JP6787530B1 - Steel - Google Patents

Abstract

Chemical composition is mass%, C: 0.01 to 0.10%, Si: 0.04 to 0.40%, Mn: 0.50 to 1.50%, Cu: 0.05 to 0.50%, Sb: 0.01 to 0.30%, Al: 0.005 to 0.050%, Mo + W: 0.01 to 0.30%, P: 0.020% or less, S: 0.0005 to 0.015%, N: 0.005% or less, O: 0.0005 to 0.0035%, Ca: Over 0% and 0.010% or less, Ni: 0 to 0.30% , Sn: 0 to 0.50%, As: 0 to 0.30%, Co: 0 to 0.30%, Cr: 0 to 0.70%, Ti: 0 to 0.050%, Nb: 0 to 0.10%, V: 0 to 0.10%, Zr: 0 to 0.050%, Ta: 0 to 0.050%, B: 0 to 0.010%, Mg: 0 to 0.010%, REM: 0 to 0.010%, balance: Fe and impurities, Si / Al: 6.0 to 16.0 , AI: 0.06 to 0.21, EI: 2.5 to 6.0 and / or Cu + Sb: 0.10 to 0.25%, Ceq: 0.180 to 0.330, and the number density of MnS contained in the steel material is less than 50 / mm2. ..

Description

The present invention relates to steel materials.

Exhaust gas containing steam, sulfur oxides, hydrogen chloride, etc. is generated in the incinerators of boiler fireplaces and waste incinerators. 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 on the steel materials constituting the exhaust gas flow path, as known as sulfuric acid dew point corrosion and hydrochloric acid dew point corrosion.

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

Japanese Unexamined Patent Publication No. 2001-164335 Japanese Unexamined Patent Publication No. 2003-213367 Japanese Unexamined Patent Publication No. 2007-239094 Japanese Unexamined Patent Publication No. 2012-57221 Japanese Unexamined Patent Publication No. 7-316745

Steel materials containing Cu, Sb, Cr, etc. exhibit excellent corrosion resistance in a sulfuric acid corrosion environment such as an exhaust gas chimney. However, in order to extend the life of boilers and incineration facilities, further improvement in corrosion resistance is expected.

In addition to exhaust gas chimneys, steel materials used in incinerator flues such as gasification and melting furnaces, heat exchangers, gas-gas heaters, desulfurization equipment, and electrostatic precipitators have corrosion resistance from the viewpoint of workability and productivity. Not only hot workability and weldability are also required.

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

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

(1) The chemical composition is mass%
C: 0.01 to 0.10%,
Si: 0.04 to 0.40%,
Mn: 0.50-1.50%,
Cu: 0.05 to 0.50%,
Sb: 0.01 to 0.30%,
Al: 0.005 to 0.050%,
Total of one or both of Mo and W: 0.01-0.30%,
P: 0.020% or less,
S: 0.0005 to 0.015%,
N: 0.005% or less,
O: 0.0005 to 0.0035%,
Ca: More than 0% and 0.010% or less,
Ni: 0-0.30%,
Sn: 0 to 0.50%,
As: 0-0.30%,
Co: 0-0.30%,
Cr: 0 to 0.70%,
Ti: 0 to 0.050%,
Nb: 0 to 0.10%,
V: 0 to 0.10%,
Zr: 0 to 0.050%,
Ta: 0 to 0.050%,
B: 0 to 0.010%,
Mg: 0-0.010%,
REM: 0-0.010%,
Remaining: Fe and impurities,
The mass ratio Si / Al between the Si content and the Al content is 6.0 to 16.0.
The AI defined by the following equation (i) is 0.06 to 0.21.
Either the EI defined by the following equation (ii) is 2.5 to 6.0, or the total content of Cu and Sb is 0.10 to 0.25% by mass. Satisfied
The Ceq defined by the following equation (iii) is 0.180 to 0.330.
The number density of MnS having a maximum length of 2.0 μm or more contained in the steel material is less than 50 / mm ² .
Steel material.
AI = ((Mo / 96) + (W / 184)) / (C / 12) ... (i)
EI = (Cu / 64) / ((Sb / 122) + (Sn / 119)) ... (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 is substituted.

(2) The chemical composition is mass%.
Sn: 0.001 to 0.50%,
The steel material according to (1) above.

(3) The chemical composition is mass%.
Ca: contains 0.00005 to 0.010%,
The XI defined by the following equation (iv) is 5.0 to 16.0.
The steel material according to (1) or (2) above.
XI = (Si / 28) / ((Al / 27) + (Ca / 40)) ... (iv)
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 is substituted.

(4) The chemical composition is mass%.
Ca: contains 0.00005 to 0.010%,
The mass ratio Ca / O between the Ca content and the O content is 1.00 or less.
The steel material according to (1) or (2) above.

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

As a result of detailed investigation of the corrosion resistance, hot workability, and weldability of the steel material in order to solve the above-mentioned problems, the present inventors have obtained the following findings.

The present inventors focused on oxides and carbides that are the starting points of corrosion, and conducted studies to improve the corrosion resistance of steel materials in an acid-corroded environment. Then, it was found that the formation of oxides, which are the starting points of corrosion, was suppressed by setting the Si / Al ratio in an appropriate range.

It was also found that Mo and W are effective in improving the corrosion resistance in an acid-corroded environment, but when they are contained in an excessive amount, their carbides become the starting point of corrosion. Therefore, a more detailed study was conducted on the relationship between the contents of Mo, W and C, and the relational expression of the acid resistance corrosion index AI defined by the following equation (i) was derived. Then, it was found that the corrosion resistance exceeding the expectation is exhibited by setting the acid corrosion resistance index AI in an appropriate range.
AI = ((Mo / 96) + (W / 184)) / (C / 12) ... (i)

Further, when Cu and Sb are contained at the same time, the corrosion resistance in an acid corrosion environment is improved, but the hot workability is lowered. On the other hand, excellent corrosion resistance and hot working can be achieved by setting at least one of the processability coefficients EI and Cu + Sb defined by the following formula (ii) and the Ceq defined by the following formula (iii) in an appropriate range. We obtained the finding that property and weldability can be obtained.
EI = (Cu / 64) / ((Sb / 122) + (Sn / 119)) ... (ii)
Ceq = C + Mn / 6 + (Cu + Ni) / 5+ (Cr + Mo + V) / 15 ... (iii)

Mn is an essential element for ensuring the strength and toughness of steel, but on the other hand, it forms MnS and deteriorates corrosion resistance in an acid-corroded environment. Further, in the present invention, since S has an effect of improving corrosion resistance by being contained together with Cu and Sb, extreme reduction is not preferable.

As a result of further studies by the present inventors in order to solve this problem, Ca is essentially added, and a part or all of S contained in the steel material is fixed as CaS, MnS is refined, and it is combined with oxygen. It was found that it can be detoxified by using MnS oxide.

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

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

C: 0.01 to 0.10%
C is an element that improves the strength of the steel material. However, when C is excessively contained, carbides increase and corrosion resistance deteriorates. Therefore, the C content is set to 0.01 to 0.10%. The C content is preferably 0.03% or more, and more preferably 0.05% or more. The C content is preferably 0.09% or less, more preferably 0.08% or less.

Si: 0.04 to 0.40%
Si is an element that contributes to deoxidation and improvement of strength and controls the morphology of oxides. However, when Si is excessively contained, the oxide increases and the corrosion resistance is impaired. Therefore, the Si content is set to 0.04 to 0.40%. The Si content is preferably 0.05% or more, more preferably 0.10% or more. Further, the Si content is preferably 0.30% or less.

Mn: 0.50 to 1.50%
Mn is an element that improves strength and toughness. However, when Mn is excessively contained, coarse MnS is generated, and the corrosion resistance and mechanical properties are deteriorated. Therefore, the Mn content is set to 0.50 to 1.50%. The Mn content is preferably 0.60% or more, and more preferably 0.80% or more. The Mn content is preferably 1.20% or less, and more preferably 1.00% or less.

Cu: 0.05 to 0.50%
Cu is an element that remarkably exhibits corrosion resistance to sulfuric acid and hydrochloric acid when contained at the same time as Sb. However, when Cu is excessively contained, the hot workability is lowered and the productivity is impaired. Therefore, the Cu content is set to 0.05 to 0.50%. The Cu content is preferably 0.10% or more, 0.15% or more, or 0.20% or more. The Cu content is preferably 0.40% or less, more preferably 0.30% or less.

Sb: 0.01 to 0.30%
Sb is an element that remarkably exhibits corrosion resistance to sulfuric acid and hydrochloric acid when contained at the same time as Cu. However, when Sb is excessively contained, the hot workability is lowered and the productivity is impaired. Therefore, the Sb content is set to 0.01 to 0.30%. The Sb content is preferably 0.03% or more, more preferably 0.06% or more, and further preferably 0.10% or more. Further, the Sb content is preferably 0.20% or less, and more preferably 0.15% or less.

Al: 0.005 to 0.050%
Al is added as an antacid. However, when Al is excessively contained, the corrosion resistance is impaired due to the increase in inclusions. Therefore, the Al content is set to 0.005 to 0.050%. The Al content is preferably 0.010% or more, and more preferably 0.020% or more. The Al content is preferably 0.045% or less, more preferably 0.040% or less.

Total of one or both of Mo and W: 0.01-0.30%
Mo and W are elements that improve corrosion resistance in an acid-corrosive environment by being contained at the same time as Cu and Sb. However, since Mo and W are expensive elements, excessive content causes a decrease in economic efficiency. Therefore, the total content of Mo and W is set to 0.01 to 0.30%. One of Mo and W may be contained alone, or both may be contained at the same time. The total content of Mo and W is preferably 0.05% or more, more preferably 0.10% or more. The total content of Mo and W is preferably 0.25% or less, and more preferably 0.20% or less.

P: 0.020% or less P is an impurity and reduces the mechanical properties and productivity of the steel material. Therefore, the upper limit of the P content is set to 0.020% or less. The P content is preferably 0.015% or less, more preferably 0.010% or less. The P content is preferably reduced as much as possible, that is, the content may be 0%, but an extreme reduction leads to an increase in steelmaking cost. Therefore, the P content may be 0.001% or more.

S: 0.0005 to 0.015%
S is generally an impurity and reduces the mechanical properties and productivity of the steel material. However, in the present invention, S has an effect of improving corrosion resistance in an acid-corrosive environment by containing Cu and Sb at the same time. Therefore, the S content is set to 0.0005 to 0.015%. The S content is preferably 0.0010% or more, 0.0050% or more, or 0.010% or more. The S content is preferably 0.013% or less, more preferably 0.011% or less.

N: 0.005% or less N is an impurity and reduces the mechanical properties and productivity of the steel material. Therefore, the upper limit of the N content is set to 0.005% or less. The N content is preferably 0.004% or less. The N content may be 0%, but an extreme reduction leads to an increase in steelmaking cost. Therefore, the N content may be 0.001% or more. Further, N has an effect of contributing to improvement of mechanical properties and the like by precipitating as a fine nitride. If the effect is desired, the N content may be 0.002% or more.

O: 0.0005 to 0.0035%
O is an element that has the effect of detoxifying MnS by binding to MnS and preventing deterioration of corrosion resistance and mechanical properties. However, when O is excessively contained, it produces a coarse oxide that is a starting point of corrosion in an acid corrosive environment. Therefore, the O content is set to 0.0005 to 0.0035%. The O content is preferably 0.0010% or more, more preferably 0.0015% or more. The O content is preferably 0.0030% or less, more preferably 0.0025% or less.

Ca: Exceeding 0% and 0.010% or less Ca is an element mainly used for controlling the morphology of sulfides, and has an effect of forming fine oxides. However, if Ca is excessively contained, the mechanical properties may be impaired. Therefore, the Ca content is set to more than 0% and 0.010% or less. The Ca content is preferably 0.00005% or more, 0.0001% or more or 0.0005% or more, more preferably 0.001% or more, still more preferably 0.002% or more. .. Further, the Ca content is preferably 0.005% or less.

In the chemical composition of the steel of the present invention, in addition to the above elements, in order to improve the corrosion resistance in an acid corrosive environment, one or more selected from Ni, Sn, As and Co are further selected in the range shown below. It may be contained. Since these elements are not always essential in steel materials, the lower limit of the content is 0%. The reasons for limiting each element will be described.

Ni: 0 to 0.30%
Ni is an element that improves corrosion resistance in an acid-corrosive environment, and in addition, has the effect of improving manufacturability in steel containing Cu. Cu has a great effect of improving corrosion resistance, but it is easily segregated, and when it is contained alone, it may promote cracking after casting. On the other hand, Ni has an effect of reducing the surface segregation of Cu. By containing Ni, in addition to suppressing segregation of Cu and cracking of slabs, the occurrence of local corrosion due to segregation is also suppressed, so that the effect of improving corrosion resistance can be obtained. Therefore, Ni may be contained if necessary. However, Ni is an expensive element, and its content in a large amount causes an increase in steelmaking cost. Therefore, the Ni content is set to 0.30% or less. The Ni content is preferably 0.25% or less. When the above effect is desired, the Ni content is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.10% or more. ..

Sn: 0 to 0.50%
Since Sn is an element that improves corrosion resistance in an acid-corrosive environment when it is contained at the same time as Cu, it may be contained if necessary. However, if Sn is contained in excess, the hot workability is lowered. Therefore, the Sn content is set to 0.50% or less. The Sn content is preferably 0.40% or less, more preferably 0.30% or less, and even more preferably 0.20% or less. When the above effect is desired, the Sn content is preferably 0.001% or more, 0.005% or more, 0.01% or more, 0.02% or more, or 0.05% or more. ..

As: 0 to 0.30%
Although As has no significant effect as compared with Sb and Sn, it may be contained as necessary because it is an element effective for improving corrosion resistance in an acid-corroded environment. However, if As is excessively contained, the hot workability is lowered. Therefore, the As content is set to 0.30% or less. The As content is preferably 0.20% or less, and more preferably 0.10% or less. When the above effect is desired, the As content is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.05% or more. ..

Co: 0 to 0.30%
Co does not have a remarkable effect as compared with Sb and Sn, but since it is an element that improves corrosion resistance in an acid-corrosive environment, it may be contained if necessary. However, if Co is excessively contained, the economic efficiency is lowered. Therefore, the Co content is set to 0.30% or less. The Co content is preferably 0.20% or less, and more preferably 0.10% or less. When the above effect is desired, the Co content is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.05% or more. ..

In the chemical composition of the steel of the present invention, in addition to the above elements, one or more selected from Cr, Ti, Nb, V, Zr, Ta, and B in order to improve mechanical properties and the like are listed below. It may be contained in the range shown. Since these elements are not always essential in steel materials, the lower limit of the content is 0%. The reasons for limiting each element will be described.

Cr: 0 to 0.70%
Since Cr is an element that enhances hardenability and strength, it may be contained if necessary. However, although Cr is an element that enhances weather resistance, it may reduce corrosion resistance in an acid-corrosive environment. Therefore, the Cr content is set to 0.70% or less. The Cr content is preferably 0.50% or less, more preferably 0.30% or less, and even more preferably 0.10% or less. When the above effect is desired, the Cr content is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.05% or more. ..

Ti: 0 to 0.050%
Since Ti is an element that forms a nitride and contributes to the refinement of crystal grains and the improvement of strength, it may be contained if necessary. However, when Ti is excessively contained, the nitride becomes coarse and the mechanical properties deteriorate. Therefore, the Ti content is set to 0.050% or less. The Ti content is preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. When the above effect is desired, the Ti content is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more. ..

Nb: 0 to 0.10%
Like Ti, Nb is an element that forms a nitride and contributes to the refinement of crystal grains and the improvement of strength, and therefore may be contained as necessary. However, when Nb is excessively contained, the nitride becomes coarse and the mechanical properties 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. When the above effect is desired, the Nb content is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more. ..

V: 0 to 0.10%
Like Ti and Nb, V is an element that forms a nitride and contributes to the refinement of crystal grains and the improvement of strength, and therefore may be contained as necessary. However, when V is excessively contained, the nitride becomes coarse and the mechanical properties 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. When the above effect is desired, the V content is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more. ..

Zr: 0 to 0.050%
Like Ti, Nb, and V, Zr is an element that forms a nitride and contributes to the refinement of crystal grains and the improvement of strength, and therefore may be contained as necessary. However, Zr is an expensive element, and a large amount of Zr causes an increase in steelmaking cost. In addition, if Zr is excessively contained, the nitride becomes coarse and the mechanical properties deteriorate. 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. When the above effect is desired, the Zr content is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more. ..

Ta: 0 to 0.050%
Ta is an element that contributes to the improvement of strength, and although the mechanism is not always clear, it also contributes to the improvement of corrosion resistance, and therefore may be contained if necessary. However, Ta is an expensive element, and its content in a large amount causes an increase in steelmaking cost. 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. When the above effect is desired, the Ta content is preferably 0.001% or more, more preferably 0.005% or more.

B: 0 to 0.010%
Since B is an element that improves hardenability and enhances strength, it may be contained if necessary. However, even if B is excessively contained, the effect may be saturated and the toughness of the base metal and HAZ may decrease. 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. When the above effect is desired, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more.

In the chemical composition of the steel of the present invention, in addition to the above elements, one or more selected from Mg and REM may be contained in the range shown below for the purpose of deoxidation and control of inclusions. Since these elements are not always essential in steel materials, the lower limit of the content is 0%. The reasons for limiting each element will be described.

Mg: 0 to 0.010%
Mg may be contained if necessary in order to form a fine oxide. However, excessive addition of Mg leads to an increase in steelmaking 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. When the above effect is desired, the Mg content is preferably 0.0001% or more, more preferably 0.0003% or more, and further preferably 0.0005% or more. ..

REM: 0-0.010%
REM (rare earth element) is an element mainly used for deoxidation, and may be contained as necessary in order to form a fine oxide. However, excessive addition of REM leads to an increase in steelmaking cost. 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. When the above effect is desired, the REM content is preferably 0.0001% or more, more preferably 0.0003% or more, and further preferably 0.0005% or more. ..

Here, REM is a general term for a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total amount of the above elements. Lanthanoids are industrially added in the form of misch metal.

In the chemical composition of the steel material of the present invention, the balance is Fe and impurities. Here, the impurity is a component mixed by raw materials such as ores and scraps and other factors in the industrial production of steel materials, and is allowed as long as it does not adversely affect the steel material according to the present invention. means.

Si / Al: 6.0 to 16.0
The Si / Al ratio (mass ratio) is an important index for suppressing oxides that tend to be corrosion origins on the surface of steel materials. In order to suppress the formation of oxides, it is effective to utilize Si, which has a weaker oxidizing power than Al, and when Si / Al is 6.0 or more, the corrosion resistance is remarkably improved. On the other hand, even if the Si / Al ratio exceeds 16.0, the effect is saturated, deoxidation becomes insufficient as the amount of Al decreases, and the corrosion resistance may decrease due to the oxide. Therefore, the Si / Al ratio is set to 6.0 to 16.0. The Si / Al ratio is preferably 6.7 or more, 8.0 or more, 8.5 or more, or 9.0 or more. The Si / Al ratio is preferably 14.0 or less, 13.5 or less, 13.0 or less, or 12.0 or less.

AI: 0.06 to 0.21
The acid resistance corrosion index AI is an index derived to suppress carbides that are likely to be corrosion origins on the surface of steel materials. Mo and W are effective in improving corrosion resistance, but if their contents are excessive, carbides that are the starting points of corrosion are likely to be formed. In order to significantly improve the corrosion resistance in an acid corrosion environment, the acid corrosion resistance index AI is set to 0.06 to 0.21. The acid resistance corrosion index AI is preferably 0.08 or more, more preferably 0.10 or more, and further preferably 0.12 or more. The acid corrosion resistance index AI is preferably 0.20 or less, more preferably 0.19 or less, and even more preferably 0.18 or less.

The acid resistance corrosion index AI is the ratio of the total number of Mo atoms and W atoms to the number of carbon atoms, as defined by the following equation (i). That is, Mo / 96, W / 184, and C / 12, respectively, are terms obtained by dividing the contents of Mo, W, and C by the mass number of each element.
AI = ((Mo / 96) + (W / 184)) / (C / 12) ... (i)

In the present invention, in order to achieve both corrosion resistance and hot workability, it is necessary to satisfy at least one of EI: 2.5 to 6.0 and Cu + Sb: 0.10 to 0.25%. Each regulation will be explained in detail.

EI: 2.5-6.0
The workability index EI is an index in consideration of the influence of Sb and Sn, which promotes a decrease in hot workability due to Cu. If the contents of Sb and Sn are too large with respect to the content of Cu, the hot workability may decrease. On the other hand, increasing the workability index EI is preferable for ensuring hot workability, but even if the value is excessive, the effect is saturated. Further, if Sb and Sn are insufficient, the effect of improving the corrosion resistance in an acid-corroded environment may be insufficient. From the viewpoint of achieving both hot workability and corrosion resistance, the workability index EI is preferably 2.5 to 6.0. The processability index EI is preferably 2.55 or more, and more preferably 2.6 or more. The workability index EI is preferably 5.9 or less, and more preferably 5.7 or less.

The processability index EI is the ratio of the number of Cu atoms to the number of Sb atoms and the number of Sn atoms, as defined by the following equation (ii). That is, Cu / 64, Sb / 122, and Sn / 119 are terms obtained by dividing the contents of Cu, Sb, and Sn by the mass number of each element, respectively.
EI = (Cu / 64) / ((Sb / 122) + (Sn / 119)) ... (ii)

Cu + Sb: 0.10 to 0.25%
The acid resistance of the steel is improved by containing Cu and Sb in a complex manner. In order to obtain this effect, the total content is preferably 0.10% or more, 0.12% or more, 0.14% or more, or 0.16% or more. On the other hand, if the total amount of Cu and Sb is too large, the hot workability may decrease. Therefore, the total content of Cu and Sb is 0.25% or less, 0.22% or less, or 0.20% or less. It is preferable to have it.

Ceq: 0.180 to 0.330
Ceq is an index showing deterioration of weldability due to an increase in hardness. If Ceq is excessive, weldability cannot be ensured. On the other hand, if Ceq is too low, the mechanical properties will be insufficient. Therefore, Ceq is set to 0.180 to 0.330. Ceq is preferably 0.200 or more, and more preferably 0.220 or more. Further, Ceq is preferably 0.330 or less, and more preferably 0.300 or less. Ceq is defined by the following equation (iii).
Ceq = C + Mn / 6 + (Cu + Ni) / 5+ (Cr + Mo + V) / 15 ... (iii)

XI: 5.0 to 16.0
Ca is an element that forms an oxide like Al. Therefore, in order to suppress the formation of oxides when Ca is contained in an amount of 0.00005% or more, Ca is also taken into consideration in addition to Al and Si, and specifically, XI defined by the following equation (iv). Is preferably 5.0 to 16.0. The XI is more preferably 6.0 or more, and even more preferably 7.0 or more. Further, the XI is more preferably 15.0 or less, and further preferably 14.0 or less.
XI = (Si / 28) / ((Al / 27) + (Ca / 40)) ... (iv)

Ca / O: 1.00 or less The Ca / O ratio (mass ratio) is an index for suppressing oxides that tend to be corrosion origins on the surface of steel materials. Ca enhances the cleanliness of steel by forming fine oxides that do not affect corrosion resistance, but when Ca is excessively contained with respect to the amount of O in steel, coarse oxides are excessively produced. , Decreases corrosion resistance. In particular, when Ca is contained in an amount of 0.00005% or more, the Ca / O ratio is preferably 1.00 or less in order to suppress the formation of excess coarse oxide. The Ca / O ratio is more preferably 0.90 or less, 0.85 or less, or 0.83 or less. The lower limit of the Ca / O ratio is not particularly limited, but if the Ca / O ratio is too low, oxides other than Ca are generated and the corrosion resistance is lowered. Therefore, the Ca / O ratio is 0.005 or more and 0.010. It is preferably more than or equal to or more than 0.015.

The element symbol in the above formulas (i) to (iv) represents the content (mass%) of each element contained in the steel material, and if it is not contained, 0 is substituted.

(B) Inclusions In the steel material according to the present invention, the number density of MnS having a maximum length of 2.0 μm or more contained in the steel material is less than 50 / mm ² .

Since MnS having a maximum length of less than 2.0 μm has almost no effect on the corrosion resistance of the steel material, inclusions having a maximum length of 2.0 μm or more are targeted in the present invention.

MnS becomes a starting point of corrosion and deteriorates corrosion resistance in an acid-corroded environment. On the other hand, an extreme reduction in the contents of Mn and S is not preferable in the steel material of the present invention from the viewpoint of improving strength, toughness and corrosion resistance, and therefore it is necessary to achieve both of them.

Therefore, as described above, in the steel material of the present invention, by containing Ca, a part or all of S contained in the steel material is fixed as CaS. In addition, MnS is detoxified by combining with oxygen to form an MnS oxide. When it becomes MnS oxide, it is detoxified and it becomes difficult to become a starting point of corrosion.

As a result, the number density of MnS having a maximum length of 2.0 μm or more contained in the steel material is limited to less than 50 / mm ² . In the following description, MnS having a maximum length of 2.0 μm or more is also simply referred to as MnS, and MnS oxide having a maximum length of 2.0 μm or more is also simply referred to as MnS oxide. The number density of MnS is preferably 40 / mm ² or less, and more preferably 30 / mm ² or less.

Further, in order to sufficiently detoxify MnS, the ratio of the number density of MnS oxides having a maximum length of 2.0 μm or more to the number density of MnS having a maximum length of 2.0 μm or more is 0.10 or more. Is preferable. The above ratio is preferably 0.12 or more, and more preferably 0.15 or more.

The number density of MnS and the number density of MnS oxide are measured by energy dispersive X-ray analysis (EDS) provided in a scanning electron microscope (SEM). The measurement magnification is 1000 times, and the maximum lengths of MnS and MnS oxides detected in the visual field are measured. Then, the number density is obtained by counting the number of inclusions having a maximum length of 2.0 μm or more and dividing by the visual field area.

The inclusions are identified by EDS, and inclusions having a total content of Mn and S of 90% by mass or more are determined to be MnS, a peak of O is detected, and the total content of Mn, S and O is contained. Inclusions having an amount of 90% by mass or more are judged to be MnS oxides.

(C) Manufacturing Method A method for manufacturing a steel material according to an embodiment of the present invention will be described. The steel material according to the present embodiment includes steel plates, shaped steels, steel pipes, and the like manufactured by hot rolling and, if necessary, cold rolling. A thick steel plate having a plate thickness of 3 mm or more, more preferably 6 mm or more is preferable.

The steel material according to the present embodiment is produced by melting steel by a conventional method, adjusting the components, hot-rolling the steel pieces obtained by casting, and cold-rolling if necessary. .. In order to control the ratio of the number densities of MnS and MnS oxides present in the steel material to the above range, it is important that the heating temperature before hot rolling is relatively low, specifically 1000. The temperature is preferably ~ 1130 ° C.

By lowering the heating temperature before hot rolling, the growth of MnS can be suppressed and the size can be reduced during rolling. Since the finely divided MnS has a relatively large surface area, it easily binds to oxygen and easily becomes an MnS oxide. In order to make the number density of MnS less than 30 / mm ² and the ratio of the number density of MnS oxides to MnS 0.12 or more, it is more preferable that the heating temperature before hot rolling is 1080 ° C. or less. ..

Next steps such as cutting or coil winding are added to the hot-rolled steel sheet after hot rolling. At that time, the temperature of the steel sheet drops, but it is desirable that the time from the completion of hot spreading to reaching 400 ° C. is 4 hours or more. Exposure to this temperature range promotes the bond between MnS and oxygen. After hot rolling, cold rolling may be performed to obtain a cold-rolled steel sheet. Further, heat treatment may be performed after cold rolling.

When a steel pipe is manufactured from the obtained steel plate, the steel pipe may be formed into a tubular shape and welded, and for example, a UO steel pipe, an electrosewn steel pipe, a forge welded steel pipe, a spiral steel pipe or the like can be obtained.

Hereinafter, the present invention will be described in more detail with reference to Examples. The conditions in the examples shown below are one-condition examples adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to this one-condition example. Further, the present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

Steels having the chemical compositions shown in Tables 1 to 3 (A1 to 29, B1 to 26, C1 to 14) are melted, and the ingots are hot-rolled under the conditions shown in Tables 4 to 6 to obtain a thickness. Manufactured a 20 mm hot-rolled steel sheet. Some steel sheets were cooled by simulating winding after hot rolling, and then cold-rolled to obtain cold-rolled steel sheets having a thickness of 3.4 mm.

A test piece for SEM observation was cut out from each of the obtained steel sheets, and the number density of inclusions was measured by the EDS provided in the SEM. The measurement magnification shall be 1000 times, the maximum lengths of MnS and MnS oxides detected in the visual field shall be measured, the number of inclusions having a maximum length of 2.0 μm or more, respectively, shall be counted and divided by the visual field area. Then, the number density was calculated.

Furthermore, various performance evaluation tests shown below were carried out using each of the obtained steel sheets.

<Sulfuric acid resistance, sulfuric acid resistance>
A test piece having a plate thickness of 3 mm, a width of 25 mm, and a length of 25 mm was collected from each steel plate from the central portion of the plate thickness and finished by wet # 400 polishing to prepare a test piece for corrosion resistance evaluation. The corrosion resistance was evaluated by a sulfuric acid immersion test and a 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.

Then, the corrosion rate was calculated from the corrosion weight loss of the test piece by the sulfuric acid immersion test and the hydrochloric acid immersion test. In the present embodiment, when the corrosion rate by sulfuric acid immersion test is less than 20.0mg / ^cm 2 / h, it determines that the excellent resistance to sulfuric acid, the corrosion rate with hydrochloric acid immersion test is 15.0 mg / ^cm 2 / When it was h or less, it was judged that the hydrochloric acid resistance was excellent.

<Hot workability>
The surface of the hot-rolled material rolled under the above conditions was visually inspected, and the hot workability was evaluated by valuing the cracked material as x and the non-cracked material as 〇.

<Welding crack>
A y-type weld crack test was performed in accordance with JIS Z 3158: 2016. Using a test piece having a thickness of 3 mm, the presence or absence of cracks on the surface and cross section was confirmed 48 hours after welding from one side with a current of 170 A.

<Tensile strength>
A tensile test piece was prepared in accordance with JIS Z 2241: 2011, and a tensile test was performed to determine the tensile strength. A 12 mm-thick test piece was taken from a hot-rolled steel sheet having a thickness of 20 mm, and a 3.4 mm-thick test piece was taken from a cold-rolled steel sheet having a thickness of 3.4 mm, and subjected to a tensile test. Those having a tensile strength of 400 MPa or more were evaluated as ◯, and those having a tensile strength of less than 400 MPa were evaluated as x.

Tables 7 to 9 summarize the measurement results of the number density of inclusions and the evaluation results of the sulfuric acid immersion test, the hydrochloric acid immersion test, the hot workability, the weld crack test and the tensile test.

As shown in Tables 7 to 9, Test Nos. That satisfy all the provisions of the present invention. In 1 to 55, excellent results were obtained in all the performance evaluation tests. On the other hand, Test No. which is a comparative example. In 56 to 72, at least one of sulfuric acid resistance, hydrochloric acid resistance, hot workability and weldability was deteriorated.

The steel material of the present invention is used for a boiler that burns heavy oil, fossil fuels such as coal, gas fuels such as liquefied natural gas, general wastes such as municipal waste, industrial wastes such as waste oil, plastics and exhaust tires, and sewage sludge. It can be used for smoke exhaust equipment. Specifically, a gas-gas heater composed of a flue duct, a casing, a heat exchanger, and two heat exchangers (heat recovery device and reheater) of smoke exhaust equipment, a desulfurization device, an electrostatic collector, and an attracting blower. , Can be suitably used for basket materials and heat transfer element plates of rotary regeneration type air preheaters.

Claims (4)

The chemical composition is mass%,
C: 0.01 to 0.10%,
Si: 0.04 to 0.40%,
Mn: 0.50-1.50%,
Cu: 0.05 to 0.50%,
Sb: 0.01 0 to 0.30%,
Al: 0.005 to 0.050%,
Total of one or both of Mo and W: 0.01-0.30%,
P: 0.020% or less,
S: 0.0005 to 0.015%,
N: 0.005% or less,
O: 0.0005 to 0.0035%,
Ca: More than 0% and 0.010% or less,
Ni: 0-0.30%,
Sn: 0 to 0.50%,
As: 0-0.30%,
Co: 0-0.30%,
Cr: 0 to 0.70%,
Ti: 0 to 0.050%,
Nb: 0 to 0.10%,
V: 0 to 0.10%,
Zr: 0 to 0.050%,
Ta: 0 to 0.050%,
B: 0 to 0.010%,
Mg: 0-0.010%,
REM: 0-0.010%,
Remaining: Fe and impurities,
The mass ratio Si / Al between the Si content and the Al content is 6.0 to 16.0.
The AI defined by the following equation (i) is 0.06 to 0.21.
Either the EI defined by the following equation (ii) is 2.5 to 6.0, or the total content of Cu and Sb is 0.10 to 0.25% by mass. Satisfied
The Ceq defined by the following equation (iii) is 0.180 to 0.330.
The XI defined by the following equation (iv) is 5.0 to 16.0.
The number density of the MnS maximum length is more than 2.0 .mu.m contained in the steel material is Ri 50 / mm ² less der, and the relative number density of more than MnS 2.0 .mu.m maximum length, the maximum length is 2. the ratio of the number density of more than MnS oxide 0μm is Ru der 0.10 or more,
Steel material.
AI = ((Mo / 96) + (W / 184)) / (C / 12) ... (i)
EI = (Cu / 64) / ((Sb / 122) + (Sn / 119)) ... (ii)
Ceq = C + Mn / 6 + (Cu + Ni) / 5+ (Cr + Mo + V) / 15 ... (iii)
XI = (Si / 28) / ((Al / 27) + (Ca / 40)) ... (iv)
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 is substituted. When the chemical composition is mass%,
Sn: 0.001 to 0.50%,
The steel material according to claim 1. When the chemical composition is mass%,
Ca: 0.00005~0.010%, it contains,
The steel material according to claim 1 or 2. When the chemical composition is mass%,
Ca: contains 0.00005 to 0.010%,
The mass ratio Ca / O between the Ca content and the O content is 1.00 or less.
The steel material according to claim 1 or 2.