JPH0129859B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a low alloy steel with high weldability and high corrosion resistance that is suitable as a material for welded structures in corrosive environments involving seawater or general atmospheric corrosive environments such as bridges. Corrosive environments involving seawater include the corrosive environment in which underwater structures are immersed in seawater, the corrosive environment in which port structures are exposed to both seawater spray and immersion in seawater, and the corrosive environment in which ship ballast tanks are exposed to high humidity and sealed conditions. There are various corrosive environments such as Of these, the ballast tanks of ships, such as tanker ballast tanks, are particularly susceptible to severe corrosion.
Moreover, since there is no conventionally excellent corrosion-resistant steel, it has been difficult to take measures to prevent corrosion. Ballast tanks are sometimes filled with seawater or emptied, and in some cases the temperature can reach around 50 degrees Celsius, so they are kept in a high-humidity sealed state. In such cases, a very thin liquid film of seawater is formed on the steel plate, and oxygen is sufficiently supplied, resulting in extremely severe local corrosion, sometimes reaching a corrosion rate of 1 mm or more per year. In the case of ballast tanks, corrosion leads to deterioration of the ship's strength and is considered an important issue from a safety standpoint. Corrosion prevention measures for such marine structures have conventionally been to use ordinary carbon steel and paint with tar epoxy or the like, and in underwater areas, cathodic protection has been taken using Zn anodes.
However, paint has a short lifespan in this environment and must be periodically repaired, and cathodic protection is completely ineffective above sea. Furthermore, since the electrodes must be replaced periodically, neither method is a complete anti-corrosion measure. Therefore, the use of inexpensive weldable corrosion-resistant steel with excellent corrosion resistance is considered to be the best anti-corrosion measure, but conventional knowledge suggests that commercially available seawater steel (0.2C-0.1P-
0.5Cu-0.5Ni steel, etc.) was known to have the best corrosion resistance. However, this type of seawater-resistant steel has poor weldability and frequently cracks during normal welding, making it impractical as a steel for welded structures such as shipbuilding. Therefore, there has been a strong desire to develop a steel that has excellent weldability and better corrosion resistance than conventional corrosion-resistant steels. Therefore, as a steel to deal with such problems, for example, Japanese Patent Application Laid-Open No. 51-71817 or
The steel described in Publication No. 52-123918 has been known for a long time. For example, the steel in JP-A-51-71817 is Si―Mn―
This steel has the P-Cu-Cr-Ni-Al system as its basic component, with additions of V, Nb, Ti, Zr, etc. Although this steel has good corrosion resistance, it does not contain expensive Ni. There is. Also, the steel in JP-A-52-123918 is Ti,
Contains Zr as an essential component and N 0.002 to 0015
%contains. This makes it a weather-resistant steel with excellent toughness in the heat-affected zone during welding with large heat input, especially in the vicinity of the bond area. In other words, the aim was to improve the toughness of the welded part by producing TiN and ZrN, and therefore N
The content is also as high as at least 0.0048% in the examples. On the other hand, the present inventors also
- No. 70911 proposes steel suitable for use in ballast tanks, etc. This steel has a low C-P-Mo system as a basic component and has excellent corrosion resistance under high humidity sealed conditions, but there was still room for improvement in terms of toughness in high heat input welded parts. . Therefore, the present inventors determined the amount of corrosion in a corrosive environment,
As a result of examining the effects of various alloying elements on weldability, we found that in steel containing P, extremely low C and extremely low N
It is possible to obtain relatively high strength and corrosion resistance twice that of conventional corrosion-resistant steel, and it also has a relatively high strength and corrosion resistance that is twice that of conventional corrosion-resistant steel.
We have obtained new knowledge that the problem, which had been given up due to the poor weldability of steel containing steel, can be solved by making it extremely low in C, N, and S. In other words, we have focused on the fact that steel manufacturing technology that reduces N and S content has recently developed, and it has become possible to obtain such low N and S steel. I found out something like this. First, P has a higher solid solubility in ferrite than in austenite, so it is uniformly dispersed in ultra-low C steel, and in steel with a pearlite structure, cementite interacts with N, P, and S during welding. As a result, the embrittlement effect caused by P increases, and pearlite parts containing P and S tend to become the starting point for brittle cracks and weld cracks. The tendency of cracks to occur can be reduced, and P
By minimizing the negative effects of
I confirmed that it can be deleted. In other words, Figure 1 shows C0.04%, Si0.2%, Mn1.2%,
This shows the relationship between S content and weld cracking resistance for steel with P0.07%, Cu0.25%, Al0.02%, and N0.0035%.It shows that the weld cracking resistance greatly improves as S content decreases. I understand. Therefore, during welding, even if the joint is heated and rapidly cooled to the austenitic region, there is no tendency for weld cold cracking or joint embrittlement caused by P, N, S, and C to occur.In fact, a high heat input welded joint has high toughness. available. In addition, P is a component that increases the hot cracking properties of weld metal, and guidance has been given to reduce it as much as possible, but by reducing the amount of C, N, and S to extremely low levels, the solidification segregation of P will be extremely reduced, resulting in cracking of weld metal. It was confirmed that there were no problems at all. This includes Fe
-P-low S-C-coagulation reaction in low N system with C content of 0.11
% or less, δ iron solidification occurs, and it is thought that one reason is that the peritectic reaction disappears;
Due to the segregation of N and S, weldability and bond toughness can be improved by reducing the amount of C in molten steel to 0.07% or less.By reducing the amount of N to 0.004% or less and the amount of S to 0.005% or less, it has become possible to improve weldability and bond toughness. In other words, even if there is segregation of C, Mn, etc., P is uniformly dispersed and solidified, and the amount of C that P does not adversely affect the hot cracking of the weld metal and heat affected zone and the bond toughness of the weld zone is 0.07% or less. N0.004% or less,
S is 0.005% or less. Furthermore, P is known to be the largest element in ferrite solid solution strengthening, but by eliminating the negative effect of P on weldability, even though it is an extremely low C steel,
It was found that relatively high strength properties can be obtained by increasing the amount of P. Furthermore, steel with a low percentage of pearlite has no local cell activity with ferrite, and P significantly increases the corrosion resistance of ferrite. Therefore, the steel made based on the findings of the present invention has sufficient weldability as a welded structural steel and is superior to conventional steels. It has achieved corrosion resistance twice that of corrosion-resistant steel. Below, the range of ingredients of the present invention is classified into six groups as shown in Table 1.

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[Table] The reason why each component element is defined as shown in Table 1 in the component range of the present invention will be explained below. First, in the invention described in item 1, it has been conventionally believed that C has no effect on corrosion resistance, but previous research has found knowledge about C at about 0.05% or more. According to the findings of the present inventors, in the case of low N steel, there is no effect of improving corrosion resistance up to 0.07%, but the lower the content is below 0.07%, the more the corrosion resistance improves for the reasons mentioned above. Moreover, when P, Cu, and Mo are contained, the effect is even greater. Furthermore, although it is well known that C is an element that generally reduces weldability, the reason for limiting the amount of C to 0.07% or less is as described above. Regarding N, it eliminates the negative effect of P on weldability and the toughness of welded joints, and lowers weld cracking susceptibility through interaction with C and S, so it is 0.004
% or less. The reason for this is as described above. Low S content eliminates the negative effects of P on weldability and the toughness of welded joints, and through interaction with C and N, lowers weld cracking susceptibility, improves the cleanliness of steel, and improves weather resistance. have. Therefore, keep it below 0.005%. The interactions with C, N, P, etc. are as described above. Al is added to refine the structure and to improve corrosion resistance, but it can further enhance the effect of corrosion resistance-improving elements such as P and Cu. In that case, the upper limit of the amount of Al added is 0.2%. Si is added to deoxidize molten steel, but it has a negative effect on corrosion resistance, and if it exceeds 1.0%, the corrosion resistance will drop considerably, so the upper limit was set at 1.0%. Mn has no effect on corrosion resistance, but is added to improve deoxidation, hot workability, and weldability. Also, since it is an ultra-low carbon steel, it is necessary to increase the strength by adding Mn to compensate for the lack of strength. For these purposes, the more Mn is added, the better, but even if it exceeds 2.5%, the effect will be small, so it is set to 2.5% or less. P is the element that has the most effect on corrosion resistance, but
In ordinary 0.15% C steel, it is an element that extremely inhibits weldability. In ultra-low carbon steels with P content of 0.07% or less, the corrosion resistance is further improved with the addition of P, and it is preferable to add P at 0.03% or more. However, if added in excess of 0.20%, 0.04
% or less, and S content less than 0.005%, weldability deteriorates, so the upper limit was set at 0.20%. Mo, Cu, Ni, Co, Cr, and W are all elements that have the effect of improving corrosion resistance.
Cu, Ni, and Cr provide greater resistance to general corrosion, while Mo, Co, and W provide greater resistance to localized corrosion. Both of them exhibit remarkable effects when added in combination with P. These elements are also effective when added singly or in combination of two or more to the invention of item 1,
The greater the amount added, the greater the effect. but,
If it exceeds 4.0%, the effect of improving corrosion resistance will be saturated, and on the contrary, an adverse effect of inhibiting toughness and weldability will appear, so the total content of one or more types was set to be 4.0% or less. Note that these elements exhibit the effect of corrosion resistance when 4.0% or less is added in combination, but when two or more are added in combination, Mo is 0.05 to 1.0% and Cu is 0.02 to 1.0%.
%, more preferably 0.02% or more and less than 0.15%, Ni
is 0.05~2.0%, Co is 0.01~1.0%, Cr is 0.1~3.0
%, W is preferably 0.01 to 1.0%. Furthermore, in the present invention, Nb, V, Ti, Zr,
Add one or more of Ta, B (group A) at 0.2% or less,
The steel of the present invention has a wide range of uses because it can be made to have a high tensile strength without impairing weldability. Furthermore, since the various elements Nb, V, and Ti fix C in the steel as carbides, they also have the function of promoting the effect of lowering C and increasing the effect of P. Furthermore, depending on the corrosive environment, such as in crude oil tanks and crude oil transportation line pipes, H caused by H ₂ S corrosion can penetrate into the steel and accumulate in inclusions, especially sulfides, in the steel, causing corrosion cracking. may occur. Also, rare earth elements (one of the elements with atomic number 57-71)
Ca, Mg, Te, Se (Group B) are elements that have the effect of reducing or altering inclusions in steel, especially sulfides, and are added to the steel of the present invention to reduce the presence of H. This is a corrosion-resistant steel that is effective even in corrosive environments. All of the above elements may be added singly or in combination, and the greater the amount added, the greater the effect on the above-mentioned weldability, strength improvement, and corrosion resistance. However, if the total amount exceeds 0.2%, the effect commensurate with the amount added will be small and the toughness will also decrease, so the total amount of one or more types is set to 0.2% or less. In addition, the above-mentioned groups A and B alone or group A +
Limiting the amount of group B compound added to 0.2% or less is effective in improving corrosion resistance and material properties, but if two or more types are added, Nb0.01-0.07% and V0.01-0.07
%, Ti0.003~0.03%, Zr0.003~0.08%, Ta0.003
~0.07%, B0.01% or less, rare earth elements 0.1% or less,
Ca0.1 or less, Mg0.05% or less, Te0.01~0.08%,
Se is preferably in the range of 0.01 to 0.08%. Furthermore, if only one type is added, the amount may be 0.2%. All of the above-mentioned steels of the present invention could be easily produced by conventional iron making, steel making, and rolling processes. Furthermore, although the steel of the present invention exhibits the above-mentioned excellent properties as it is as it is normally hot-rolled, its performance may not be even better even if it is subjected to heat treatment such as quenching, tempering, or normalizing after hot-rolling. However, it is not inferior. Next, the effects of the steel of the present invention will be specifically described below based on examples. Examples Table 2 shows the chemical composition and corrosion resistance of the test materials. Corrosion resistance was determined by the results of a one-year actual ship test in a tanker ballast tank to determine seawater resistance, and a two-year atmospheric exposure test conducted in a semi-rural area to determine weather resistance. Both are shown as corrosion ratios when the amount of corrosion of SM steel is set as 100. In Table 2, samples No. 1 to 4 are comparative materials,
No. 5 to No. 25 are the steels of the present invention. In Tables 2 and 3, No. 4, which has a low C content and contains P, Cu, and Mo, has good seawater resistance, but because it has a high N content and excessive P and S content, it is shown in Table 3. The simulated heat-affected zone vEo (Sharpey impact value at 0°C), which is an indicator of high heat input weldability, is low. However, the low C, low N, and low S steels of the present invention all have high corrosion resistance, weld crack stop preheating temperature, and toughness of the reproduced heat affected zone.
It exhibits properties suitable for high weldability and corrosion-resistant steel. As described above, the steel of the present invention has extremely excellent corrosion resistance and weldability, and can be manufactured easily and inexpensively, making it very useful industrially.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between S content and weld cracking resistance.

Claims (1)

[Claims] 1 C0.07% or less, Si1.0% or less, Mn2.5% or less,
Contains P0.03~0.20%, Al0.003~0.2%, and S
and P-containing highly weldable, corrosion-resistant steel in which N is limited to 0.005% or less and 0.004% or less, respectively, and the remainder is substantially iron. 2 C0.07% or less, Si1.0% or less, Mn2.5% or less,
P0.03~0.20%, Al0.003~0.2%, Mo, Cu, Ni,
Contains a total of 4% or less of one or more of Co, Cr, and W,
Furthermore, S and N are limited to 0.005% or less and 0.004% or less, respectively, and the remainder is substantially iron, a P-containing highly weldable and corrosion-resistant steel. 3 C0.07% or less, Si1.0% or less, Mn2.5% or less,
P0.03~0.20%, Al0.003~0.2%, Nb, V, Ti,
Contains 0.2% of one or more of Zr, Ta, B (group A), and further contains 0.005% or less of S and N, 0.004
% or less, with the remainder consisting essentially of iron.
High weldability corrosion resistant steel. 4 C0.07% or less, Si1.0% or less, Mn2.5% or less,
P0.03~0.20%, Al0.003~0.2%, Nb, V, Ti,
1 or more and 0.2% or less of Zr, Ta, B (group A),
1 of rare earth elements, Ca, Mg, Te, Se (group B)
Contains 0.2% or more of species or more, and the total of Group A + Group B is 0.2% or less, and S and N are each 0.005%.
Hereinafter, P-containing high weldability and corrosion-resistant steel is limited to 0.004% or less, with the remainder essentially consisting of iron. 5 C0.07% or less, Si1.0% or less, Mn2.5% or less,
P0.03~0.20%, Al0.003~0.2%, Mo, Cu, Ni,
Contains a total of 4% or less of one or more of Co, Cr, and W,
Contains at least 0.2% of Nb, V, Ti, Zr, Ta, and B (group A), and further contains S and N, respectively.
Highly weldable and corrosion-resistant P-containing steel, limited to 0.005% or less, 0.004% or less, with the remainder consisting essentially of iron. 6 C0.07% or less, Si1.0% or less, Mn2.5% or less,
P0.03~0.20%, Al0.003~0.2%, Mo, Cu, Ni,
Contains a total of 4% or less of one or more of Co, Cr, and W,
One or more of Nb, V, Ti, Zr, Ta, B (group A), 0.2% or less, rare earth elements, Ca, Mg, Te, Se (B
Contains 0.2% or more of one or more of the following groups), and the total of Group A + Group B is 0.2% or less, and S and N
is limited to 0.005% or less and 0.004% or less, respectively, and the remainder is substantially iron-containing high weldability and corrosion-resistant steel.