JPS6032709B2 - P-containing high weldability corrosion resistant steel - 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 of seawater immersion that seawater structures experience, the corrosive environment of both seawater splash and seawater immersion that seawater structures experience, and the high-temperature sealed environment that ships' ballast tanks experience. There are various corrosive environments such as corrosive environments.

Among these, the corrosion of ships' ballast tanks, typified by tanker ballast tanks, is particularly severe, and since there is currently no excellent corrosion-resistant steel, it has been difficult to take measures to prevent corrosion. Ballast tanks are filled with seawater and emptied, and in some cases can reach temperatures as high as 500m, so they are sealed at high temperatures. In this good case, a very thin liquid film of seawater forms on the steel plate and sufficient oxygen is supplied, resulting in very severe local corrosion, sometimes reaching a corrosion rate of more than one side 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. Conventional anti-corrosion measures for such marine structures include painting ordinary carbon steel with tarepoxy or the like, and electrolytic corrosion protection using Zn anodes in underwater areas.

However, paint has a short lifespan in this environment and must be periodically repaired, and cathodic protection is completely ineffective at 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.
Next-0.1P-0. to u-0. It was known that steel (such as Shu I steel) had the best corrosion resistance. However, this type of seawater-resistant steel has poor weldability, and weld cracks occur frequently during normal welding.
It was not practical as a steel for welded structures in shipbuilding and other areas.

Therefore, there has been a strong desire to develop a steel that has excellent acid-soluble properties and better corrosion resistance than conventional corrosion-resistant steels. Therefore, as a steel to deal with such problems, for example, Tokkan Sho 51
Steels described in Japanese Patent No. 171817 or Tokukan Sho 52-1123918 have been known.

For example, the steel of JP-A-51-71817 is Si-Mn-
The basic component is P-Cu-Cr-Ni-N, and other V
, Nb, Ti, Zr, etc., and although this steel has good corrosion resistance, it also contains expensive Ni.
Further, the copper disclosed in JP-A-52-123918 contains Ti and Zr as essential components, and 0.002 to 0.015% N.
contains. This makes it a film-resistant steel with excellent welding heat-affected zone, especially near the bond area, under high heat input. That is, the aim is to improve the toughness of the welded part by generating TIN and ZrN, and therefore the N content is as high as at least 0.0048% in the examples. In response to this, the present inventors have also proposed a steel suitable for use in ballast tanks and the like in Japanese Patent Laid-Open No. 70911/1983. This steel has a low C-P-Mo system as its basic component and has excellent corrosion resistance under high-temperature sealed conditions, but there is still room for improvement in terms of the toughness of the high heat input weld pocket. Ta.

Therefore, the present inventors investigated the amount of corrosion in a corrosive environment and the influence of various alloying elements on weldability.As a result, steel containing P has relatively high strength despite extremely low C and N. Achieving twice the corrosion resistance of conventional corrosion-resistant steels, and solving the problem of conventional high-P and N-containing steels, which had been given up due to their poor weldability, by making them extremely low in C and N. We obtained new knowledge that

In other words, focusing on the recent development of steelmaking technology that reduces N and the fact that it has become possible to obtain low-N steel, we have conducted various studies on steels that reduce N as much as possible, and have found the following. Ta. 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 containing pearlite structure, due to the interaction between cementite, N, and P during welding, The arm-forming effect of P increases, and pearlite parts containing P tend to become brittle cracks and the starting point of weld cracks, but by reducing the pearlite part by turning it into a trough, it is possible to reduce the occurrence of cracks at the ferrite-carbide interface. It was confirmed that the negative effects of P can be reduced by minimizing the negative effects of P. Therefore, during welding, even if the austenite is heated and rapidly cooled by V, there is no problem of weld cold cracking or vertical joint verticalization caused by P, N, and C, and in fact, high toughness can be obtained with high heat input welded joints. .

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 extremely low C,
By converting to N, the solidification corner breakage of P is extremely reduced.
It was also confirmed that there were no problems with weld metal cracking.

This is because the coagulation reaction in the Fe-P-C-low N system has a C content of 0.
If it is less than 11%, 6-iron solidification occurs, which is thought to be due to the absence of peritectic reaction, but due to the casing of C and N during solidification of the steel ingot, the amount of C in molten steel is 0.07% or less. By reducing the content to .004% or less, it became possible to improve weldability and poundability. In other words, even if there is segregation of C, Mn, etc., P is uniformly dispersed and solidified, and P does not adversely affect hot cracking in the weld metal and heat affected zone and bond toughness of the weld zone.
The amount is 0.07% or less NO. 0.004% or less.

Furthermore, although P is known to be the most important element for ferrite storm welding strengthening, by eliminating the negative effect of P on weldability, it is possible to change relatively high strength properties by increasing the amount of P, even in extremely low C steels. I solved it.

Steels with a low percentage of pearlite have no local cell activity with ferrite, and P significantly increases the corrosion resistance of ferrite. Therefore, the steel made based on the knowledge of the present invention has sufficient weldability as a welded structural steel and has the same properties as conventional steels. It has achieved corrosion resistance twice that of corrosion-resistant steel.

Below, the component ranges of the present invention are classified into *3 groups as shown in Table 1.

Table 1 Component range of the steel of the present invention (many) 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 basic component system, C was conventionally thought to have 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 below 0.07%, the lower the content, 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 acidity, the reason for limiting the amount of C to 0.07% or less is as described above.

Si is added to deoxidize molten steel, but it has an adverse effect on corrosion resistance, and in particular, if it exceeds 1.0%, the corrosion resistance is considerably reduced, so the upper limit was set at 1.0%.

Although it has no effect on Mn' corrosion resistance, it improves deoxidation, hot workability,
Added to improve weldability. Furthermore, 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 there is, the better.
Even if it is added in excess of 5%, the effect is small, so 2.5%
The following.

P is the element that has the most effect on corrosion resistance, but the normal 0
．． In 15% C steel, it is an element that extremely inhibits weldability.

In ultra-low carbon steel of 0.07% or less, when P is added, Mo is 0.05-1.0% and Cu is 0.02-1.0%.
, more preferably 0.02% or more and less than 0.15%, N
i is 0.05~2-0%, C〇 is ○-01~1.0%,
W is preferably 0.01 to 1-0%. At the same time, corrosion resistance is further improved, and it is preferable to add 0.03% or more.

However, if added in excess of 0.02%, 0.04%
% or less, the weldability deteriorates, so the upper limit is set to 0.
．． It was set at 20%.

Al is added for deoxidation and grain refinement, and from the viewpoint of corrosion resistance, but it also has the effect of further increasing the effect of corrosion resistance improving elements such as P and Cu.

However, if more than 0.2% is added, the cleanliness of the steel will be impaired and the toughness will deteriorate, so the upper limit was set at 0.2%.
On the other hand, if the content is less than 0.003%, these effects cannot be obtained, so the lower limit is set to 0.003%.

Mo, CuNi, Co, and W are all elements that have the effect of improving corrosion resistance. Among these, Cu and Ni increase resistance to general corrosion, and Mo, Co, and W increase resistance to local corrosion. do. 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. However, 4.0%
If the content exceeds the above, the effect of improving corrosion resistance will be saturated, and on the contrary, there will be an adverse effect of inhibiting grain quality and weldability. Note that these elements are added in a combined amount of 4.0% or less to exhibit the effect of corrosion resistance, but the combined addition of two or more types of N eliminates the negative effects of P on weldability and the toughness of welded joints, and improves the mutual interaction with C. This action lowers the susceptibility to weld cracking, so it is kept to 0.004% or less.

The reason for this is as described above. The above is the basic component system in the present invention, but the invention further includes Nb
, V, Ti, Zr, Ta, B (group A), one or more 0
．． By adding 2% or less, it is possible to increase the tensile strength without impairing weldability, thereby expanding the applications of the steel of the present invention.

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. Depending on the corrosive environment, for example, in crude oil tanks or crude oil transportation line pipes, heat caused by 2S corrosion may penetrate into the steel and accumulate inclusions in the steel, especially sulfides, causing corrosion cracks. There is. In addition, rare earth elements (one or more elements with atomic numbers 57-71 and Y
), Ca is an element (group B) that has the effect of reducing or altering inclusions in steel, especially sulfides, and is added to the steel of the present invention to create corrosion-resistant steel that is effective even in a corrosive environment involving sunlight. That is. 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 rutting property will be reduced.
．． 2% or more.

In addition, the amount of the above-mentioned group A alone or the combination of groups A+B is 0.
．． Limiting it to 2% is effective in improving corrosion resistance and material properties, but if two or more types are added, Nbo. 01-0.07
%, VO. 01-0.07%, Tio. 003~0.0
3%, Zro. 003~0. % female, Tao. 003~0
．． 07%, BO. 01% or less, rare earth elements 0.1% or less, Cao. A range of 1% or less is desirable.

Moreover, when only one type is added, it may be added up to 0.2%. All of the above-mentioned steels of the present invention can be easily manufactured by conventional gun steel manufacturing and rolling.

Furthermore, although the steel of the present invention exhibits the above-mentioned excellent properties as it is in a normal heat-bonded state, its performance will not be even better even if it is subjected to heat treatment such as quenching, reinforcing, or taming, which is common after hot boating. Even if there is, it will not be inferior. Next, the effects of the steel of the present invention will be specifically described below based on examples.

implementation row Table 2 shows the chemical composition and corrosion resistance of the test materials.

Corrosion resistance was determined by the results of a one-year backboard 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, sample No. 1 to 4 are comparative materials, and No. 9～No
．． 78 is the steel of the present invention.

In Tables 2 and 3, N containing P, Cu, and Mo has a low C content.
o. 4 has good seawater resistance, but because the N content is high and the P content is excessive, vEo (Charpy impact value at 0°C) of the reproduced heat affected zone, which is an index of high heat input weldability in Table 3, is low. However, both the low C and low N steels of the present invention have high corrosion resistance, high weld cracking stop preheating temperature, and toughness of the reproduced heat affected zone, and exhibit characteristics suitable for highly weldable corrosion resistant steels. In conclusion, 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.

Table 2 (Continued from Table 2) *Clean ballast. Results of a one-year actual ship test in a tank, corrosion ratio when conventional SM material is set as 100.

** Results of two-year atmospheric exposure test in Sagamihara City, S
Let the corrosion weight loss of M material be 100.

Table 3 *Based on JIS Z3158** Maximum heating temperature 13.50℃, 800~50
Cooled at 0°C for 80 seconds, then heated in a heated oven (80KJ, plate thickness 25 m).

Claims (1)

[Claims] 1 C 0.07% or less, Si 1.0% or less, Mn 2.5
% or less, P0.03-0.20%, Al0.003-0
．． Contains 2%, further contains 4% or less of one or more of Mo, Cu, Ni, Co, and W in total, and further contains 0.004 N.
% or less, 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
．． Contains 2% and further contains 4% or less of one or more of Mo, Cu, Ni, Co, W in total, Nb, V, Ti, Zr
, Ta, and B in a total of 0.2% or less, N is further limited to 0.004% or less, and the remainder is substantially iron. 3 C0.07% or less, Si1.0% or less, Mn2.5
% or less, P0.03-0.20%, Al0.003-0
．． Contains 2% and further contains 4% or less of one or more of Mo, Cu, Ni, Co, W in total, Nb, V, Ti, Zr
, Ta, and B (Group A) in total of 0.2% or less, rare earth elements, and Ca (Group B) in total of 0.2% or less.
2% or less, and the total of group A + group B is 0.2% or less, and N is further limited to 0.004% or less, and the remainder is substantially iron.