JP7385106B2 - steel material - Google Patents

Description

The present invention relates to steel materials.

Furnaces at thermal power plants and incinerators at waste incineration facilities generate exhaust gas containing water vapor, sulfur oxides, hydrogen chloride, and the like. When this exhaust gas is cooled in an exhaust gas chimney or the like, it condenses into sulfuric acid and hydrochloric acid, which causes significant corrosion to the steel materials forming the exhaust gas flow path, known as sulfuric acid dew point corrosion and hydrochloric acid dew point corrosion.

To address these problems, sulfuric acid/hydrochloric acid dew point corrosion resistant steels and highly corrosion resistant stainless steels have been proposed. For example, Patent Documents 1 to 4 propose steel materials with excellent sulfuric acid dew point corrosion resistance to which Cu, Sb, Co, Cr, etc. are added. Furthermore, Patent Document 5 proposes a highly corrosion-resistant stainless steel to which Cr, Ni, and the like are added.

Japanese Patent Application Publication No. 2001-164335 Japanese Patent Application Publication No. 2003-213367 Japanese Patent Application Publication No. 2007-239094 JP2012-57221A Japanese Patent Application Publication No. 7-316745

Steel materials containing Cu, Sb, Cr, Co, etc. exhibit excellent corrosion resistance in a sulfuric acid corrosive environment such as an exhaust gas chimney. However, further improvements in corrosion resistance are expected in order to extend the lifespan of power generation equipment and incineration equipment.

In addition to the flue gas stack, the steel materials used for the incinerator flue, such as the gasification melting furnace, heat exchanger, gas-to-gas heater, desulfurization equipment, and electrostatic precipitator, are installed as steel plates or steel pipes, so they are not only corrosion resistant. In addition, hot workability and weldability are also required.

An object of the present invention is to solve the above problems and provide a steel material that has excellent corrosion resistance in sulfuric acid corrosive environments and hydrochloric acid corrosive environments, and has excellent hot workability and weldability.

The present invention has been made to solve the above problems, and its gist includes the following steel materials.

(1) The chemical composition is in mass%,
C: 0.010% or more and less than 0.20%,
Si: 0.10 to 0.50%,
Mn: 0.10-1.00%,
S: 0.0005-0.015%,
Cu: 0.10-0.50%,
Cr: 0.10-1.00%,
Sb: 0.03 to 0.15%,
Ni: 0.01-0.50%,
Al: 0.005-0.10%,
P: 0.025% or less,
O: 0.0005-0.0035%,
N: 0.0010-0.0080%,
The remainder: Fe and impurities,
Al/O is 3.0 to 35.0,
CI defined by the following formula (i) is 4.0 to 30.0,
DI defined by the following formula (ii) is 0.96 to 3.00,
Ceq defined by the following formula (iii) is 0.180 to 0.380,
Steel material.
CI=(Cu/64)/(S/32)...(i)
DI=(Cu/64)/(Ni/59)...(ii)
Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15...(iii)
However, the element symbol in the above formula represents the content (mass %) of each element contained in the steel material, and if it is not contained, 0 shall be substituted.

(2) The chemical composition is in mass % instead of a part of the Fe,
Ti: 0.15% or less,
Mo: 0.50% or less,
W: 0.50% or less,
Sn: 0.30% or less,
As: 0.30% or less,
Co: 0.30% or less, and Bi: 0.010% or less,
Contains one or more selected from
The steel material described in (1) above.

(3) The chemical composition is in mass % instead of a part of the Fe,
Nb: 0.10% or less,
V: 0.10% or less,
Zr: 0.050% or less,
Ta: 0.050% or less, and B: 0.010% or less,
Contains one or more selected from
The steel material described in (1) or (2) above.

(4) The chemical composition is in mass % instead of a part of the Fe,
Ca: 0.010% or less,
Mg: 0.010% or less, and REM: 0.010% or less,
Contains one or more selected from
The steel material according to any one of (1) to (3) above.

According to the present invention, it is possible to provide a steel material that has corrosion resistance in an acid corrosion environment and has excellent both hot workability and weldability.

In order to solve the above-described problems, the present inventors conducted a detailed investigation into the corrosion resistance, hot workability, and weldability of steel materials, and as a result, they came to the following findings.

When the cause of corrosion that occurred in corrosion-resistant steel containing Cu, Sb, and Cr was investigated, it was found that the origin was oxides and inclusions generated on the surface of the steel material. Therefore, in order to suppress the generation of oxides, it is important to set Al/O, which is the ratio based on mass of Al content to O content, to 3.0 to 35.0. I got this knowledge.

Furthermore, by reducing the S content, the generation of inclusions can be suppressed. However, it has been found that when Cu and S are contained simultaneously, a hardly soluble CuS film is formed on the surface of the steel material, and the corrosion resistance of the steel material is improved more than expected. In other words, the balance of Cu and S content is important, and by setting the acid corrosion index CI = (Cu/64)/(S/32) to a value of 4.0 to 30.0, the steel plate surface While forming a CuS film, it is possible to suppress the generation of inclusions that tend to become a starting point for corrosion.

On the other hand, although Cu is effective for corrosion resistance, it causes surface red brittleness, and Cu that exceeds the solid solubility limit in steel segregates at grain boundaries and the surface of the steel base, causing surface cracks. Therefore, as a result of repeated studies on the Cu content, we decided to include Cu and Ni at the same time, and the value of the red heat embrittlement index DI = (Cu/64)/(Ni/59) was set to 0.96 to 3.00. By doing so, it was revealed that while maintaining corrosion resistance, surface cracking could be suppressed and stable hot workability could be maintained.

Furthermore, in order to satisfy the above conditions and obtain a steel material with excellent weldability, it is important to set the carbon equivalent Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15 to 0.180 to 0.380. I found that.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be explained in detail.

(A) Chemical composition The reasons for limiting each element are as follows. In addition, in the following description, "%" regarding content means "mass %".

C: 0.010% or more and less than 0.20% C is an element that improves the strength of steel materials. However, when C is contained excessively, carbides increase and corrosion resistance deteriorates. Therefore, the C content is set to 0.010% or more and less than 0.20%. The C content is preferably 0.030% or more, more preferably 0.050% or more. Further, the C content is preferably 0.15% or less, more preferably 0.10% or less.

Si: 0.10~0.50%
Si is an element that contributes to deoxidation and improvement of strength, and controls the morphology of oxides. However, when Si is contained excessively, oxides increase and corrosion resistance is impaired. Therefore, the Si content is set to 0.10 to 0.50%. The Si content is preferably 0.15% or more, more preferably 0.20% or more. Further, the Si content is preferably 0.40% or less, more preferably 0.30% or less.

Mn: 0.10-1.00%
Mn is an element that improves strength and toughness. However, when Mn is contained excessively, coarse MnS is generated, resulting in deterioration of corrosion resistance and mechanical properties. Therefore, the Mn content is set to 0.10 to 1.00%. The Mn content is preferably 0.30% or more, more preferably 0.50% or more. Further, the Mn content is preferably 0.85% or less, more preferably 0.70% or less.

S: 0.0005-0.015%
S is an element that exhibits better corrosion resistance than expected by forming CuS in combination with Cu. However, when S is contained excessively, hot workability and mechanical properties of the steel material are reduced. Therefore, the S content is set to 0.0005 to 0.015%. The S content is preferably 0.0010% or more, more preferably 0.0050% or more. Further, the S content is preferably 0.008% or less.

Cu: 0.10~0.50%
Cu is an extremely important element that, when contained simultaneously with Sb, significantly exhibits corrosion resistance against sulfuric acid and hydrochloric acid. However, when Cu is contained excessively, hot workability decreases and manufacturability is impaired. Therefore, the Cu content is set to 0.10 to 0.50%. The Cu content is preferably 0.15% or more, more preferably 0.20% or more. Further, the Cu content is preferably 0.40% or less, more preferably 0.30% or less.

Cr:0.10~1.00%
Cr is an element that improves corrosion resistance like Cu and Sb. In particular, by containing Cr at the same time as Cu and Sb, excellent corrosion resistance is exhibited in an acidic environment with high temperature and high concentration. However, when Cr is contained excessively, corrosion resistance is impaired due to an increase in nitrides, which serve as starting points for corrosion. Therefore, the Cr content is set to 0.10 to 1.00%. It is preferable that the Cr content is 0.15% or more. Further, the Cr content is preferably 0.80% or less, more preferably 0.70% or less.

Sb: 0.03-0.15%
As mentioned above, Sb is an extremely important element that improves corrosion resistance when contained together with Cu. However, when Sb is contained excessively, hot workability decreases. Therefore, the Sb content is set to 0.03 to 0.15%. It is preferable that the Sb content is 0.08% or more. Moreover, it is preferable that the Sb content is 0.13% or less.

Ni: 0.01~0.50%
Ni exhibits the effect of increasing manufacturability in steel containing Cu. Cu has a great effect on improving corrosion resistance, but it tends to segregate, and when contained alone, it may promote cracking after casting. On the other hand, Ni has the effect of reducing surface segregation of Cu. By containing Ni, in addition to suppressing Cu segregation and slab cracking, occurrence of local corrosion due to segregation is also suppressed, so the effect of improving corrosion resistance is significantly exhibited. However, Ni is an expensive element, and containing a large amount increases steel manufacturing costs. Therefore, the Ni content is set to 0.01 to 0.50%. The Ni content is preferably 0.05% or more, more preferably 0.10% or more, and even more preferably 0.15% or more. Further, the Ni content is preferably 0.30% or less, more preferably 0.25% or less.

Al: 0.005-0.10%
Al is added as a deoxidizing agent. However, when Al is contained excessively, corrosion resistance is impaired due to an increase in inclusions. Therefore, the Al content is set to 0.005 to 0.10%. It is preferable that the Al content is 0.020% or more. Further, the Al content is preferably 0.050% or less.

P: 0.025% or less P is an impurity and reduces the mechanical properties and manufacturability of steel materials. Therefore, an upper limit is set for the P content to be 0.025% or less. The P content is preferably 0.020% or less, more preferably 0.015% or less. Note that it is preferable to reduce the P content as much as possible, that is, the content may be 0%, but an extreme reduction will lead to an increase in steel manufacturing costs. Therefore, the P content is preferably 0.001% or more.

O: 0.0005-0.0035%
O is an element that has the effect of making MnS harmless by combining with MnS and preventing deterioration of corrosion resistance and mechanical properties. However, when O is contained excessively, coarse oxides are generated which become a starting point for corrosion in an acidic environment. Therefore, the O content is set to 0.0005 to 0.0035%. The O content is preferably 0.0030% or less, more preferably 0.0025% or less. Further, the O content is preferably 0.0010% or more, more preferably 0.0015% or more.

N: 0.0010-0.0080%
Since N reduces the mechanical properties and manufacturability of steel, the N content is set to 0.0010 to 0.0080%. The N content is preferably 0.0070% or less, more preferably 0.0060% or less. Note that although it is preferable to reduce the N content as much as possible, an extreme reduction will lead to an increase in steel manufacturing costs. Furthermore, fine nitrides contribute to improving mechanical properties. Therefore, the N content is preferably 0.0015% or more, more preferably 0.0020% or more.

In addition to the above-mentioned elements, the chemical composition of the steel of the present invention may further contain one or more selected from Ti, Mo, W, Sn, As, Co, and Bi within the range shown below. The reasons for limiting each element will be explained.

Ti: 0.15% or less Ti is an element that improves corrosion resistance. In particular, since excellent corrosion resistance is exhibited in an acidic environment by containing Ti at the same time as Cu and Cr, Ti may be included as necessary. However, when Ti is contained excessively, corrosion resistance is impaired due to an increase in nitrides that cause corrosion. Therefore, the Ti content is set to 0.15% or less. The Ti content is preferably 0.07% or less, more preferably 0.05% or less. In addition, especially when it is desired to obtain the above effects, the Ti content is preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.03% or more. preferable.

Mo: 0.50% or less Mo is an element that improves corrosion resistance in an acidic environment when contained simultaneously with Cu and Sb, and therefore may be contained as necessary. However, Mo is an expensive element, and its inclusion in large amounts increases steel manufacturing costs. Therefore, the Mo content is set to 0.50% or less. The Mo content is preferably 0.30% or less. In addition, especially when improving the corrosion resistance against hydrochloric acid, the Mo content is preferably 0.01% or more, and more preferably 0.10% or more.

W: 0.50% or less Similar to Mo, W is an element that improves corrosion resistance in an acidic environment by containing it simultaneously with Cu and Sb, so it may be included as necessary. However, W is an expensive element, and its inclusion in large amounts increases steel manufacturing costs. Therefore, the W content is set to 0.50% or less. The W content is preferably 0.30% or less. In addition, especially when improving the corrosion resistance against hydrochloric acid, the W content is preferably 0.01% or more, and more preferably 0.10% or more.

Sn: 0.30% or less Sn is an element that improves corrosion resistance in an acidic environment, so it may be included if necessary. However, when Sn is contained excessively, hot workability decreases. Therefore, the Sn content is set to 0.30% or less. It is preferable that the Sn content is 0.20% or less. In addition, when it is desired to obtain the above effects, the Sn content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more. .

As: 0.30% or less Although As does not have a remarkable effect compared to Sb and Sn, it is an element effective in improving corrosion resistance in an acidic environment, so it may be included if necessary. However, when As is contained excessively, hot workability decreases. Therefore, the As content is set to 0.30% or less. The As content is preferably 0.20% or less, more preferably 0.10% or less. In addition, when it is desired to obtain the above effects, the As content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more. .

Co: 0.30% or less Although Co has no remarkable effect compared to Sb and Sn, it is an element that improves corrosion resistance in an acidic environment, so it may be included if necessary. However, when Co is contained excessively, economical efficiency decreases. Therefore, the Co content is set to 0.30% or less. The Co content is preferably 0.20% or less, more preferably 0.10% or less. In addition, when it is desired to obtain the above effects, the Co content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more. .

Bi: 0.010% or less Although Bi has no remarkable effect compared to Sb and Sn, it is an element that improves corrosion resistance in an acidic environment, so it may be included if necessary. However, when Bi is contained excessively, hot workability decreases. Therefore, the Bi content is set to 0.010% or less. The Bi content is preferably 0.007% or less, more preferably 0.005% or less. In addition, when it is desired to obtain the above effects, the Bi content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more. .

In addition to the above-mentioned elements, the chemical composition of the steel of the present invention further contains one or more selected from Nb, V, Zr, Ta, and B in the range shown below in order to improve mechanical properties etc. You may let them. The reasons for limiting each element will be explained.

Nb: 0.10% or less Nb is an element that forms nitrides, and may be included as necessary for the purpose of making crystal grains finer and improving strength. However, when Nb is contained excessively, mechanical properties may deteriorate. Therefore, the Nb content is set to 0.10% or less. The Nb content is preferably 0.050% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. In addition, when it is desired to obtain the above effects, the Nb content is preferably 0.001% or more, more preferably 0.005% or more.

V: 0.10% or less V, like Nb, is an element that forms nitrides, and may be included as necessary, mainly to improve strength through precipitation strengthening. However, when V is contained in excess, mechanical properties may deteriorate. Therefore, the V content is set to 0.10% or less. The V content is preferably 0.050% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. In addition, when it is desired to obtain the above-mentioned effect, it is preferable that the V content is 0.005% or more.

Zr: 0.050% or less Zr, like Nb and V, is an element that forms nitrides, and may be included as necessary, mainly for improving strength by precipitation strengthening. However, Zr is an expensive element, and containing a large amount increases steel manufacturing costs. Therefore, the Zr content is set to 0.050% or less. The Zr content is preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. In addition, when it is desired to obtain the above effects, it is preferable that the Zr content is 0.005% or more.

Ta: 0.050% or less Ta is an element that contributes to improving strength, and although the mechanism is not necessarily clear, it also contributes to improving corrosion resistance, so it may be included as necessary. However, Ta is an expensive element, and its inclusion in large amounts increases steel manufacturing costs. Therefore, the Ta content is set to 0.050% or less. The Ta content is preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. In addition, when it is desired to obtain the above effects, the Ta content is preferably 0.001% or more, more preferably 0.005% or more.

B: 0.010% or less Since B is an element that improves hardenability and increases strength, it may be included as necessary. However, even if B is contained in excess, the effect may be saturated and the toughness of the base material and HAZ may deteriorate. Therefore, the B content is set to 0.010% or less. The B content is preferably 0.0050% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. In addition, when it is desired to obtain the above effects, the B content is preferably 0.0003% or more, more preferably 0.0005% or more.

In the chemical composition of the steel of the present invention, in addition to the above-mentioned elements, one or more selected from Ca, Mg and REM may be contained in the range shown below for the purpose of deoxidation and control of inclusions. You can. The reasons for limiting each element will be explained.

Ca: 0.010% or less Ca is an element mainly used to control the form of sulfides, and may be included as necessary to form fine oxides. However, when Ca is contained excessively, mechanical properties may be impaired. Therefore, the Ca content is set to 0.010% or less. The Ca content is preferably 0.005% or less. In addition, when it is desired to obtain the above effects, the Ca content is preferably 0.0005% or more, more preferably 0.001% or more, and even more preferably 0.002% or more. .

Mg: 0.010% or less Mg may be included if necessary in order to form a fine oxide. However, adding excessive Mg causes an increase in steel manufacturing cost. Therefore, the Mg content is set to 0.010% or less. The Mg content is preferably 0.005% or less, more preferably 0.003% or less. In addition, when it is desired to obtain the above effects, the Mg content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more. .

REM: 0.010% or less REM (rare earth element) is an element mainly used for deoxidation, and may be included as necessary to form fine oxides. However, adding REM in excess increases steel manufacturing costs. Therefore, the REM content is set to 0.010% or less. The REM content is preferably 0.005% or less, more preferably 0.003% or less. In addition, when it is desired to obtain the above effects, the REM content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more. . Here, REM represents Sc, Y, and lanthanoid, and REM content represents the total content of these elements.

In the chemical composition of the steel of the present invention, the balance is Fe and impurities. Here, impurities are components that are mixed in from raw materials such as ore, scrap, and other factors during the industrial production of steel, and are allowed within the range that does not adversely affect the steel material according to the present invention. means.

Al/O: 3.0-35.0
As mentioned above, Al and O affect the corrosion resistance of steel materials. When the O content is excessive, Al/O becomes small, and when the Al content is excessive, Al/O becomes large, and in either case, oxides are generated and tend to become corrosion starting points. Therefore, Al/O is set to 3.0 to 35.0. Al/O is preferably 3.5 or more, more preferably 4.0 or more. Further, Al/O is preferably 30.0 or less, more preferably 25.0 or less.

CI: 4.0-30.0
Cu and S affect the corrosion resistance of steel materials. When the CI defined by the following formula (i) exceeds 30.0, it becomes difficult to form a CuS film on the surface of the steel material, making it impossible to obtain sufficient corrosion resistance. However, when CI is less than 4.0, inclusions tend to become corrosion starting points, and corrosion resistance may deteriorate. Therefore, CI is set to 4.0 to 30.0. The CI is preferably 18.0 or more, more preferably 20.0 or more. Further, the CI is preferably 28.0 or less, more preferably 26.0 or less.
CI=(Cu/64)/(S/32)...(i)

DI: 0.96-3.00
Cu and Ni affect the hot workability of steel materials. When the Ni content is low relative to the Cu content and the DI defined by the following formula (ii) exceeds 3.00, hot workability decreases. However, although it is preferable to make the DI small in order to ensure hot workability, the effect is saturated if it is less than 0.96. Furthermore, if Cu is insufficient, the effect of improving corrosion resistance will be insufficient. Therefore, DI is set to 0.96 to 3.00. The DI is preferably 1.20 or more, more preferably 1.40 or more. Further, the DI is preferably 2.80 or less, more preferably 2.60 or less.
DI=(Cu/64)/(Ni/59)...(ii)

Ceq:0.180~0.380
Furthermore, C, Mn, Cu, Ni, Cr, Mo, and V affect the weldability of steel materials. Ceq defined by the following formula (iii) is an index indicating deterioration of weldability due to increase in hardness of steel material, and is set to 0.380 or less in order to ensure weldability. However, when Ceq is less than 0.180, mechanical properties become insufficient. Therefore, Ceq is set to 0.180 to 0.380. Ceq is preferably 0.190 or more, more preferably 0.200 or more. Moreover, it is preferable that Ceq is 0.340 or less.
Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15...(iii)
However, in formula (iii), if Mo and V are not contained in the steel, it is calculated as 0.

(B) Manufacturing method There are no particular restrictions on the manufacturing method of the steel materials according to the present invention, but for example, steel plates, shaped steel, steel pipes, etc. that are manufactured by hot rolling and, if necessary, cold rolling, etc. is included.

When manufacturing steel materials, steel is melted using a conventional method, the components are adjusted, and the resulting steel slab is hot rolled and, if necessary, cold rolled. After hot rolling, the product may be cooled with water as it is, or may be cooled with air and then reheated and quenched. After hot rolling, it may be wound into a coil. After hot rolling, it may be cold rolled and further heat treated.

When manufacturing a steel pipe, a steel plate may be formed into a tubular shape and welded, and UO steel pipes, electric resistance welded steel pipes, forge welded steel pipes, spiral steel pipes, etc. can be made. Seamless steel pipes manufactured by subjecting steel slabs to hot extrusion or piercing rolling are also included in the steel materials of the present invention.

Hereinafter, the present invention will be explained in more detail with reference to Examples. Note that the conditions in the examples shown below are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of conditions. Further, the present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Steel having the chemical composition shown in Tables 1 and 2 was melted, and the steel ingot was heated at 1150° C. for 2 hours, then hot rolled and air cooled to obtain a steel plate with a thickness of 20 mm.

Using each of the obtained steel plates, various performance evaluation tests described below were conducted.

<Sulfuric acid resistance, hydrochloric acid resistance>
A test piece with a thickness of 4 mm, a width of 25 mm, and a length of 25 mm was taken from the center of each steel plate and finished with wet #400 polishing to serve as a test piece for corrosion resistance evaluation. Corrosion resistance was evaluated by sulfuric acid immersion test and hydrochloric acid immersion test. In the sulfuric acid immersion test, the test piece was immersed in a 50% sulfuric acid aqueous solution at 70°C for 6 hours, and in the hydrochloric acid immersion test, the test piece was immersed in a 10% hydrochloric acid aqueous solution at 80°C for 5 hours.

Thereafter, the corrosion loss of the test piece was calculated by the sulfuric acid immersion test and the hydrochloric acid immersion test. Based on Comparative Example AA, those whose corrosion loss was 50% or less are marked ◎, those whose corrosion loss was more than 50% but not more than 70% are marked ○, and those whose corrosion loss exceeded 70% are marked ×. Sulfuric acid resistance and hydrochloric acid resistance were evaluated.

<Hot workability>
The appearance of the surface of the hot-rolled material rolled under the above conditions was visually observed, and hot workability was evaluated by rating "x" if cracks had occurred and "○" if no cracks had occurred.

<Weld crack>
A y-type weld cracking test was conducted in accordance with JIS Z 3158:2016. Using a test piece with a thickness of 20 mm, welding was performed from both sides at a current of 170 A, and after 48 hours had elapsed, the presence or absence of cracks on the surface and cross section was confirmed.

<Tensile strength>
A tensile test piece was prepared in accordance with JIS Z 2241:2011, a tensile test was performed, and the tensile strength was determined. Those with a tensile strength of 400 MPa or more were rated as ○, and those with a tensile strength of less than 400 MPa were rated as ×.

Table 3 also shows the measurement results of the sulfuric acid immersion test, hydrochloric acid immersion test, hot workability, weld cracking test, and tensile test.

As shown in Table 3, steels A to Z have chemical compositions, Al/O, CI, DI, and Ceq within the scope of the present invention, and therefore have excellent corrosion resistance to hydrochloric acid and sulfuric acid, hot workability, weldability, and tensile strength. All results were good. On the other hand, steels AA to AK have chemical compositions, Al/O, CI, DI, and Ceq that are outside the scope of the present invention, and therefore have poor corrosion resistance to hydrochloric acid and sulfuric acid, hot workability, weldability, and tensile strength. This resulted in deterioration of at least one of the following.

Steel AA was the steel used as the standard for evaluation in the hydrochloric acid corrosion test and the sulfuric acid corrosion test, but due to its high Al/O content, its corrosion resistance to hydrochloric acid and sulfuric acid was lower than that of the steel of the invention example. Since steel AB had a low Al/O content, its corrosion resistance was reduced. In addition, steel AC with CI less than the specified value, steel AD with CI exceeding the specified value, steel AE with DI less than the specified value, steel AI with low Cu content, steel AJ with low Sb content, steel AJ with excessive Cr content. The corrosion resistance of the steel AK to hydrochloric acid and sulfuric acid was also lower than that of the steel of the present invention example.

Furthermore, since the DI of steel AF exceeds the specified value, hot workability was reduced. Steel AG had an excessive Ceq value, resulting in weld cracking, and steel AH had an excessively low Ceq value, resulting in insufficient tensile strength.

The steel material of the present invention can be used in boilers that burn fossil fuels such as heavy oil and coal, gas fuels such as liquefied natural gas, general waste such as municipal waste, industrial waste such as waste oil, plastics, and used tires, and sewage sludge. Can be used for smoke exhaust equipment. Specifically, the flue duct, casing, heat exchanger of the smoke exhaust equipment, gas-gas heater consisting of two heat exchangers (heat recovery device and reheater), desulfurization device, electric precipitator, induced blower. It can be suitably used for basket materials, heat transfer element plates, fin materials, etc. of rotary regenerative air preheaters.

Claims (4)

The chemical composition is in mass%,
C: 0.010% or more and less than 0.20%,
Si: 0.10 to 0.50%,
Mn: 0.10-1.00%,
S: 0.0005-0.015%,
Cu: 0.10-0.50%,
Cr: 0.10-1.00%,
Sb: 0.03 to 0.15%,
Ni: 0.01-0.50%,
Al: 0.005-0.10%,
P: 0.025% or less,
O: 0.0005-0.0035%,
N: 0.0010-0.0080%,
The remainder: Fe and impurities,
Al/O is 3.0 to 35.0,
CI defined by the following formula (i) is 4.0 to 30.0,
DI defined by the following formula (ii) is 0.96 to 3.00,
Ceq defined by the following formula (iii) is 0.180 to 0.380,
Steel material.
CI=(Cu/64)/(S/32)...(i)
DI=(Cu/64)/(Ni/59)...(ii)
Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15...(iii)
However, the element symbol in the above formula represents the content (mass %) of each element contained in the steel material, and if it is not contained, 0 shall be substituted. Al content is 0.005 to 0.041%,
O content is 0.0005 to 0.0030%,
The chemical composition is replaced by a part of the Fe in mass %,
Ti: 0.15% or less,
Mo: 0.50% or less,
W: 0.50% or less,
Sn: 0.30% or less,
As: 0.30% or less , and Bi: 0.010% or less,
Contains one or more selected from
The steel material according to claim 1. The chemical composition is replaced by a part of the Fe in mass %,
Nb: 0.10% or less,
V: 0.10% or less,
Zr: 0.050% or less,
Ta: 0.050% or less, and B: 0.010% or less,
Contains one or more selected from
The steel material according to claim 1 or claim 2. The chemical composition is replaced by a part of the Fe in mass %,
Ca: 0.010% or less,
Mg: 0.010% or less, and REM: 0.010% or less,
Contains one or more selected from
The steel material according to any one of claims 1 to 3.