JP6338032B1 - Sulfuric acid dew-point corrosion steel - Google Patents

Abstract

An object of the present invention is to provide a sulfuric acid dew point corrosion-resistant steel that simultaneously realizes excellent sulfuric acid dew point corrosion resistance and manufacturability and is excellent in bendability and fatigue resistance. The present invention has a predetermined component composition, and the content of S, Cu, Sn and Sb in the component composition is the following formula (1), and the content of Cu, Ni, Sn, Sb and Co is 2) It relates to a sulfuric acid dew point corrosion steel satisfying the relationship of the formulas. 0.50 ≦ [% Cu] / (10 × [% S] + [% Sn] + [% Sb]) ≦ 5.00 (1) 0.50 ≦ ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn]) ≦ 2.50 (2) where [% S], [% Cu], [% Ni], [ % Sn], [% Sb], and [% Co] are the contents (mass%) of S, Cu, Ni, Sn, Sb, and Co in the component composition, respectively.

Description

  The present invention relates to a sulfuric acid dew point corrosion steel used as a constituent material for heat exchangers, tanks, plants, etc. in an environment where sulfuric acid is in contact with or in which sulfuric acid dew point is generated, and in particular, excellent sulfuric acid dew point corrosion resistance and manufacturability. And sulfuric acid dew-point corrosion steel having excellent bendability and fatigue resistance.

  In boilers that burn fuels such as heavy oil and coal containing sulfur, and heat exchangers and flues of thermal power plants, sulfur oxides contained in exhaust gas condense into sulfuric acid as the temperature decreases, causing severe corrosion. The so-called “sulfuric acid dew point corrosion” that occurs is a problem.

As a solution to this sulfuric acid dew point corrosion problem, sulfuric acid dew point corrosion steel has been developed and already put into practical use.
As such a sulfuric acid dew point corrosion steel, Sb that improves sulfuric acid corrosion resistance, and also Cu, which is an element that improves acid resistance, are utilized to propose a technology that improves acid resistance as well as sulfuric acid corrosion resistance. Has been.
For example, Patent Document 1 discloses that
“In mass%, C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, Cu: 0.1 to 1%, Mo: 0.001 -1%, Sb: 0.01-0.2%, P: 0.05% or less, S: 0.05% or less, the balance being Fe and inevitable impurities, and acid corrosion resistance index Low alloy steel excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance, characterized in that AI (AI / 10000 = 0.0005 + 0.045 × Sb% −C% × Mo%) is 0 or more.
Is disclosed.

On the other hand, when Cu having a melting point lower than Fe or Sb which is easily segregated is added, slab cracks and slab surface scratches occur during hot working such as casting and rolling, and care is required to avoid deterioration of product quality. Therefore, a decrease in productivity and an increase in cost are problems.
As a solution to such a problem, Patent Document 2 aims to improve hot workability by reducing the amount of S and adding Mo and B.
“In weight percent, C: 0.01 to 0.15%, Si: 0.1 to 0.5%, Mn: 0.1 to 0.5%, P: 0.03% or less, S: 0.0. 005% or less, Cu: 0.2 to 1.0%, Ni: 0.5% or less, Cr: 2.0% or less, Al: 0.1% or less, V: 0.2% or less, Nb: 0 2% or less, Ti: 0.2% or less, the total of one or two of Sn and Sb is 0.01 to 1.0%, and B is 0.001 to 0.01% and Mo is 0.00. An acid dew-point corrosion resistant steel excellent in hot workability, characterized by containing one or more of 01 to 0.5%, the balance being Fe and inevitable impurities.
Is disclosed.

JP 2003-213367 A Japanese Patent Laid-Open No. 10-110237

By the way, the concentration of sulfuric acid generated in the sulfuric acid dew point environment varies depending on the temperature. For example, sulfuric acid concentration: about 20% by mass at low temperature: 40 ° C., sulfuric acid concentration: about 50% by mass at medium temperature: 70 ° C., high temperature: 100 The sulfuric acid concentration is 70 to 80% by mass at ℃ to 140 ° C.
For this reason, when applying a sulfuric acid dew point corrosion steel to an actual installation, the material which shows high corrosion resistance in various sulfuric acid dew point corrosion environments is calculated | required.

  However, when the low alloy steel of Patent Document 1 is applied to an actual facility, the acid resistance, particularly hydrochloric acid resistance, is superior to conventional sulfuric acid dew point corrosion steel, but the sulfuric acid dew point corrosion resistance is There is a problem in that satisfactory characteristics cannot always be obtained, and corrosion resistance (sulfuric acid dew point corrosion resistance) against a high concentration of sulfuric acid at a high temperature is particularly low.

  Further, even in the sulfuric acid dew point corrosion resistant steel disclosed in Patent Document 2, the desired sulfuric acid dew point corrosion resistance may not be obtained even when S is reduced or Mo is added.

  The present invention was developed in view of the above situation, and provides sulfuric acid dew point corrosion steel that simultaneously realizes excellent sulfuric acid dew point corrosion resistance and manufacturability, and is also excellent in bendability and fatigue resistance. The purpose is to do.

In order to achieve the above object, the inventors first investigated the effect of each additive element in the sulfuric acid dew point corrosion environment and examined the effect in detail.
Specifically, each additive element that improves sulfuric acid dew point corrosion resistance has an effect on manufacturability, and also bendability and fatigue resistance, and each additive element that improves manufacturability, bendability, and fatigue resistance. In order to investigate the effects on sulfuric acid dew point corrosion resistance, steels with various composition are manufactured to achieve both sulfuric acid dew point corrosion resistance and manufacturability, as well as to obtain excellent bendability and fatigue resistance. Thus, effective combinations of additive elements were examined.

As a result, the following findings were obtained.
1) From the standpoint of achieving both sulfuric acid dew point corrosion resistance and manufacturability, it is effective to add Ni and Co in addition to Cu, Sn, Sb and S.
2) There is an optimal range for the contents of Cu, Sn, Sb and S, and by controlling these within the range, while ensuring the manufacturability, further bendability and fatigue resistance, Excellent sulfuric acid dew point corrosion resistance can be obtained.
3) Improve sulfuric acid dew point corrosion resistance Cu, Sn and Sb contain appropriate amounts of Ni and Co, while maintaining sulfuric acid dew point corrosion resistance, greatly improving manufacturability, especially hot workability. Can be improved. At the same time, excellent bendability and fatigue resistance can be obtained.
The present invention has been completed through further studies based on the above findings.

  That is, the gist configuration of the present invention is as follows.

1. % By mass
C: 0.050 to 0.150%,
Si: 0.10 to 0.80%,
Mn: 0.50 to 1.00%,
P: 0.050% or less,
S: 0.0020 to 0.0200%,
Cu: 0.20 to 0.50%,
Ni: 0.10 to 0.80%,
Cr: 0.20 to 1.50%,
Sn: 0.005 to 0.100%,
Sb: 0.050 to 0.300%,
Co: 0.002 to 0.020%,
Al: 0.001 to 0.050% and N: 0.0005 to 0.0050%
And the balance has a component composition consisting of Fe and inevitable impurities,
Sulfuric acid dew point corrosion resistance in which the content of S, Cu, Sn and Sb in the above component composition satisfies the relationship of the following formula (1), and the content of Cu, Ni, Sn, Sb and Co satisfies the following formula (2) respectively. steel.
0.50 ≦ [% Cu] / (10 × [% S] + [% Sn] + [% Sb]) ≦ 5.00 (1)
0.50 ≦ ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn]) ≦ 2.50 (2)
Here, [% S], [% Cu], [% Ni], [% Sn], [% Sb] and [% Co] are respectively S, Cu, Ni, Sn, Sb and Co in the component composition. Content (mass%).

2. 2. The sulfuric acid dew-point corrosion resistant steel according to 1, wherein the component composition is in mass% and further contains Ti: 0.005 to 0.050%.

3. 3. The sulfuric acid dew-point corrosion resistant steel according to 2 above, wherein the contents of C, Ti and N in the component composition satisfy the relationship of the following formula (3).
0.30 ≦ [% Ti] / (0.2 × [% C] + [% N]) ≦ 2.50 (3)
Here, [% C], [% Ti] and [% N] are the contents (mass%) of C, Ti and N in the component composition, respectively.

4). It has a steel structure in which the area ratio of the ferrite phase in the entire steel structure is 75% or more, the area ratio of the pearlite phase is less than 25%, and the total area ratio of the structure other than the ferrite phase and the pearlite phase is less than 5%. ,
4. The sulfuric acid dew-point corrosion resistant steel according to any one of 1 to 3, wherein the maximum Vickers hardness is 200 or less and the average Vickers hardness is 80 or more.

5. The sulfuric acid dew point corrosion steel according to any one of 1 to 4,
In the cathodic polarization curve showing the relationship between the current density and the potential in a sulfuric acid aqueous solution having a temperature of 70 ° C. and a concentration of 50% by mass, the potential at the current density of 0.1 A / cm ² is shown. Is Va (V),
The above Va is related to the potential Vg (V) when the current density in the cathode polarization curve in the sulfuric acid aqueous solution of the reference steel of the sulfuric acid dew-point corrosion steel is 0.1 A / cm ² , and the following (4) Sulfuric acid dew-point corrosion steel that satisfies the formula.
Vg−Va> 0.03 (4)

According to the present invention, a sulfuric acid dew point corrosion-resistant steel having excellent sulfuric acid dew point corrosion resistance and manufacturability and excellent bendability and fatigue resistance can be obtained.
The sulfuric acid dew point corrosion steel of the present invention can be suitably used as a constituent material for tanks, plants, etc. under various sulfuric acid dew point corrosion environments. Such a tank, plant, etc. can be manufactured.

This shows the relationship between the value of [% Cu] / (10 × [% S] + [% Sn] + [% Sb]) and the corrosion rate in the sulfuric acid immersion test of steel. This shows the relationship between the value of ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn]) and the evaluation of manufacturability. [% Cu] / (10 × [% S] + [% Sn] + [% Sb]) and ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [ % Sn]) is a plot of sulfuric acid dew point corrosion resistance and manufacturability evaluation results. An example of the cathode polarization curve in a sulfuric acid aqueous solution having a temperature of 70 ° C. and a concentration of 50% by mass is shown.

  Hereinafter, the present invention will be specifically described. First, the reason why the steel component composition is limited to the above range will be described. In addition, although the unit of element content in the component composition of steel is “mass%”, hereinafter, it is simply indicated by “%” unless otherwise specified.

C: 0.050 to 0.150%
C is an element that increases the strength of steel. In order to obtain a desired strength, the C content is 0.050% or more. On the other hand, when the amount of C exceeds 0.150%, the sulfuric acid dew point corrosion resistance is deteriorated, and the weldability and the toughness of the weld heat affected zone are deteriorated. Therefore, the C content is in the range of 0.050 to 0.150%. Preferably, it is 0.060 to 0.100% of range.

Si: 0.10 to 0.80%
Si is a component added as a deoxidizer and has the effect of increasing the strength of steel. For this reason, the amount of Si shall be 0.10% or more. However, if the Si content exceeds 0.80%, the toughness of the steel deteriorates. Therefore, the Si amount is set to a range of 0.10 to 0.80%. Note that Si contributes to the improvement of the resistance to sulfuric acid dew point corrosion by forming an anticorrosion film in a sulfuric acid aqueous solution environment. In order to obtain such an effect of improving the resistance to sulfuric acid dew point corrosion, the Si content is preferably 0.25% or more.

Mn: 0.50 to 1.00%
Mn is an element that increases the strength of steel. In order to obtain a desired strength, the amount of Mn is set to 0.50% or more. On the other hand, when the amount of Mn exceeds 1.00%, the toughness and weldability of steel are reduced. Therefore, the amount of Mn is made into the range of 0.50 to 1.00%. From the viewpoint of maintaining strength and suppressing the formation of inclusions that degrade the sulfuric acid dew point corrosion resistance, the Mn content is preferably in the range of 0.50 to 0.70%.

P: 0.050% or less P is a harmful element that segregates at grain boundaries and lowers the toughness of steel. In particular, when the P content exceeds 0.050%, the toughness is significantly reduced. Therefore, the P content is 0.050% or less.
Although P is desirably reduced as much as possible, a reduction to less than 0.005% causes an increase in manufacturing cost. Therefore, the lower limit of the amount of P is preferably 0.005%.

S: 0.0020 to 0.0200%
S is an element that contributes to the formation of a Cu ₂ S film in the presence of Cu, suppresses the corrosion reaction on the steel surface, and improves the resistance to sulfuric acid dew point corrosion. On the other hand, S forms MnS which is a non-metallic inclusion, and this MnS is a harmful element that lowers the local corrosion resistance due to the origin of local corrosion. Therefore, from the viewpoint of ensuring sulfuric acid dew point corrosion resistance, the S amount is set to 0.0020% or more. On the other hand, from the viewpoint of avoiding a decrease in local corrosion resistance, the amount of S is set to 0.0200% or less. In addition, from the viewpoint of further improving the sulfuric acid dew point corrosion resistance, the S amount is preferably 0.0050% or more.

Cu: 0.20 to 0.50%
Cu is an essential element that improves acid resistance in a corrosive environment caused by acid. Here, when the amount of Cu is less than 0.20%, the effect is small. On the other hand, if the amount of Cu exceeds 0.50%, the acid resistance improving effect is saturated and the productivity, in particular, hot workability is deteriorated. Therefore, the amount of Cu is made 0.20 to 0.50% of range.

Ni: 0.10 to 0.80%
Ni is an element that suppresses deterioration of hot workability due to the addition of Cu or Sb. However, when the amount of Ni is less than 0.10%, the effect is small. On the other hand, if the Ni content exceeds 0.80%, the effect of suppressing deterioration of hot workability is saturated and the cost is increased. Therefore, the Ni content is in the range of 0.10 to 0.80%.

Cr: 0.20 to 1.50%
Although Cr does not greatly contribute to the effect of improving the sulfuric acid dew point corrosion resistance in a normal temperature environment, it is an element that improves the sulfuric acid dew point corrosion resistance when the use environment becomes a high temperature of 120 ° C. or higher. When the Cr content is less than 0.20%, these effects are small. On the other hand, when the Cr content exceeds 1.50%, these effects are saturated and the cost is increased. Therefore, the Cr content is in the range of 0.20 to 1.50%. Preferably, it is 0.40 to 1.50% of range.

Sn: 0.005-0.100%
Sn has a function of suppressing corrosion in an acid environment by forming a dense rust tank. However, if the Sn amount is less than 0.005%, this effect cannot be sufficiently obtained. On the other hand, when the Sn content exceeds 0.100%, hot workability and toughness are deteriorated. Therefore, the Sn amount is in the range of 0.005 to 0.100%.

Sb: 0.050-0.300%
Sb is an element that concentrates on the steel surface as a Cu compound by complex addition with Cu and improves acid resistance. However, when the Sb content is less than 0.050%, the effect is small. On the other hand, when the amount of Sb exceeds 0.300%, the effect is saturated and manufacturability, particularly hot workability is deteriorated. Therefore, the Sb amount is in the range of 0.050 to 0.300%. From the viewpoint of achieving both sulfuric acid dew point corrosion resistance and manufacturability, the Sb content is preferably in the range of 0.100 to 0.200%.

Co: 0.002 to 0.020%
Co is an element that suppresses deterioration of hot workability due to the addition of Cu, Sn, and Sb together with Ni. Co is an element that contributes to the improvement of the resistance to sulfuric acid dew point corrosion even in a small amount. However, when the Co content is less than 0.002%, the effect is small. On the other hand, when the amount of Co exceeds 0.020%, the cost increases. Therefore, the amount of Co is set to a range of 0.002 to 0.020%. Preferably it is 0.002 to 0.010% of range.

Al: 0.001 to 0.050%
Al is an element added as a deoxidizer. From the viewpoint of obtaining such an effect, the Al amount needs to be 0.001% or more. On the other hand, if the Al content exceeds 0.050%, the toughness of the steel decreases. Therefore, the Al content is in the range of 0.001 to 0.050%. Preferably, it is 0.010 to 0.050% of range.

N: 0.0005 to 0.0050%
N is an element that degrades the toughness of steel in a solid solution state, and is preferably reduced as much as possible, but is acceptable if the N content is 0.0050% or less. On the other hand, it is technically difficult to completely remove N, and reduction more than necessary causes an increase in manufacturing cost. Therefore, the lower limit of the N amount is 0.0005%.

Further, it is not sufficient that each component satisfies the above range, and the contents of S, Cu, Sn and Sb are the following formula (1), and the contents of Cu, Ni, Sn, Sb and Co are the following (2 It is important to satisfy the relationship
0.50 ≦ [% Cu] / (10 × [% S] + [% Sn] + [% Sb]) ≦ 5.00 (1)
0.50 ≦ ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn]) ≦ 2.50 (2)
Here, [% S], [% Cu], [% Ni], [% Sn], [% Sb] and [% Co] are respectively S, Cu, Ni, Sn, Sb and Co in the component composition. Content (mass%).
Hereinafter, the experiment that led to this knowledge will be described.

[Experiment]
C: 0.050 to 0.150%, Si: 0.10 to 0.80%, Mn: 0.50 to 1.00%, P: 0.050% or less, Cr: 0.20 to 1.50 %, Al: 0.001 to 0.050% and N: 0.0005 to 0.0050%, and the contents of S, Cu, Ni, Sn, Sb and Co were changed in various ways (the balance) Fe and unavoidable impurities) were melted in a converter and a steel slab having a thickness of 200 mm was formed by a continuous casting method. After cooling this steel slab, it was reheated to 1200 ° C. and hot-rolled to obtain a hot rolled steel sheet having a thickness of 4.5 mm.
In the hot rolling, the rolling reduction: 97.75%, the finishing temperature: 850 ° C., the winding temperature: 560 ° C., and the average cooling rate from 800 ° C. to 650 ° C. is 3.0 to 8.0 ° C./s. It was made to be within the range.

In order to investigate the influence of each additive element in the sulfuric acid dew point corrosion environment from the hot-rolled steel sheet thus obtained, a corrosion test piece having a width of 20 mm × length of 30 mm × thickness of 3 mm was cut out, and the cut out corrosion test piece was subjected to a sulfuric acid aqueous solution ( (Corrosion weight loss was measured in a sulfuric acid immersion corrosion test immersed in a temperature of 70 ° C. and a concentration of 50 mass%) for 6 hours, and the corrosion rate of each specimen was calculated from the corrosion weight loss.
The sulfuric acid dew point corrosion resistance was evaluated according to the following criteria.
Pass (○): 280 g / (m ² · hr) or less Fail (×): Corrosion rate exceeds 280 g / (m ² · hr) Also, the surface scratch depth during steel slab casting should be colored on the surface The scratches were confirmed by the above, and the manufacturability (hot workability) was evaluated based on the following criteria by observing by visually observing and cutting out the cross section.
Pass (◯): Surface scratch depth is less than 0.2 mm Fail (×): Surface scratch depth is 0.2 mm or more The evaluation results of the sulfuric acid dew point corrosion resistance and manufacturability are [% Cu] / ( 10 × [% S] + [% Sn] + [% Sb]) and / or ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn]) It shows to FIGS.

As shown in FIG. 1, by controlling [% Cu] / (10 × [% S] + [% Sn] + [% Sb]) in the range of 0.50 to 5.00, excellent resistance It turns out that the improvement effect of sulfuric acid dew point corrosion property is acquired. In addition, as shown in FIG. 2, ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn]) is in a range of 0.50 to 2.50. It can be seen that excellent manufacturability can be obtained by controlling to.
Then, as shown in FIG. 3, [% Cu] / (10 × [% S] + [% Sn] + [% Sb]) is set to a range of 0.50 to 5.00, and ([% Ni ] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn]) within a range of 0.50 to 2.50, excellent sulfuric acid dew point corrosion resistance It can be seen that both productivity can be achieved.
From the above experimental results, the inventors can satisfy both the above formulas (1) and (2) at the same time to achieve both excellent sulfuric acid dew point corrosion resistance and manufacturability, and further bendability and resistance. The inventors have found that sufficient fatigue properties can be obtained, and have developed the present invention.

0.50 ≦ [% Cu] / (10 × [% S] + [% Sn] + [% Sb]) ≦ 5.00
As described above, appropriate amounts of S, Sn, and Sb are added according to the amount of Cu. Specifically, [% Cu] / (10 × [% S] + [% Sn] + [% Sb]) is added. By adjusting to the range of 0.50-5.00, the sulfuric acid dew point corrosion-resistant improvement effect is acquired, ensuring manufacturability and also bendability and fatigue resistance.
For this reason, about content of S, Cu, Sn, and Sb, it is the relationship of 0.50 <= [% Cu] / (10 * [% S] + [% Sn] + [% Sb]) <= 5.00. It is necessary to satisfy.
The value of [% Cu] / (10 × [% S] + [% Sn] + [% Sb]) is preferably 3.50 or less, more preferably 3.00 or less, and even more preferably 2.50. It is as follows.

0.50 ≦ ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn]) ≦ 2.50
Further, as described above, appropriate amounts of Ni and Co are added according to the amount of Cu, the amount of Sn, and the amount of Sb. Specifically, ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn]) is adjusted in the range of 0.50 to 2.50, while maintaining the sulfuric acid dew point corrosion resistance, the manufacturability, particularly the hot workability is greatly improved. Improvement effect is obtained.
Therefore, the contents of Cu, Ni, Sn, Sb and Co are set to 0.50 ≦ ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn). ]) ≦ 2.50 must be satisfied.
The value of ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn]) is preferably 0.55 or more, more preferably 0.60 or more. It is.
From the viewpoint of improving productivity, only the lower limit of ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn]) is specified. However, if the amount of Ni increases, the resistance to sulfuric acid dew point corrosion may be adversely affected. Therefore, ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn) ]) Is also specified here.

Although the basic components have been described above, the elements described below can be appropriately contained as necessary.
Ti: 0.005 to 0.050%
Ti is an element added for the purpose of improving the strength and toughness of steel. However, if the amount of Ti is less than 0.005%, a desired effect cannot be obtained. On the other hand, if the amount of Ti exceeds 0.050%, the effect of improving the strength and toughness of the steel is saturated. Therefore, when Ti is contained, the Ti amount is in the range of 0.005 to 0.050%.

Further, it is preferable that the contents of C, Ti and N satisfy the relationship of the following formula (3).
0.30 ≦ [% Ti] / (0.2 × [% C] + [% N]) ≦ 2.50 (3)
Here, [% C], [% Ti] and [% N] are the contents (mass%) of C, Ti and N in the component composition, respectively.

0.30 ≦ [% Ti] / (0.2 × [% C] + [% N]) ≦ 2.50
The inventors appropriately control the relationship among the amounts of Ti, C, and N in the above component composition. Specifically, [% Ti] / (0.2 × [% C] + [% N]) It was found that the fatigue resistance can be greatly improved by controlling the value in the range of 0.30 to 2.50.
For this reason, in said component composition, it is suitable to further satisfy the relationship of said Formula (3) about content of Ti, C, and N.
The value of [% Ti] / (0.2 × [% C] + [% N]) is more preferably 0.40 or more and 2.00 or less, more preferably 0.50 or more and 1.50. Hereinafter, it is more preferably 0.50 or more and 1.10 or less.

Components other than the above are Fe and inevitable impurities.
The inevitable impurities referred to here are elements that are inevitably mixed from steel raw material ore and scrap, etc., and are not added consciously and do not affect the effects of the present invention. It refers to an impurity component. Examples of such inevitable impurities include O (oxygen), and the upper limit is about 0.0050%.

Next, a suitable steel structure of the sulfuric acid dew point corrosion steel of the present invention will be described.
As a suitable steel structure of the sulfuric acid dew point corrosion steel of the present invention, the area ratio of the ferrite phase occupying the entire steel structure is 75% or more, the area ratio of the pearlite phase is less than 25%, and the remainder other than the ferrite phase and the pearlite phase. Examples include steel structures having a total area ratio of less than 5%.
In order to obtain such a structure, the hot rolling conditions described later are appropriately controlled. In particular, the average cooling rate in the temperature range of 800 ° C. to 650 ° C. is 1.0 ° C./s or more and 20.0 ° C. It is important to keep it below / s.

Area ratio of ferrite phase: 75% or more The sulfuric acid dew-point corrosion steel may be used after being bent depending on the shape of the final product. Here, if the area ratio of the ferrite phase is less than 75%, cracks may occur during bending. Therefore, the area ratio of the ferrite phase in the entire steel structure is preferably 75% or more. More preferably, it is 80% or more. The area ratio of the ferrite phase may be 100%.

Perlite phase area ratio: less than 25% The sulfuric acid dew-point corrosion steel may be used after being bent depending on the shape of the final product. Here, if the area ratio of the pearlite phase is 25% or more, there is a possibility that cracking may occur during bending. Therefore, the area ratio of the pearlite phase in the entire steel structure is preferably less than 25%. More preferably, it is 20% or less. The area ratio of the pearlite phase may be 0%.

  Examples of the remaining structure other than the ferrite phase and the pearlite phase include a bainite phase and the like. When a bainite phase, a martensite phase, or the like is mixed, there is a concern about cracking during bending. Therefore, the total area ratio of the remaining structure other than the ferrite phase and the pearlite phase is preferably less than 5%.

On the other hand, if the maximum Vickers hardness exceeds 200, cracks tend to occur during bending, and fatigue resistance tends to deteriorate. However, when the average Vickers hardness is less than 80, it is difficult to ensure a predetermined strength.
For this reason, it is preferable that the maximum Vickers hardness is 200 or less and the average Vickers hardness is 80 or more.

Furthermore, in the sulfuric acid dew-point corrosion steel of the present invention, the current density: 0.1 A / cm in the cathodic polarization curve showing the relationship between the current density and the potential in a sulfuric acid aqueous solution having a temperature of 70 ° C. and a concentration of 50% by mass. When the potential at ² is Va (V), Va is the current density in the cathodic polarization curve in the sulfuric acid aqueous solution of the reference steel of the sulfuric acid dew-point corrosion steel: 0.1 A / cm ² It is preferable that the following expression (4) is satisfied in relation to the potential Vg (V).
Vg−Va> 0.03 (4)

That is, corrosion of steel in sulfuric acid aqueous solution proceeds by a reduction reaction of hydrogen ions and a dissolution reaction of iron in sulfuric acid aqueous solution. FIG. 4 shows the relationship between the cathodic polarization curve representing the relationship between the current density and potential of the reduction reaction of hydrogen ions in a sulfuric acid aqueous solution at a temperature of 70 ° C. and a concentration of 50% by mass, and the relationship between the current density and potential of the iron dissolution reaction. An example of the anodic polarization curve to represent is shown. In FIG. 4, the point where the cathodic polarization curve and the anodic polarization curve intersect is the point where corrosion actually proceeds.
Here, the inventors obtained the cathode polarization curves of various steels under various conditions, and further investigated the relationship between the cathode polarization curves and sulfuric acid dew point corrosion resistance.
As a result, it is effective to suppress the cathodic reaction to improve the sulfuric acid dew point corrosion resistance. The sulfuric acid dew point corrosion resistance is cathodic polarization in a sulfuric acid aqueous solution having a temperature of 70 ° C. and a concentration of 50% by mass. The current density in the curve was found to be closely related to the potential at 0.1 A / cm ² .
As a result of further investigation, when the potential at the current density of 0.1 A / cm ² is Va (V) in the cathode polarization curve of the target steel, Va is a standard that is so-called general steel. Steel temperature: 70 ° C., concentration: 50% by mass of sulfuric acid aqueous solution in cathodic polarization curve Current density: Vg (V), which is the potential at 0.1 A / cm ² , as described above (4 It has been found that it is preferable to satisfy the formula (1), and by satisfying such a relationship, the resistance to sulfuric acid dew point corrosion is further enhanced.
For this reason, it is preferable to satisfy the relationship of Vg−Va> 0.03. More preferably, Vg−Va> 0.05. The upper limit of Vg-Va is not particularly limited, but is usually about 0.15.
Note that when potential measurement is performed using the Hg / Hg (SO ₄ ) reference electrode, both Va and Vg show negative values. Even in this case, Va is made relatively smaller than Vg. This is very important.
In addition, the current density in the cathode polarization curve: 0.1 A / cm ² potential was selected. If the current density is smaller than this, noise may occur depending on the measurement conditions. This is because the cathode reaction itself is rate-limiting and it may be difficult to accurately measure the potential.

  Further, the reference steel here is mass%, C: 0.050 to 0.150%, Si: 0.10 to 0.80%, Mn: 0.50 to 1.00%, P: 0. 0.050% or less, S: 0.0020 to 0.0200%, Al: 0.001 to 0.050% and N: 0.0005 to 0.0050%, with the balance being Fe and inevitable impurities Steel having a component composition (in particular, Cu: less than 0.02%, Ni: less than 0.02%, Cr: less than 0.02%, Sn: less than 0.005%, Sb: less than 0.010%, Co : Steel having a component composition suppressed to less than 0.002% and Ti: less than 0.005%). In the case of steel having such a component composition, the cathodic polarization curves in a sulfuric acid aqueous solution having a temperature of 70 ° C. and a concentration of 50% by mass are substantially the same.

Next, the suitable manufacturing method of the sulfuric-acid dew-point corrosion steel of this invention is demonstrated.
The sulfuric acid dew-point corrosion steel of the present invention is obtained by finishing a steel material adjusted to the above component composition into various shapes such as a thin steel plate, a thick steel plate, and a shaped steel. After melting by a generally known method such as a furnace, electric furnace, vacuum degassing apparatus, etc., a steel slab is formed by a continuous casting method, etc., and this steel slab is hot-rolled by reheating immediately or after cooling. A method is mentioned. Moreover, when making it a cold-rolled steel sheet, pickling, cold rolling, and annealing are further performed to obtain a product.

In addition, as hot rolling conditions, from the viewpoint of ensuring required mechanical properties, that is, strength (hardness), bendability, and fatigue resistance, the rolling reduction is 50 to 99%, and the finishing temperature is 650 to 950 ° C. The winding temperature is preferably 400 to 650 ° C., and the average cooling rate from 800 ° C. to 650 ° C. is preferably 1.0 to 20.0 ° C./s.
Further, from the viewpoint of satisfying the above expression (4), it is preferable that the average cooling rate from 800 ° C. to 650 ° C. is 1.0 to 10.0 ° C./s.

Steel having the component composition shown in Table 1 (the balance is Fe and inevitable impurities) was melted in a converter and formed into a steel slab having a thickness of 200 mm by a continuous casting method. After cooling this steel slab, it was reheated to 1200 ° C. and hot-rolled to obtain a hot rolled steel sheet having a thickness of 4.5 mm.
In the hot rolling, the reduction rate: 97.75%, the finishing temperature: 850 ° C., the winding temperature: 560 ° C., and the average cooling rate from 800 ° C. to 650 ° C. are as shown in Table 2.
About the hot-rolled steel sheet thus obtained, the area ratio and Vickers hardness of each phase in the steel structure were measured, and sulfuric acid dew point corrosion resistance, manufacturability, bendability and fatigue resistance were evaluated by the following methods. . These results are shown in Table 2.

・ Measurement of area ratio of each phase in steel structure Using 3% Nital reagent (3% nitric acid + ethanol) to corrode vertical section (depth position of 1/4 thickness) parallel to rolling direction of hot rolled steel sheet Then, the part was observed and photographed with an optical microscope with a magnification of 100 times, and the area ratio of ferrite and pearlite was determined using the photographed tissue photograph. Here, the area ratios of ferrite and pearlite were measured using a point count method (according to ASTM E562-83 (1988)) by observing 5 visual fields. Further, the area ratio of the remaining structure other than the above-described ferrite and pearlite can be obtained by subtracting the total area ratio of ferrite and pearlite from 100%.

-Measurement of Vickers hardness Vickers hardness is an arbitrary value in the surface layer (position 0.5 mm from the surface) of the hot-rolled steel sheet obtained as described above under the condition of load: 9.8 N according to JIS Z 2244. These 20 points were measured, and the average value and the maximum value were obtained.

・ Sulfuric acid dew point corrosion resistance From the hot-rolled steel sheet obtained as described above, a corrosion test piece having a width of 20 mm × length of 30 mm × thickness of 3 mm was cut out, and the cut out corrosion test piece was converted into a sulfuric acid aqueous solution (temperature: 70 ° C., concentration). : 50 mass%) for 6 hours in a sulfuric acid immersion corrosion test, the corrosion weight loss was measured, and the corrosion rate of each specimen was calculated from the corrosion weight loss.
And the sulfuric acid dew point corrosion resistance in medium temperature was evaluated with the following references | standards.
Pass, particularly excellent (◎): Corrosion rate is less than 250 g / (m ² · hr) Pass (○): Corrosion rate is 250 g / (m ² · hr) or more and 280 g / (m ² · hr) or less Fail (× ): Corrosion rate exceeds 280 g / (m ² · hr)

Separately, a corrosion test piece having a width of 20 mm, a length of 30 mm, and a thickness of 3 mm was cut out from the hot-rolled steel sheet obtained as described above, and the cut out corrosion test piece was converted into a sulfuric acid aqueous solution (temperature: 140 ° C., concentration: 80 The sample was subjected to a sulfuric acid immersion corrosion test that was immersed in (mass%) for 3 hours, the corrosion weight loss was measured, and the corrosion rate of each specimen was calculated from the corrosion weight loss.
The sulfuric acid dew point corrosion resistance at high temperatures was evaluated according to the following criteria.
Pass, particularly excellent (◎): Corrosion rate is less than 92 g / (m ² · hr) Pass (○): Corrosion rate is 92 g / (m ² · hr) to 97 g / (m ² · hr) or less Fail (× ): Corrosion rate is over 97 g / (m ² · hr)

-Manufacturability Manufacturability was evaluated according to the following criteria by checking the surface scratch depth at the time of steel slab casting by coloring scratches on the surface and visually observing and cutting out the cross section.
Pass, particularly excellent (◎): no observation of surface scratches Pass (◯): surface scratch depth is less than 0.2 mm Fail (x): surface scratch depth is 0.2 mm or more

-Bendability From the hot-rolled steel sheet obtained as described above, a test piece having a width of 50 mm, a length of 100 mm, and a thickness of 3.2 mm was cut out, and three sheets of the same thickness were sandwiched inside the cut out test piece. Then, 180 ° bending (3T bending) was performed, and the state of the bent portion was visually observed, and the bendability was evaluated according to the following criteria.
Pass (○): No crack Fail (×): Crack

・ Fatigue resistance Fatigue resistance is measured by taking a sample so that the longitudinal direction is perpendicular to the rolling direction of the steel sheet.
In accordance with Z 2275 (1978), a plane bending fatigue test was performed under the conditions of double swing (stress ratio: −1) and frequency: 10 Hz.
In the swing plane bending fatigue test, the stress at which no fracture was observed until 1 million cycles was measured, and the fatigue resistance was evaluated according to the following criteria using this stress as the fatigue strength.
Pass, particularly excellent (◎): Fatigue strength is 200 MPa or more Pass (◯): Fatigue strength is 150 MPa or more and less than 200 MPa Fail (×): Fatigue strength is less than 150 MPa

Further, a test material having a size of 10 mm × 10 mm was cut out from the hot-rolled steel sheet obtained as described above, and the end surface and the back surface of the cut-out test piece were covered with a protective coating to be protected. This test material was immersed in a sulfuric acid aqueous solution (temperature: 70 ° C., concentration: 50 mass%) for 10 minutes, and then the potential was swept to the cathode side at a rate of 1 mV / sec to about 0.4 V to obtain a cathode polarization curve. Collected. Using the obtained cathode polarization curve, the potential Va (V) at a current density of 0.1 A / cm ² was obtained by drawing, and the reference steel No. 1 was obtained. The current difference in the cathode polarization curve of 18 was determined as a potential difference from the potential Vg (V) at 0.1 A / cm ² . In measuring the potential, a Hg / Hg (SO ₄ ) reference electrode was used. The results are also shown in Table 2.

From Table 2, it can be seen that all of the inventive examples are excellent in sulfuric acid dew point corrosion resistance, manufacturability, bendability and fatigue resistance.
On the other hand, in all the comparative examples, at least one of sulfuric acid dew point corrosion resistance, manufacturability, bendability, and fatigue resistance could not satisfy the desired characteristics.

Claims (4)

% By mass
C: 0.050 to 0.150%,
Si: 0.10 to 0.80%,
Mn: 0.50 to 1.00%,
P: 0.050% or less,
S: 0.0020 to 0.0200%,
Cu: 0.20 to 0.50%,
Ni: 0.10 to 0.80%,
Cr: 0.20 to 1.50%,
Sn: 0.005 to 0.100%,
Sb: 0.050 to 0.300%,
Co: 0.002 to 0.020%,
Al: 0.001 to 0.050% and N: 0.0005 to 0.0050%
And the balance has a component composition consisting of Fe and inevitable impurities,
The contents of S, Cu, Sn, and Sb in the above component composition satisfy the relationship of the following formula (1), and the contents of Cu, Ni, Sn, Sb, and Co satisfy the following formula (2), respectively :
It has a steel structure in which the area ratio of the ferrite phase in the entire steel structure is 75% or more, the area ratio of the pearlite phase is less than 25%, and the total area ratio of the structure other than the ferrite phase and the pearlite phase is less than 5%. ,
A sulfuric acid dew-point corrosion resistant steel having a maximum Vickers hardness of 200 or less and an average Vickers hardness of 80 or more .
0.50 ≦ [% Cu] / (10 × [% S] + [% Sn] + [% Sb]) ≦ 5.00 (1)
0.50 ≦ ([% Ni] + 5 × [% Co]) / ([% Cu] + [% Sb] + 10 × [% Sn]) ≦ 2.50 (2)
Here, [% S], [% Cu], [% Ni], [% Sn], [% Sb] and [% Co] are respectively S, Cu, Ni, Sn, Sb and Co in the component composition. Content (mass%).   The sulfuric acid dew point corrosion-resistant steel according to claim 1, wherein the component composition further includes Ti: 0.005 to 0.050% by mass. The sulfuric acid dew-point corrosion resistant steel according to claim 2, wherein the contents of C, Ti and N in the component composition satisfy the relationship of the following formula (3).
0.30 ≦ [% Ti] / (0.2 × [% C] + [% N]) ≦ 2.50 (3)
Here, [% C], [% Ti] and [% N] are the contents (mass%) of C, Ti and N in the component composition, respectively. The sulfuric acid dew-point corrosion steel according to any one of claims 1 to 3 ,
In the cathodic polarization curve showing the relationship between the current density and the potential in a sulfuric acid aqueous solution having a temperature of 70 ° C. and a concentration of 50% by mass, the potential at the current density of 0.1 A / cm ² is shown. Is Va (V),
The above Va is related to the potential Vg (V) when the current density in the cathode polarization curve in the sulfuric acid aqueous solution of the reference steel of the sulfuric acid dew-point corrosion steel is 0.1 A / cm ² , and the following (4) Sulfuric acid dew-point corrosion steel that satisfies the formula.
Vg−Va> 0.03 (4)

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