JPH10330880A - Steel for welded structure, excellent in seawater corrosion resistance, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for welded structure, excellent in seawater corrosion resistance in marine environments under exposure to seawater and air. SOLUTION: The steel for welded structure, excellent in seawater corrosion resistance, has a composition consisting of, by weight, <=0.15% C, <=0.7% Si, 0.20-1.50% Mn, 0.03-0.15% P, <=0.020% S, 0.010-0.10% Al, <=0.10% Cr, 0.4-4.0% Ni, <=0.4% Cu, 0.10-1.50% Mo, and the balance Fe with inevitable impurities. It is further desirable that Ni and Mo satisfy the relation of Ni+3Mo>=1.2% and the carbon equivalent (Ceq) and weld cracking parameter (Pcm), represented by Ceq=C+Si/24+Mn/6+Cr/5+Ni/40+Mo/4+V/14 and Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B, are regulated to <=0.4% and <=0.20%, respectively. Moreover, it is desirable that Ni, Mn, and Mo satisfy the relations of Ni+3Mo>=1.2% and Mn×Mo<=0.4%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a high corrosion resistance and high weldability suitable for the construction of a welded steel structure used in an environment directly exposed to seawater, such as a pier, seawall, or ship, It relates to a low alloy steel having good toughness.

[0002]

2. Description of the Related Art Steel structures such as piers, seawalls, and ships are repeatedly exposed to seawater, so that the steel structure is extremely susceptible to corrosion. Especially in the marine environment,
Corrosion is much greater in the tidal zone of the tide where seawater is repeatedly attached and dried and in the splash zone where seawater is repeatedly attached and dried than in seawater.

Therefore, in order to provide marine structures used in such an environment with durability for several decades,
It is necessary to apply a durable coating such as polyethylene lining to the steel material to be used, or to use titanium clad steel or the like in which the surface of a steel plate is coated with a highly corrosion-resistant metal such as titanium. However, such an anticorrosion method is not only expensive in the material itself, but also extremely difficult to perform welding, and causes an increase in construction cost.

On the other hand, there is a so-called seawater-resistant steel as an inexpensive and relatively easy-to-weld corrosion-resistant steel material. The seawater-resistant steel is a steel material having improved corrosion resistance in seawater and on the sea by containing an appropriate amount of an element effective against seawater corrosion, such as copper, phosphorus, chromium, nickel, tungsten, and aluminum. However, although the seawater-resistant steel on the market has a higher corrosion resistance effect than ordinary carbon steel, its corrosion rate is at least as high as that of carbon steel.
It can be reduced to about 1/2, which is not sufficient in terms of long-term durability.

In order to further improve the corrosion resistance, it is necessary to contain these effective elements in larger amounts. For such seawater resistant steel, see, for example,
Japanese Patent No. 8441 discloses a steel containing about 5 to 6% of Al and Cr, and Japanese Patent Application Laid-Open No. HEI 5-117812 discloses a steel containing 5 to 45% of Mn or the same amount of Mn and Cr simultaneously. Have been.

However, the inclusion of a large amount of elements effective against seawater corrosion is not a favorable direction for weldability, and if it is intended to impart sufficient corrosion resistance to a steel material by increasing the content of the element, the welding is difficult. Defects are more likely to occur, and inspection and repair of the welds requires a great deal of labor and cost. In other words, it has been difficult to produce inexpensively a steel material for a welded structure having sufficient seawater corrosion resistance in a marine environment by the conventional technology.

[0007]

SUMMARY OF THE INVENTION An object of the present invention has been made in view of the above circumstances, and it is an object of the present invention to have sufficient seawater corrosion resistance in a marine environment and excellent weldability. Another object is to provide a steel material having high toughness. This facilitates the construction of steel structures,
In addition, since the toughness is good, it is possible to enhance safety against destruction.

[0008]

Means for Solving the Problems In order to achieve the above object, the inventors have made intensive studies on the component composition of steel materials. The present inventors have invented a weather-resistant steel having high corrosion resistance, which is suitable for a welding structure material in a corrosive environment in which flying salt is involved, such as a bridge or a steel tower that is previously constructed in a coastal region (Japanese Patent Laid-Open No. H10-163873). 8-134587). Corrosion resistance in the marine environment is a more severe environment than the weather resistance required for steel materials used in coastal areas, but the present invention examines the present invention in detail and achieves both high corrosion resistance and weldability. And found that it is possible to produce a steel material having good toughness.

In the first invention, C: 0.15% or less, Si: 0.7% or less, Mn: 0.20% to 1.50%, P: 0.03 to 0.1% by weight.
15%, S: 0.020% or less, Al: 0.010 to 0.10%, Cr: 0.10%
Below, Ni: 0.4 ~ 4.0%, Cu: 0.4% or less, Mo: 0.10% ~
It is a welded structural steel with excellent seawater corrosion resistance, characterized by containing 1.50% and the balance being Fe and inevitable impurities. According to the present invention, seawater corrosion resistance of a basic steel material can be improved.

[0010] The second invention is the above steel, wherein the steel further comprises Ni.
Mo satisfies the relationship of Ni + 3Mo ≧ 1.2%, and the carbon equivalent (Ceq) shown below is 0.4% or less, and the welding crack susceptibility index (Pc
m): A welded structural steel excellent in seawater corrosion resistance, characterized by being 0.20% or less. Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + Mo / 4 + V / 14 Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B According to the present invention, the seawater corrosion resistance and weldability of steel can be further improved at the same time.

[0011] A third aspect of the present invention is the steel, wherein the steel further comprises Mn.
Mo is a welded structural steel excellent in seawater corrosion resistance, characterized by satisfying the relationship of Mn × Mo ≦ 0.4%. According to the present invention, the toughness of a steel material having excellent seawater corrosion resistance can be improved.

A fourth aspect of the present invention is the above steel, wherein the steel further comprises Ni,
Mn and Mo satisfy the relationship of Ni + 3Mo ≧ 1.2% and Mn × Mo ≦ 0.4%, and the carbon equivalent (Ceq) shown below is 0.4% or less, and the welding crack susceptibility index (Pcm) is 0.20% or less. It is a welded structural steel excellent in seawater corrosion resistance. Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + Mo / 4 + V / 14 Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B According to the present invention, a steel material having both seawater corrosion resistance, weldability, and toughness can be obtained.

According to a fifth aspect of the present invention, the steel having the chemical composition according to any one of the first to fourth aspects of the present invention is subjected to continuous casting or decomposition rolling, and then the obtained steel slab is reheated and hot rolled. In this case, the accumulated rolling reduction at 950 ° C. or less is 20% or more, and the hot rolling is completed in a temperature range of 750 ° C. or more and 900 ° C. or less. It is a manufacturing method. According to the present invention, the toughness of the steel material can be stably increased, so that the safety of the manufactured steel structure against destruction can be improved.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic concept of the present invention will be described below. (1) Since Cr has an effect of improving the corrosion resistance of steel, it has often been added to conventionally produced seawater-resistant steel. However, as a result of investigations by the present inventors, as shown in FIG. 5, an increase in the Cr content certainly improves the seawater corrosion resistance evaluated at the maximum drilling depth. However, it has been found that when Ni and Mo are simultaneously contained in a region where the Cr content is about 0.5% or less, Cr has an effect of rather promoting local corrosion.

Therefore, by minimizing the Cr content, the corrosion resistance of steel in an environment directly exposed to seawater can be improved. In addition, Cr generally increases the hardness of the welded portion and easily causes so-called low-temperature cracking, thereby deteriorating weldability. Therefore, from the viewpoint of weldability, it is desirable to reduce the Cr content as much as possible.

(2) As shown in FIG. 6, the effect of improving the seawater corrosion resistance is observed even when Ni and Mo are used alone. However, when an appropriate amount of Ni and Mo are simultaneously contained, the corrosion resistance of the steel material is remarkably improved. I found to do. From this, it is possible to improve the seawater corrosion resistance and limit the weld cracking susceptibility to a practical range by limiting the contents of Ni and Mo and suppressing the Cr content to 0.1% or less. Can be maintained.

(3) Further, the contents of Mn and Mo are limited,
In addition, by finishing hot rolling at an appropriate temperature,
Good toughness can be imparted to steel. Next, in the present invention, the chemical components of steel materials and the reasons for limiting the production method will be described. C is contained to secure a predetermined strength, but 0.1
If it exceeds 5%, the weldability and toughness deteriorate, so the upper limit is made 0.15%.

Si is contained as a deoxidizing agent in the steelmaking stage and as an element for improving the strength of the steel material. However, if contained excessively, the toughness is remarkably reduced, so the upper limit is made 0.7%.

Mn is 0.20% in order to secure a predetermined strength.
Although it must be contained as described above, if it is contained excessively, a bainite structure is likely to be generated, and mechanical properties, particularly toughness, are deteriorated. Therefore, the upper limit is made 1.5%.

P is an important element in the present invention,
Since it has the effect of improving the strength of steel and the effect of improving corrosion resistance, it is contained in a necessary amount. But 0.
If the content is less than 03%, there is no effect on the improvement of the corrosion resistance, and if the content exceeds 0.15%, the weldability deteriorates, so the range is made 0.03 to 0.15%.

Since S is an element harmful to corrosion resistance, its upper limit is made 0.02%. Al is used as a deoxidizer in the steelmaking stage
The content is 0.01% or more, but if it is contained excessively, inclusions serving as corrosion starting points are likely to be generated. Therefore, the upper limit is set to 0.1%.

In a salty environment, Cr promotes pitting corrosion and impairs seawater corrosion resistance. Also,
Increases cold cracking susceptibility during welding. Therefore, the upper limit of the content is set to 0.1%.

Ni is an important element in the present invention,
The coexistence with Mo has the effect of improving the corrosion resistance in a salty environment. However, less than 0.4% has no effect. On the other hand, if the content exceeds 4.0%, it is disadvantageous in terms of economy, and since the bainite structure is easily generated, the mechanical properties, particularly the toughness deteriorates, so the range is 0.4 to 0.4%.
4.0%.

Cu has an effect of improving corrosion resistance and is contained in a required amount. However, if the content exceeds 0.4%, the effect is saturated, and a flaw is easily generated at the time of hot rolling. Since it is disadvantageous, the upper limit is set to 0.4%.

Mo is an important element in the present invention,
The coexistence with Ni has the effect of improving the corrosion resistance in a salty environment. However, the addition of less than 0.1% has no effect. On the other hand, when the content exceeds 1.5%, it is disadvantageous in terms of economy, and a bainite structure is easily generated, and mechanical properties, particularly toughness are deteriorated. Therefore, the range is set to 0.1% to 1.5%.

Ni + 3Mo ≧ 1.2%. Mo, when included together with an appropriate amount of Ni, significantly improves corrosion resistance. The details of the effect of Ni and Mo on seawater corrosion resistance are not clear, but are considered as follows. It is considered that Mo is enriched in the vicinity of the interface of the rust and the denseness of the rust near the interface of the rust, and has an effect of preventing corrosion factors such as moisture and salt from coming into contact with the steel surface.

On the other hand, Mo has the property of making rust brittle and tends to cause defects such as cracks, while Ni has the property of improving the properties of rust and making defects such as cracks less likely to occur. Since a synergistic effect is exhibited by these two different properties, it is considered that by containing an appropriate amount of Mo together with Ni, the corrosion resistance is remarkably improved. From the viewpoint of corrosion resistance, an effect appears when 0.1% or more of Mo is contained together with 0.4% or more of Ni. However, if the amount of Ni + 3Mo is less than 1.2%, the effect is not sufficient, so the range is set to 1.2% or more.

Carbon equivalent (Ceq): 0.4% or less, weld crack susceptibility index (Pcm): 0.20% or less. Here, Ceq and Pcm are represented by the following equations. Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + Mo / 4 + V / 14 Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B In the construction of steel structures, the carbon equivalent (Ceq) and the weld cracking are set to a practical level, that is, 50 to 70 ° C or less, to prevent welding low temperature cracking in welding. Sensitivity index (Pc
The upper limit of m) is 0.4% and 0.2%, respectively.

It is assumed that Mn × Mo ≦ 0.4%. Since Ni and Mo both enhance the hardenability of steel, under normal hot rolling conditions, a bainite structure is easily formed and the toughness becomes insufficient. The steel of the present invention is a steel containing both Ni and Mo.To eliminate such disadvantages, the upper limit of the Mn content is set to 1.50%, and Mo is contained in a range satisfying the relationship of Mn × Mo ≦ 0.4%. I do.

Next, a method for producing steel having the above characteristics will be described. The steel of the present invention is produced by melting in a converter or an electric furnace, and then casting the steel ingot by slab by continuous casting or ingot rolling, and then reheating the steel slab to form a steel plate or a shaped steel by hot rolling. Can be manufactured. At this time, in order to improve the toughness of the steel material, hot rolling is performed at 95%.
The cumulative draft at 0 ° C. or less is set to 20% or more, and the end temperature of hot rolling is suppressed to a relatively low temperature of 750 ° C. to 900 ° C. Thereby, generation of a coarse bainite structure that impairs toughness can be suppressed, generation of a pearlite structure can be promoted, and the structure can be refined. As a result,
A fine ferrite-pearlite structure can be obtained,
Achieve good toughness.

[0031]

Next, embodiments of the present invention will be described. Table 1 shown in FIG. 7 shows the chemical compositions of the test materials according to the present invention steel and the comparative steel, the crack prevention preheating temperature by the oblique y-type welding crack test, and the maximum hole corrosion depth by the combined cycle corrosion test. Here, the combined cycle test is performed under the conditions assuming a tidal zone and a splash zone in the ocean.

In this example, a steel ingot having the composition shown in Table 1 was melted and heated to 1200 ° C. to start hot rolling.
Rolling was completed at 850 ° C. at a cumulative rolling reduction of 30% at 50 ° C. or less to obtain a steel sheet having a thickness of 25 mm. After the completion of the rolling, it was air-cooled to room temperature. The obtained steel sheet is JIS Z 315
8 and the combined cycle corrosion test shown in FIG.

FIG. 1 shows the value of the maximum piercing depth of the test material according to the combined cycle corrosion test together with the crack prevention preheating temperature. Comparative steels a to j and m and n have a maximum perforation depth of 0.45 mm or more and have poor seawater corrosion resistance. The comparative steels k, l, o, and p have relatively small maximum perforation depth and are excellent in seawater corrosion resistance, but their crack prevention preheating temperatures are 100 ° C, 150 ° C, 100 ° C, and 150 ° C, respectively. And weldability is not good.

On the other hand, invention steels A, B, D, E, G, J, K
Has a maximum perforation depth of 0.40 mm or less and a crack prevention preheating temperature of room temperature. Therefore, both seawater corrosion resistance and weldability are good. Invented steels C, F, H, I
Has a preheating temperature of 50 ° C or 70 ° C to prevent welding cracks, but has a maximum drilling depth of 0.30mm, 0.31mm, and 0.
20 mm and 0.21 mm. This is significantly better in corrosion resistance than d, l, j, o, which have comparable weldability in the comparative steel. Further, steels H and I containing Cu within the scope of the present invention have a smaller maximum hole depth than the other invention steels, and the effect of containing Cu is recognized.

Table 2 shown in FIG.
This is the result of exposing seawater to a for one year, removing rust on the surface of the test piece, and measuring the depth of the hole. The method of manufacturing the test steel sheet is the same as described above. The seawater exposure test piece was cut into a width of 40 mm and a length of 6000 mm. The test piece was exposed vertically to a seawater exposure test site at the Nippon Kokan Fukuyama Steel Works in Fukuyama City, Hiroshima Prefecture, so that the center of the test piece in the longitudinal direction was at the level of the average seawater level.
The exposure period is one year.

The results of the seawater exposure test are shown in FIG. 3 as the relationship between the distance from the average sea level and the perforated depth. Corrosion is
The most severe in the so-called splash zone, the perforation depth reaches 0.91 mm for comparative steel a, while it is only 0.33 for invention steel G.
mm, and invention steel G is extremely excellent in corrosion resistance. In addition, inventive steel G shows superior corrosion resistance in both the middle of the sea and the tidal zone compared to comparative steel a.

Table 3 shown as FIG. 9 shows the relationship between the chemical composition of each test material, its rolling conditions and toughness. Invention steel G
From -0-0 to G-5-3, the effect of rolling conditions on toughness was investigated with the same chemical composition. In addition, invention steel G
For -2-2 and H to R and comparative steels q to u, the effect of the composition on the toughness was investigated with the rolling conditions kept constant. The toughness was evaluated by the fracture surface transition temperature (vTrs) and the energy transition temperature (vTre) in the Charpy impact test.

Steel having a finish rolling temperature exceeding 900 ° C. G-0-0
And steel G-1-0 have an energy transition temperature (vTre) exceeding 0 ° C,
Not enough toughness. In addition, even if the finish rolling temperature is 900 ° C or less, steel G-2-1 and steel G-3-1 whose cumulative draft at 950 ° C or less is less than 20% have a vTre exceeding 0 ° C and a toughness of Is not enough. Finish rolling temperature is below 900 ℃, 950 ℃
Steel G-2-2, G-3-2,
Both steel G-4-2 and steel G-4-3 have a vTre of -20 ° C or less and are excellent in toughness. On the other hand, comparative steels q to u
vTre exceeds 0 ° C and toughness is not enough. Invention steel G ~ I and L
~ R all have a vTre of 0 ° C or less and are excellent in toughness.

FIG. 4 shows the effect of the contents of Mn and Mo on toughness. The numbers in the figure are the energy transition temperatures (vTre) of the Charpy test. Mn is 0.2% to 1.5% and
When Mo is in the range of 0.1% to 1.5%, vTre is 0 ° C. or less, and particularly in the range of Mn × Mo ≦ 0.4%, vTre is -20 ° C. or less, and it is clear that the material has particularly excellent toughness.

[0040]

According to the present invention, a structural steel for welding having high corrosion resistance and practical weldability in an environment directly exposed to seawater, such as a pier, seawall, or ship, can be economically produced. can do. Further, the present invention is to produce a steel material in a hot-rolled state, and the hot-rolling process is not only a general thick-plate rolling process, but also a shaped steel rolling process, and further, a thin-plate hot rolling process. The present invention is also applicable to hot strip mill rolling as a rolling process.

[0041]

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram showing a preheating temperature for preventing cracking of an inventive steel and a comparative steel by an oblique y-type welding crack test and a maximum depth of perforated corrosion generated by a combined cycle corrosion test.

FIG. 2 is a view showing test conditions of a combined cycle corrosion test.

FIG. 3 is a graph showing a comparison of corrosion amounts of inventive steel G and comparative steel a exposed to seawater for one year.

FIG. 4 is a diagram showing the relationship between the Mn and Mo contents and the energy transition temperature in the Charpy impact test of the invention steel and the comparative steel.

FIG. 5 is a diagram showing the effect of the Cr content on the maximum depth of perforated corrosion generated by the combined cycle corrosion test.

FIG. 6 is a diagram showing the effect of Ni and Mo contents on the maximum depth of perforated corrosion generated by a combined cycle corrosion test.

FIG. 7 is a table showing the results of the chemical composition of the invention steel and the comparative steel, the crack prevention preheating temperature, and the maximum depth of perforated corrosion generated by the combined cycle corrosion test.

FIG. 8 is a table showing the results of a one-year seawater exposure test of invention steels and comparative steels.

FIG. 9 is a diagram showing, as Table 3, results of chemical compositions, hot rolling conditions, and transition temperatures in a Charpy impact test of invention steels and comparative steels.

Claims (5)

[Claims] (1) C: 0.15% or less, Si: 0.7% by weight
Below, Mn: 0.20% to 1.50%, P: 0.03 to 0.15%, S:
0.020% or less, Al: 0.010 to 0.10%, Cr: 0.10% or less, Ni: 0.4 to 4.0%, Cu: 0.4% or less, Mo: 0.10%
Welded structural steel with excellent seawater corrosion resistance, characterized in that it contains up to 1.50% and the balance is Fe and inevitable impurities. 2. The welding structural steel according to claim 1, further comprising Ni and Mo.
Satisfies the relationship of Ni + 3Mo ≧ 1.2%, the carbon equivalent (Ceq) shown below is 0.4% or less, and the weld crack susceptibility index (P
cm): 0.20% or less, the welded structural steel excellent in seawater corrosion resistance according to claim 1, characterized in that: Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + Mo / 4 + V / 14 Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B 3. The welded structural steel according to claim 1, further comprising Mn and Mo.
Is characterized by satisfying the relationship of Mn × Mo ≦ 0.4%,
The welded structural steel according to claim 1, which is excellent in seawater corrosion resistance. 4. The welding structural steel according to claim 1, further comprising Ni, Mn.
And Mo satisfy the relationship of Ni + 3Mo ≧ 1.2% and Mn × Mo ≦ 0.4%, and the carbon equivalent (Ceq) shown below is 0.4%
The steel for welded structures having excellent seawater corrosion resistance according to claim 1, wherein the weld crack susceptibility index (Pcm) is 0.20% or less. Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + Mo / 4 + V / 14 Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B 5. The steel for welding structure according to any one of claims 1 to 4, wherein after continuous casting or disassembly rolling, the obtained steel slab is reheated and hot-rolled at 950 ° C. or lower. A method for producing a welded structural steel excellent in seawater corrosion resistance, characterized in that the cumulative rolling reduction is 20% or more and hot rolling is completed in a temperature range of 750 ° C or more and 900 ° C or less.