and Al are added in combination. 1.5% for a reliable effect
The above additions are desirable. On the other hand, if it exceeds 10.0%, it will cause cost increase and the effect will be saturated, so 10.0%
Was set as the upper limit.

Cr: Cr is an element effective in improving the corrosion resistance, but if the added amount is less than 1.5%, it has an adverse effect. In the present invention, it is 1.5% or more in order to obtain a reliable effect. did. On the other hand, if it exceeds 10.0%, the effect is saturated and it causes a cost increase, so 10.0% is made the upper limit, and the range is made 1.5 to 10.0%. Al: Addition of 0.05% or more of Al improves the anticorrosion property of the passivation film formed on the steel surface together with Cr. This effect is N
It is remarkably improved by the combined addition of i and Cr. On the other hand, 5.0
%, The surface cracking tends to occur during hot rolling, so the content is made 0.05 to 5.0%.

Sol. Al: sol. Al is required to be 0.03% or more in order to suppress the decrease in pH in the pit and to suppress the deep growth of the pit by elution into the pit. The oxide-based inclusions that remain in the steel are difficult to dissolve in water and do not act on the suppression effect. Also, 4.95
%, The effect will be saturated, so 0.03 to 4.95%
And

O: O inevitably mixes in molten steel to deteriorate low temperature toughness. If the amount is large, the amount of Ca that binds to Ca and is effective in controlling the morphology of MnS is reduced, and coarse oxide inclusions are formed, which is not preferable from the viewpoint of low temperature toughness. Therefore, the upper limit is set to 0.005%.
N: N, like O, inevitably mixes in molten steel and deteriorates low temperature toughness. From the viewpoint of low temperature toughness, the content is limited to 0.005% or less, but the lower the better.

Each of P and Cu is an element added for the purpose of further improving the corrosion resistance by improving the anticorrosion action of the rust layer formed in the use environment. P: P is 0.0
When added in an amount of 1% or more, the corrosion resistance of the steel material is improved by the mechanism that the rust colloid generated is made fine and the rust layer is made dense, but the effect is not more than 0.
Since 1% is sufficient, the range is set to 0.01 to 0.1%.

Cu: When Cu is added in an amount of 0.05% or more,
The mechanism that finely disperses the dissolution active points on the surface of the steel material in the atmospheric environment, promotes the uniform dissolution of the steel material, and makes the generated rust colloid particles dense, improves the anticorrosion performance of the rust layer, but the effect is 2 If it exceeds 0.0%, it will be saturated, so the range was made 0.05 to 2.0%. Mo and W are elements added for the purpose of further improving the corrosion resistance by the action of suppressing the generation and growth of pitting corrosion by Cl ⁻ ions.

Mo: If Mo is added in an amount of 0.05% or more,
Suppresses local pH drop in the pit during use
Although it has the effect of suppressing the growth of pitting corrosion by a mechanism,
The effect is saturated when it exceeds 2.0%, so 0.05-
The range was 2.0%. W: Add 0.01% or more of W
Then, like Mo, Cl of the protective film on the steel surface ^-ion
Resistance to, but the effect is limited to the upper limit
3.5% is sufficient, so the range is 0.01-0.35
%.

V, Nb, Ti and Ta are elements that are further added for the purpose of ensuring workability. V: When V is added in an amount of 0.01% or more, it is effective in fixing C and N, suppressing an increase in strength due to C and N, and ensuring workability. 0.2% is sufficient as the upper limit of the effect, so
The range was 0.01 to 0.2%.

Nb, Ta, Ti: When Nb and Ta are added by 0.05% or more and Ti is added by 0.005% or more,
It is effective to fix C and N and suppress the strength increase due to C and N to secure workability. Since the upper limit of the effect is 0.1%, Nb and Ta are 0.05%.
.About.0.1%, and Ti is 0.005 to 0.1%. B is an element added for the purpose of further improving the corrosion resistance by the unique action shown below.

B: When B is added in an amount of 0.0003% or more, the protective function of the protective film on the steel surface is improved by the mechanism of suppressing the cathodic reduction dissolution rate of the protective film on the steel surface. Since the upper limit of the effect is 0.0040%, the range is made 0.0003 to 0.0040%. Ca and REM are elements added for the purpose of improving the low temperature toughness by controlling the morphology of MnS.

Ca: When 0.001% or more of Ca is added, the corrosion resistance is improved by the mechanism of suppressing the pH decrease due to the dissolution of the steel material on the steel surface and suppressing the dissolution rate of iron. Is controlled to improve the low temperature toughness, but the effect is 0.01% as the upper limit amount, so the range was made 0.001 to 0.01%. RE
When M: REM is added in an amount of 0.001% or more, it controls the morphology of MnS and improves the low temperature toughness, but the effect is 0.005% as the upper limit amount, so the range is 0.0
It was set to 010 to 0.0050%.

Next, the reasons for limiting the heating and rolling conditions will be described. The reason for limiting the heating temperature to 1000 to 1200 ° C. is to keep the austenite grains at the time of heating small and to make the rolling structure finer. 1200 ° C. is the upper limit temperature at which the austenite grains during heating do not coarsen,
If the heating temperature exceeds this, the austenite grains become coarse, and the structure after cooling transformation also coarsens, so the low temperature toughness deteriorates. On the other hand, if the heating temperature is too low, the additive alloy is not sufficiently solution-treated, the internal quality of the steel is deteriorated, and the finishing temperature of rolling is lowered too much, so that improvement in low temperature toughness cannot be expected. Therefore, it is necessary to set the lower limit to 1000 ° C.

Next, regarding the finishing temperature, the finishing temperature is 900
In the high temperature rolling over ℃, recrystallized austenite grains grow and the grain refinement becomes insufficient, and in the low temperature rolling under 750 ° C, the ferrite structure undergoes processing, so the low temperature toughness deteriorates. Therefore, the finishing temperature is 900
Limited to ~ 750 ° C. Further, the rolling reduction from 950 ° C. was set to 50% or more. This is because if the rolling reduction is less than 50%, recrystallization of austenite partially occurs, resulting in a mixed grain structure and degrading the low temperature toughness.

For cooling after hot rolling, either air cooling or water cooling (controlled cooling) may be applied.

[0031]

EXAMPLES The effects of the present invention will be specifically shown by the following examples. The chemical composition of the test steel is shown in Table 1 and Table 2 (continued from Table 1), the heating temperature, the finishing temperature, the rolling reduction, and the cooling conditions are shown in Table 3, and the strength, toughness, and corrosion resistance of the obtained steel sheet are shown in Table 4. (Continued from Table 3).

In Tables 1 and 2, I to X are steels having a limited composition of the present invention, and A to H are out of the range. Comparative steel A is JI
S G3114: Corresponding to weather resistant hot rolled steel for welded structure, B is JIS G3125: Corresponding to high weather resistant rolled steel, C is as high as Cr content in stainless steel,
E is ASTM A543: Ni-Cr-Mo for pressure vessels
It is equivalent to steel. These steels were hot-rolled according to the rolling conditions shown in Table 3. Test pieces were cut out from the steel sheet thus manufactured and subjected to a tensile test, a Charpy test and a weather resistance test.

In the weather resistance test, in order to reproduce a shaded corrosive environment in which a large amount of chlorine ions are present, such as in a coastal area, 5% saline solution is applied once a day by an exposure stand in which the test piece is not exposed to direct sunlight. An accelerated weathering test for spraying (salt water spraying / shade exposure test) was carried out for one year, and the corrosion weight loss and pitting corrosion coefficient (maximum pitting depth / sheet thickness reduction amount) were evaluated. As is clear from Table 4, steel sheet I obtained by applying the present invention
1, J1, K1, L1, M1, N1, O1, P1, Q
It can be seen that 1, R1, S1, T1, U1, V1, W1 and X1 all have excellent corrosion resistance, strength and toughness as compared with the comparative steel. On the other hand, A1, B1, D1, E
1, F1, G1 and H1 have a large amount of corrosion loss because the composition is out of the limiting components, and C1 has a large pitting coefficient. Also, comparative steel I2
Since J2 has a low heating temperature and finishing temperature, J2 has a high heating temperature and a small reduction rate from 950 ° C, and K2 has a high finishing temperature and a small reduction rate from 950 ° C. Since the rolling reduction is small, the toughness is low in all cases.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

[0038]

As described above, according to the present invention, Ni-Al-Cr steel excellent in general corrosion resistance and pitting corrosion resistance is hot-rolled under the limited conditions, so that it is possible to obtain high corrosion resistance in a shaded corrosion environment with many chloride ions. For structural members such as bridges and buildings that require corrosion resistance and low temperature toughness, Ni-Cr for stainless steel and pressure vessels
Since it is possible to provide a high weather resistance steel that is cheaper than Mo steel, has excellent low temperature toughness, and has corrosion resistance superior to that of weather resistant steel, the industrial effect of the present invention is extremely large.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location C22C 38/54

Claims (7)

[Claims] 1. C: 0.01 to 0.10% by weight, S
i: 0.03 to 0.35%, Mn: 0.1 to 1.5%,
S: 0.01% or less, Ni: 1.5 to 10.0%, C
r: 1.5 to 10.0%, Al: 0.05 to 5.0%,
O: 0.005% or less, N: 0.005% or less,
The heating temperature of the steel consisting of the balance Fe and unavoidable impurities is set to 1
000-1200 ° C, finishing temperature 900-750 ° C, 9
A method for producing a steel sheet exhibiting excellent corrosion resistance and low temperature toughness in a shaded environment of high sea salt particles, characterized in that a rolling reduction from 50 ° C. is 50% or more, and after rolling, air cooling or water cooling is performed. 2. Al: 0.05-5.0% of so
l. Al: 0.03 to 4.95%, The method for producing a steel sheet having excellent corrosion resistance and low temperature toughness in a high sea salt particle shade environment according to claim 1. 3. Further, P: 0.01 to 0.1%, Cu:
The method for producing a steel sheet having excellent corrosion resistance and low temperature toughness in a shaded environment of high sea salt particles according to claim 1 or 2, characterized in that it contains 0.05 to 2.0% of one or two kinds. 4. Further, Mo: 0.05-2.0%, W:
0.01-3.5% of 1 type or 2 types are contained, The high sea salt particle of any one of Claims 1-3 characterized by the above-mentioned. The steel plate which shows the outstanding corrosion resistance and low temperature toughness in shade environment. Production method. 5. Further, V: 0.01 to 0.2%, Nb:
0.05-0.1%, Ta: 0.05-0.1%, T
i: 0.005 to 0.1% of one type or two or more types of high sea salt particles according to any one of claims 1 to 4, which have excellent corrosion resistance and low temperature toughness in a shaded environment. The manufacturing method of the steel plate shown. 6. Further, B: 0.0003 to 0.0040.
%, The method for producing a steel sheet exhibiting excellent corrosion resistance and low temperature toughness in a high sea salt particle shade environment according to any one of claims 1 to 5. 7. Ca: 0.001-0.01%,
REM: 0.0010 to 0.0050% 1 type or 2 types
A method for producing a steel sheet exhibiting excellent corrosion resistance and low temperature toughness in a shade environment of high sea salt particles according to any one of claims 1 to 6, wherein the steel sheet contains a seed.