JPH108197A - Steel plate excellent in corrosion resistance in partially heated zone and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel plate, having excellent corrosion resistance in a base material part as well as having sufficient corrosion resistance in a heated zone in an as-partially-heated state by welding, etc., while obviating the necessity of extreme reduction ion carbon content and tempering heat treatment, and to provide its production method. SOLUTION: This steel plate has a composition consisting of, by weight, <=0.30% C, 0.2-2.0% Mn, <0.016% S, 0.1-1.0% Cu, 0.6[Cu] to 1.0[Cu]% Ni, 0.003-0.001% N, >(48[N]/14+48[S]/32) to <(48[N]/14+48[S]/32+0.2)% Ti, and the balance Fe with inevitable impurities and also has a structure composed essentially of ferrite and bainite. Further, the steel plate can be produced by subjecting a steel slab with the composition to hot rolling at a finishing temp. between 950 deg.C and the Ar3 point, to cooling under the condition of <=5sec time of stay at 700-600 deg.C, and then to coiling at 550-450 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet having excellent corrosion resistance against corrosion generated in a partially heated portion such as a weld.

[0002]

2. Description of the Related Art ERW carbon steel pipes are less expensive than seamless steel pipes. However, when used in severe corrosive environments such as seawater, tap water, and acidic water, the problem is that the welds are selectively corroded. is there. Such corrosion of the partially heated portion is caused by the formation of a macro battery due to the formation of a quenched structure due to rapid heating and quenching during welding, and the formation of a micro battery by the S concentrated portion generated by re-dissolution of sulfide. It is said that.

Therefore, Japanese Patent Publication No. 53-28845, Japanese Patent Publication No. 53-10525 and Japanese Patent Publication No. 56-28984 disclose Cu and Ni which are effective in reducing the potential difference between the welded portion and the base material.
C as a steel component or suppresses sulfide re-dissolution
a and REM are added.

However, in order to suppress the corrosion of the welded portion, not only Ca, REM, Cu, Ni, etc. are added, but also the quenched structure of the welded portion is suppressed as much as possible, so that the welded portion and the base metal portion are not added. It is desirable to reduce the potential difference.

[0005] From this point of view, Japanese Patent Publication No. 59-50747
By tempering the welded part,
No. 9096 aims to improve the corrosion resistance by eliminating the quenched structure by making the steel component extremely low carbon.

[0006]

However, tempering involves an increase in the number of manufacturing steps, increases costs, and has problems such as deformation. On the other hand, extremely low carbonization causes high manufacturing costs, and when increasing strength, it is necessary to add a large amount of expensive alloying elements, and the addition of a large amount of alloying elements promotes the formation of a quenched structure. Will be. Further, there are few conventional techniques that take into account even the corrosion resistance of the base material portion, and there is a problem in use in a severely corrosive environment.

[0007] The present invention has been made in view of such a problem, and it is possible to improve the strength by carbon without performing ultra-low carbon, and to perform partial heat treatment such as welding without performing tempering heat treatment. It is an object of the present invention to provide a steel sheet in which a heating portion can have sufficient corrosion resistance and a base material portion has excellent corrosion resistance.

[0008]

SUMMARY OF THE INVENTION The present invention provides a relatively coarse precipitate, Ti, which is generally said to deteriorate corrosion resistance.
The use of N suppresses the formation of a quenched structure in the welded part and its surrounding area, as much as possible, thereby suppressing the progress of selective corrosion and ensuring sufficient corrosion resistance even when heated. This makes it possible to apply it to the heating section. further,
The formation of Cu scabs is suppressed, and the main constituent of the structure is ferrite and bainite having a low C concentration, thereby improving the corrosion resistance of the base material.

That is, the steel sheet of the present invention contains C
: 0.30% or less, Mn: 0.2 to 2.0%, S:
Less than 0.016%, Cu: 0.1 to 1.0%, Ni:
0.6 [Cu] to 1.0 [Cu]%, N: 0.003 to 0.0
10%, Ti: more than (48 [N] / 14 + 48 [S] / 32) to (48 [N] / 1
4 + 48 [S] /32+0.2)%, or P
: 0.01 to 0.10%, Si: 0.01 to 2.0
%, Nb: 0.01-0.04%, Cr: 0.1-1.
0%, B: 0.0002 to 0.0020%, V: 0.0
Group A consisting of 1 to 0.20%, Mo: 0.01 to 0.20%, REM: 0.0004 to 0.0020%, C
a: It contains one or two or more elements from the group B consisting of 0.0004 to 0.0020%, the balance being Fe and unavoidable impurities, and having a structure mainly composed of ferrite and bainite. is there.

In the case where improvement in strength is desired, in the above components, C: 0.065 to 0.30%, Ti: (48 [N] / 14 +
48 [S] / 32) It is better to set to more than (48 [N] / 14 + 48 [S] /32+0.1)%.

Here, the reason for limiting the steel component (unit: weight%) of the present invention will be described first. In addition, [element symbol]
Indicates the content (%) of the element.

C: 0.30% or less C is an element effective for increasing the strength of the steel sheet, or if added too much, the corrosion resistance is deteriorated. However, as the C content is smaller, the corrosion resistance is improved. Therefore, when strength is not required, the content is preferably 0.1% or less, more preferably 0.040% or less. Note that C
Although the lower limit of the content is not particularly limited, the level which can be industrially produced is about 0.0010%, and in the present invention, a sufficient corrosion resistance effect can be obtained with such a level. In the present invention, it is not necessary to reduce the carbon content to an extremely low level in order to avoid the formation of a quenched structure. To obtain a steel sheet having a high strength level of 440 N / mm ² or more, 0.05% or more, preferably 0% or more. .06
What is necessary is just to contain C amount of 5% or more.

Mn: 0.2 to 2.0% Mn is effective for increasing the strength. If it is less than 0.2%, the effect is too small. Even if added up to 2.0%, the effect of the present invention is not impaired, so the upper limit is made 2.0%.

S: less than 0.016% S reduces the corrosion resistance of the steel sheet.
Less than 016%. It is preferably less than 0.010%, more preferably less than 0.002% in consideration of the corrosion resistance of the base material.

Cu: 0.10 to 1.0% Cu suppresses the expansion of the potential difference between the heating portion and the base material portion.
It is desirable to add 0.10% or more. However, Cu
When adding Ni, expensive Ni must be added at the same time, and the effect is saturated even if 0.3% or more is added. Therefore, the upper limit is 1.0%.

Ni: 0.6 [Cu] to 1.0 [Cu]% Ni suppresses Cu scab and suppresses the formation of a micro battery,
It has the effect of improving corrosion resistance. From the viewpoint of improving corrosion resistance, it is necessary to add 60% or more of the Cu content. However, since Ni is expensive, only the same amount as Ni is added.

N: 0.003% to 0.010% N combines with Ti to form TiN, suppresses austenite coarsening during heating such as welding, and prevents the formation of an extremely hardened structure in a heated portion. Since the formation is suppressed, the formation of the macro battery is suppressed, and the corrosion resistance of the heating section is improved by adding the compound appropriately. From this point of view, 0.003% or more, preferably 0.1% or more.
Add 005% or more. When the C content is set to 0.05% or more, preferably 0.065% or more for increasing the strength, it is preferable to add 0.0035% or more. However, if it is added in excess of 0.010%, the amount of TiN precipitates serving as corrosion starting points increases, and the corrosion resistance of the base material deteriorates. Therefore, the upper limit is made 0.010%.

Ti: more than (48 [N] / 14 + 48 [S] / 32) to (48 [N]
/14+48[S]/32+0.2)% or less Ti combines with N and S, and the remainder becomes solid solution Ti. Solid solution T
i has the effect of improving the corrosion resistance of the heating section, and
In the absence of any, the corrosion resistance is greatly degraded. It is considered that the reason is that the sulfide does not completely turn into TiS, and MnS that is easily dissolved during heating such as welding tends to remain. It is said that MnS promotes pitting corrosion because when dissolved during welding, S forms a solid solution in the surrounding ground iron. For this reason, the amount of solid solution Ti calculated by [Ti]-(48 [N] / 14 + 48 [S] / 32) needs to take a value larger than zero. Preferably, the amount of solid solution Ti is 0.02% or more. On the other hand, if the amount of solid solution Ti is excessively large, such as 0.20% or more, the corrosion resistance is deteriorated. It is considered that the reason for this is that the heating portion becomes martensite due to excessive Ti, so that a macro battery is formed between the heating portion and the base material portion, so that corrosion easily proceeds. For this reason, the amount of solid solution Ti is less than 0.2%, preferably less than 0.13%.

When manufacturing a high-strength steel sheet, as described above,
The C content is preferably set to 0.05% or more, preferably 0.065% or more. In this case, as the amount of solid solution C increases, it becomes easy to form martensite due to excess Ti, so The upper limit of the amount is set to less than 0.10%, preferably less than 0.06%.

The steel sheet of the present invention contains the above components as essential components, and the balance is composed of Fe and unavoidable impurities.
In order to further improve the strength and corrosion resistance without impairing the effects of the present invention, one or more elements from the following groups P, A, and B can be contained.

P: 0.01 to 0.10%, P has an effect of improving corrosion resistance. If it is less than 0.01%, the effect is too small, while if it exceeds 0.10%, the workability is deteriorated.

Group A: Si: 0.01-2.0%,
Nb: 0.01 to 0.04%, Cr: 0.1 to 1.0
%, B: 0.0002 to 0.0020%, V: 0.01
-0.20%, Mo: 0.01-0.20% These elements have a strength improving effect. If the amount is less than the lower limit of each element, the effect is too small, while if it exceeds the upper limit, the workability deteriorates. Mo also has a corrosion resistance improving effect in the above range.

Group B; REM: 0.0004-0.0
020%, Ca: 0.0004 to 0.0020% These elements have an effect of controlling the form of carbide to improve stretch flangeability. If the amount is less than the lower limit of each element, the effect is too small, while if it exceeds the upper limit, the workability deteriorates.

The structure of the steel sheet of the present invention is mainly composed of F (ferrite) and B (bainite). F and B
The third phase other than the above is allowed to have an area ratio of less than 5% and preferably less than 2% as a range having no problem in the effect of the present invention.

The corrosion resistance when the structure of the steel sheet is F + B is good, but when F + M (martensite) or F + P (pearlite), the corrosion resistance is deteriorated. Here, P means lamellar pearlite, and if it is difficult to distinguish between bainite and pearlite, it is determined to be bainite in the present invention. The reason that the corrosion resistance of F + P or F + M deteriorates is presumed to be that martensite or pearlite has a relatively high C concentration in the structure compared to bainite, so that corrosion proceeds selectively and a micro battery is formed.

The steel sheet of the present invention is prepared by hot rolling a slab having the above-mentioned components at a finishing temperature of 950 ° C. to _three Ar points.
Cooling time of less than 5 seconds at 00 ° C,
It is obtained by winding at 450 ° C.

The finishing temperature: 950 ° C. to Ar ferrite transformation proceeds at a high speed is less than ₃ points Ar ₃ point, not obtained or a sufficient corrosion resistance to facilitate the formation of pearlite or martensite. On the other hand, since it is difficult to set the temperature to be higher than 950 ° C. with ordinary hot rolling equipment, the upper limit is set to 950 ° C.
° C.

700 to 700 in the cooling process after finish rolling
600 ° C. residence time: 5 seconds or less At 700 to 600 ° C., the ferrite transforms into ferrite at a high speed. Therefore, when the residence time (time during which the steel sheet is at 700 to 600 ° C. during cooling) exceeds 5 seconds, C Is concentrated to a high concentration, and after transformation in the hot-rolling cooling and winding system,
This is because it becomes perlite or martensite.

Winding temperature: 550 to 450 ° C. When the film is wound at a temperature exceeding 550 ° C., the ferrite transformation proceeds slowly but after the winding, C is concentrated in austenite, and after the transformation, pearlite and martensite are removed. Become. On the other hand, when the temperature is lower than 450 ° C., martensite is generated even if C is not concentrated in austenite. In this case, there is much residual strain in martensite,
Even if C is not so concentrated, corrosion proceeds selectively. In the case where the structure is F + P, corrosion progresses selectively because C is concentrated to near the eutectoid composition. Corrosion progresses particularly when C is concentrated to become martensite.

[0030]

【Example】

Example A A slab having the components shown in Tables 1 and 2 was hot-rolled at a finishing temperature of 890 ° C, cooled at 700 to 600 ° C for 3 seconds, and wound at 500 ° C. In addition, sample No. A in Table 1
Using the steel of the component No. 4 under various hot rolling conditions shown in Table 3, hot rolled steel sheets of the sample No. K group were obtained.

The structure of these hot-rolled steel sheets was examined, the parts were welded by butt welding, and the portions raised by welding were ground to prepare corrosion test pieces. The test piece was subjected to a constant potential corrosion test (3% NaCl, 30 ° C., −550 mV, 4
8 hr), and the perforated depth was measured. The hole depth (Dm) of the base metal part is 0.35 mm, and the hole depth (Dw) of the welded part
Since it is desired to suppress Dm to 1.2 times or less, a material satisfying these conditions is defined as an example of the present invention. The results of the microstructure observation and the measurement results of the perforated depths of the base metal and the weld are also shown in the same table.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

In Table 1, samples Nos. A1 to A8 were obtained by investigating the effect of the Ti content on the perforated depth, and the results are shown in FIG. From FIG. 1, it can be seen that an appropriate amount of Ti is necessary in order to have good puncture resistance. As a result of detailed investigation, it was found that Ti
Since the concentration was presumed to affect the corrosion resistance, as shown in FIG. 2, the amount of solid solution Ti calculated by calculation ([Ti] −Ti lower limit (ie, 48 [N] / 14 + 48 [S] / 32)) was arranged on the horizontal axis. From this, it can be seen that the solid solution Ti contributes to the improvement of the corrosion resistance of the welded portion of the steel sheet. It is presumed that the corrosion resistance or the deterioration rapidly when the amount of solute Ti is too large is due to the increase of precipitates and the tendency of the welded portion to become martensite and the macro battery to be easily formed.

Sample Nos. B1 to B5 were obtained by investigating the effect of the N content on the perforated depth, and the results are shown in FIG. It can be seen that even if the solute Ti is within the range of the present invention, sufficient corrosion resistance cannot be obtained unless N is within an appropriate range. This is presumed to be due to the fact that TiN suppresses coarsening of the austenite structure, so that martensite hardly occurs in the welded portion. The reason why the corrosion resistance deteriorates when N is excessive is presumed to be that the amount of TiN serving as a starting point of corrosion increases.

Samples Nos. C1 to C4 were obtained by investigating the effect of the Ni content on the perforated depth, and the results are shown in FIG. If the Ni content is less than 0.18%, the corrosion resistance rapidly deteriorates. It is known that Ni has an effect of suppressing pitting corrosion, but there is no report that such a rapid change occurs. From the results shown in FIG.
It is presumed that scabs occur and the range becomes microcells, so that the corrosion resistance rapidly deteriorates. It is said that it is only necessary to add the same amount of Ni from half of Cu in order to suppress Cu baldness, but in FIG. 4, the Cu content is below 0.30% and the Ni content is 0.18%. From the viewpoint of corrosion resistance, it is understood that at least about 60% of Cu needs to be added.

Sample Nos. D1 to D3 and E1 to E3
Is an investigation of the effect of the Cu content on the perforation depth, and the results of Nos. D1 to D3 and No. E are shown in FIG. Cu is considered to suppress the expansion of the potential difference between the welded portion and the base material portion, and from the results in FIG. 5, it is necessary to add 0.1% or more to ensure sufficient corrosion resistance of the welded portion. I understand. However, it can be seen that the effect tends to saturate even if it exceeds 0.3%.

Samples Nos. H1 to H3 in Table 2 were obtained by investigating the effect of the S content on the perforation depth.
o. The results of H1 to H3 and No. A4 are shown in FIG. Sulfide is a starting point for corrosion, so it is better to use as little as possible.
It can be seen that by setting the content to less than 0.016%, good corrosion resistance can be obtained.

Samples Nos. F1 to F3 were obtained by investigating the effect of the C content on the perforation depth. C formed carbides and became a starting point of corrosion. Since a martensitic structure is easily generated in the part, it is desirable that the amount is small, but it is understood that sufficient corrosion resistance can be obtained even at 0.07% without excessive reduction in the present invention. However, in the present invention, up to 0.30% can be added from the effect of suppressing the formation of martensite in a weld by TiN.

In addition, sample Nos. G1 to G12, I1 to 3
Is a result of confirming that various alloying elements including Mn do not impair the effects of the present invention, and in the scope of the present invention, the drilling depth of the welded portion is the same as the drilling depth of the base metal portion. 1.
It is less than twice.

Samples Nos. K1 to K13 in Table 3 were obtained by investigating the effect of the structure on the drilling depth by changing the hot rolling conditions in various ways.
In the case of the embodiment in which B (bainite) was used, the corrosion resistance was good, but in the case of F + M (martensite) or F +
It can be seen that when P (pearlite) is used, the corrosion resistance deteriorates. As described above, pearlite is lamellar pearlite, and bainite is another material that is difficult to distinguish between bainite and pearlite. Also, the organization is F
It can be seen that it is necessary to suppress precipitation of ferrite as much as possible and to perform a winding treatment at 450 to 550 ° C. to make + B. However, as is clear from Sample Nos. K2, K7 and K11, even if P or M is present, good corrosion resistance is obtained when the amount is small.

Example B A slab having the components shown in Tables 4 and 5 was hot-rolled at a finishing temperature of 880 ° C., cooled at a residence time of 700 to 600 ° C. for 3 seconds, and wound at 500 ° C. In addition, sample No. A in Table 4
Using the steel of the component No. 4 under various hot rolling conditions shown in Table 6, sample No. K group hot rolled steel sheets were obtained. Example B
Is aimed at increasing the C content and increasing the strength as compared with Example A.

With respect to these hot-rolled steel sheets, the structure observation and the perforated depth were measured in the same manner as in Example A. The hole depth (Dm) of the base metal part is 0.50 mm and the hole depth (Dw) of the welded part is 1.2 times or less of Dm. And The results of the microstructure observation and the measurement results of the perforated depths of the base metal and the weld are also shown in the same table.

[0045]

[Table 4]

[0046]

[Table 5]

[0047]

[Table 6]

Sample Nos. A1 to A8 were obtained by investigating the effect of the Ti content on the drilling depth.
Nos. C1 to C4 are of the Ni content,
Nos. D1 to D5 were obtained by investigating the effects of the Cu content, Nos. H1 to H3 by the S content, and Nos. F1 to F4 by the effects of the C content on the drilling depth. G1 to G12 and Nos. I1 and I2 are examples in which it was confirmed that various alloying elements including Mn did not impair the effects of the present invention. The perforated depth is 1.2 times or less the perforated depth of the base material.
The tensile strength of No. F1 was 44 kgf / mm ² , No. F2 was 52 kgf / mm ² , No. F3 was 68 kgf / mm ² , and No. F4
Is 81 kgf / mm ² , which indicates that a C content of 0.05% or more enhances the strength.

K1 to K13 in Table 6 were obtained by investigating the effect of the structure on the perforation depth by changing the hot rolling conditions in various ways. The structure of the steel sheet was F (ferrite) + B (bainite). In the case of the embodiment described above, the corrosion resistance is good, but when M (martensite) or P (pearlite) is generated instead of B, the corrosion resistance is deteriorated.

[0050]

As described above, according to the steel sheet of the present invention, the presence of an appropriate amount of solute Ti and the suppression of austenite coarsening due to TiN precipitates cause quenching in a heated part such as a welded part or its periphery. Generation of a structure can be suppressed as much as possible, and sufficient corrosion resistance can be imparted to the partially heated portion even without heating, without performing tempering heat treatment.
Further, since the effect of suppressing the formation of the quenched structure is not due to the extremely low carbon content, the C content can be allowed up to 0.30% as long as the corrosion resistance is not impaired. Is also easy. Furthermore, by making the main component of the structure ferrite and bainite with a low C concentration,
The formation of a micro battery can be suppressed, and the barb caused by Cu added for improving the corrosion resistance of the welded portion is suppressed by Ni, so that the base material portion has good corrosion resistance. Further, according to the production method of the present invention, it is possible to easily produce a steel sheet excellent in corrosion resistance of the heating section.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of total Ti content on drilling depth.

FIG. 2 is a graph showing the effect of the solid solution Ti content on the perforation depth.

FIG. 3 is a graph showing the effect of N content on drilling depth.

FIG. 4 is a graph showing the effect of Ni content on drilling depth.

FIG. 5 is a graph showing the effect of the Cu content on the perforation depth.

FIG. 6 is a graph showing the effect of the S content on the perforation depth.

Claims (4)

[Claims] 1. C .: 0.30% or less in weight%, M:
n: 0.2 to 2.0%, S: less than 0.016%, C
u: 0.1 to 1.0%, Ni: 0.6 [Cu] to 1.0 [Cu]
%, N: 0.003 to 0.010%, Ti: (48 [N] /
14 + 48 [S] / 32) more than (48 [N] / 14 + 48 [S] /32+0.2)
%, The balance being Fe and inevitable impurities, and having a structure mainly composed of ferrite and bainite. 2. C: 0.065 to 0.30%, Ti:
(48 [N] / 14 + 48 [S] / 32) More than (48 [N] / 14 + 48 [S] / 32 +
The steel sheet excellent in corrosion resistance of the partially heated part according to claim 1, which contains less than 0.1)%. 3. In addition to the components described in claim 1 or 2,
Further, P: 0.01 to 0.10%, Si: 0.01 to
2.0%, Nb: 0.01 to 0.04%, Cr: 0.1
-1.0%, B: 0.0002-0.0020%, V:
Group A consisting of 0.01 to 0.20%, Mo: 0.01 to 0.20%, REM: 0.0004 to 0.0020
%, Ca: The steel sheet having excellent corrosion resistance of the partially heated portion according to claim 1 or 2, which contains one or more elements from the group B consisting of 0.0004 to 0.0020%. 4. A slab having the steel component according to claim 1 is hot-rolled at a finishing temperature of 950 ° C. to ₃ points of Ar, and a residence time at 700 to 600 ° C. is 5 seconds or less. A method for producing a steel sheet having excellent corrosion resistance in a partial heating part, comprising cooling and winding at 550 to 450 ° C.