KR20220066368A - steel - Google Patents

Abstract

Chemical composition, in 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: More than 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 steel materials is less than 50/mm ² Steel materials.

Description

steel

The present invention relates to steel materials.

In the furnace of a boiler, the incinerator of a waste incineration facility, etc., exhaust gas containing water vapor|steam, sulfur oxides, hydrogen chloride, etc. is generate|occur|produced. When this exhaust gas is cooled in an exhaust gas chimney or the like, it condenses to form sulfuric acid and hydrochloric acid, and as is known as sulfuric acid dew point corrosion and hydrochloric acid dew point corrosion, significant corrosion is caused to steel materials constituting the exhaust gas flow path.

In response to this problem, sulfuric acid/hydrochloric acid dew point corrosion-resistant steel and high corrosion-resistant stainless steel have been proposed. For example, in patent documents 1 - 4, the steel materials excellent in sulfuric acid dew point corrosion resistance to which Cu, Sb, Co, Cr, etc. were added are proposed. Moreover, in patent document 5, the highly corrosion-resistant stainless steel which added Cr, Ni, etc. is proposed.

Japanese Patent Laid-Open No. 2001-164335 Japanese Patent Laid-Open No. 2003-213367 Japanese Patent Laid-Open No. 2007-239094 Japanese Patent Laid-Open No. 2012-57221 Japanese Patent Laid-Open 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 prolong the life of boilers and incineration facilities, further improvement of corrosion resistance is expected.

In addition to the exhaust gas chimney, steel materials used for incinerator flues, such as gasification melting furnaces, heat exchangers, gas-gas heaters, desulfurization devices, and electric dust collectors, from the viewpoint of workability and productivity, not only have corrosion resistance, but also Processability and weldability are also required.

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

This invention was made in order to solve the said subject, and makes the following steel materials a summary.

(1) the chemical composition, in mass %,

C: 0.01-0.10%,

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: 0.005-0.050%,

The sum of one or both of Mo and 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: Exceeds 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: 0-0.050%,

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;

The mass ratio of Si content and Al content Si/Al is 6.0 to 16.0,

AI defined by the following (i) formula is 0.06 to 0.21,

EI defined by the following formula (ii) is 2.5 to 6.0, or the total content of Cu and Sb satisfies at least one of 0.10 to 0.25% by mass%,

Ceq defined by the following formula (iii) is 0.180 to 0.330,

The number density of MnS with a maximum length of 2.0 μm or more included in the steel material is less than 50/mm ² ,

steel.

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 said formula shows content (mass %) of each element contained in steel materials, and shall substitute 0 when it does not contain.

(2) the chemical composition is, in mass%,

Sn: containing 0.001 to 0.50%,

The steel material as described in said (1).

(3) the chemical composition is, in mass%,

Ca: containing 0.00005 to 0.010%,

XI defined by the following formula (iv) is 5.0 to 16.0,

The steel material as described in said (1) or (2).

XI = (Si/28)/((Al/27) + (Ca/40)) ... (iv)

However, the element symbol in the said formula shows content (mass %) of each element contained in steel materials, and shall substitute 0 when it does not contain.

(4) the chemical composition is, in mass%,

Ca: containing 0.00005 to 0.010%,

The mass ratio of Ca content to O content Ca/O is 1.00 or less,

The steel material as described in said (1) or (2).

ADVANTAGE OF THE INVENTION According to this invention, it has favorable corrosion resistance in an acid corrosion environment, and it becomes possible to provide the steel material excellent in both hot workability and weldability.

MEANS TO SOLVE THE PROBLEM In order to solve the above-mentioned subject, the present inventors came to acquire the following knowledge as a result of investigating the corrosion resistance, hot workability, and weldability of steel materials in detail.

MEANS TO SOLVE THE PROBLEM The present inventors investigated in order to improve the corrosion resistance of steel materials in an acid corrosion environment, paying attention to the oxide and carbide used as the origin of corrosion. And it turned out that generation|occurrence|production of the oxide used as the origin of corrosion is suppressed by making Si/Al ratio into an appropriate range.

Moreover, although Mo and W are effective for the improvement of corrosion resistance in an acid corrosion environment, when they were contained excessively, it discovered that these carbides became corrosion origins. Therefore, a more detailed examination was conducted about the relationship between the contents of Mo, W, and C, and the relational expression of the acid corrosion resistance index (AI) defined by the following formula (i) was derived. And the knowledge that corrosion resistance exceeding expectation is expressed by making acid-resistant corrosion-resistant index (AI) into an appropriate range was acquired.

AI=((Mo/96)+(W/184))/(C/12) ...(i)

In addition, when Cu and Sb are contained simultaneously, corrosion resistance in an acid corrosive environment is improved, but hot workability is reduced. On the other hand, the knowledge that excellent corrosion resistance, hot workability and weldability are obtained by making at least one of the workability coefficients EI and Cu+Sb defined by the following formula (ii), and Ceq defined by the following formula (iii) within an appropriate range. got it

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 in ensuring the strength and toughness of steel, but on the other hand, it forms MnS and deteriorates corrosion resistance in an acid corrosive environment. Moreover, in this invention, since S has the effect of improving corrosion resistance by containing it 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 essential, and some or all of S contained in steel materials is fixed as CaS, and MnS is refined and combined with oxygen, It discovered that it could be made harmless by setting it as 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 addition, in the following description, "%" with respect to content means "mass %".

C: 0.01 to 0.10%

C is an element which improves the intensity|strength of steel materials. However, when C is contained excessively, carbides increase and corrosion resistance deteriorates. Therefore, the C content is set to 0.01 to 0.10%. It is preferable that it is 0.03 % or more, and, as for C content, it is more preferable that it is 0.05 % or more. Moreover, it is preferable that it is 0.09 % or less, and, as for C content, it is more preferable that it is 0.08 % or less.

Si: 0.04 to 0.40%

Si is an element which contributes to deoxidation and the improvement of strength, and controls the form of an oxide. However, when Si is contained excessively, oxides increase, impairing corrosion resistance. Therefore, Si content shall be 0.04-0.40%. It is preferable that it is 0.05 % or more, and, as for Si content, it is more preferable that it is 0.10 % or more. Moreover, it is preferable that Si content is 0.30 % or less.

Mn: 0.50 to 1.50%

Mn is an element that improves strength and toughness. However, when Mn is contained excessively, coarse MnS is produced, and corrosion resistance and mechanical properties deteriorate. Therefore, the Mn content is set to 0.50 to 1.50%. It is preferable that it is 0.60 % or more, and, as for Mn content, it is more preferable that it is 0.80 % or more. Moreover, it is preferable that it is 1.20 % or less, and, as for Mn content, it is more preferable that it is 1.00 % or less.

Cu: 0.05 to 0.50%

When Cu is contained simultaneously with Sb, it is an element which expresses the corrosion resistance with respect to sulfuric acid and hydrochloric acid remarkably. However, when Cu is contained excessively, hot workability falls and productivity is impaired. Therefore, Cu content shall be 0.05 to 0.50 %. The Cu content is preferably 0.10% or more, 0.15% or more, or 0.20% or more. Moreover, it is preferable that it is 0.40 % or less, and, as for Cu content, it is more preferable that it is 0.30 % or less.

Sb: 0.01 to 0.30%

When Sb is contained simultaneously with Cu, it is an element which expresses the corrosion resistance with respect to sulfuric acid and hydrochloric acid remarkably. However, when Sb is contained excessively, hot workability falls and 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 still more preferably 0.10% or more. Moreover, it is preferable that it is 0.20 % or less, and, as for Sb content, it is more preferable that it is 0.15 % or less.

Al: 0.005-0.050%

Al is added as a deoxidizer. However, when Al is contained excessively, corrosion resistance is impaired by an increase in inclusions. Therefore, Al content shall be 0.005-0.050%. It is preferable that it is 0.010 % or more, and, as for Al content, it is more preferable that it is 0.020 % or more. Moreover, it is preferable that it is 0.045 % or less, and, as for Al content, it is more preferable that it is 0.040 % or less.

Sum of one or both of Mo and W: 0.01 to 0.30%

Mo and W are elements which improve corrosion resistance in an acid corrosion environment by making them contain simultaneously with Cu and Sb. However, since Mo and W are expensive elements, excessive containing causes a fall of economical efficiency. Therefore, the total content of Mo and W is set to 0.01 to 0.30%. Mo and W may contain one independently, and may contain both simultaneously. It is preferable that it is 0.05 % or more, and, as for the total content of Mo and W, it is more preferable that it is 0.10 % or more. Moreover, it is preferable that it is 0.25 % or less, and, as for the total content of Mo and W, it is more preferable that it is 0.20 % or less.

P: 0.020% or less

P is an impurity and reduces the mechanical properties and productivity of steel materials. Therefore, an upper limit is placed on the P content to be 0.020% or less. It is preferable that it is 0.015 % or less, and, as for P content, it is more preferable that it is 0.010 % or less. In addition, it is preferable to reduce the P content as much as possible, ie, the content may be 0%, but extreme reduction results in an increase in the 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 steel materials. However, in this invention, S has the effect of improving the corrosion resistance in an acid corrosion environment by containing Cu and Sb simultaneously. 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. Moreover, it is preferable that it is 0.013 % or less, and, as for S content, it is more preferable that it is 0.011 % or less.

N: 0.005% or less

N is an impurity and reduces the mechanical properties and productivity of steel materials. Therefore, an upper limit is placed on the N content to be 0.005% or less. The N content is preferably 0.004% or less. In addition, although 0% of N content may be sufficient, extreme reduction causes increase of steelmaking cost. Therefore, the N content may be 0.001% or more. Moreover, N has the effect of contributing to the improvement of mechanical properties etc. by precipitating as a fine nitride. In the case of obtaining the effect, the N content may be 0.002% or more.

O: 0.0005 to 0.0035%

O is an element having an effect of detoxifying MnS and preventing deterioration of corrosion resistance and mechanical properties by bonding with MnS. However, when O is contained excessively, a coarse oxide serving as a starting point of corrosion in an acid corrosive environment is produced. Therefore, the O content is set to 0.0005 to 0.0035%. It is preferable that it is 0.0010 % or more, and, as for O content, it is more preferable that it is 0.0015 % or more. Moreover, it is preferable that it is 0.0030 % or less, and, as for O content, it is more preferable that it is 0.0025 % or less.

Ca: Exceeds 0% and 0.010% or less

Ca is an element mainly used for controlling the form of sulfides, and has an effect of forming fine oxides. However, when Ca is contained excessively, mechanical properties may be impaired. Therefore, the Ca content exceeds 0% and is made 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, and still more preferably 0.002% or more. Moreover, it is preferable that Ca content is 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 corrosion resistance in an acid corrosive environment, at least one selected from Ni, Sn, As and Co is further contained within the range shown below. you can do it In addition, since these elements are not necessarily essential in steel materials, the lower limit of content is 0 %. The reason for limitation of each element is demonstrated.

Ni: 0 to 0.30%

Ni is an element that improves corrosion resistance in an acid corrosive environment, and in addition, in steel containing Cu, it has an effect of improving manufacturability. Although Cu has a great effect of improving corrosion resistance, it is easy to segregate, and when it is contained alone, it may promote cracking after casting. On the other hand, Ni has the effect of reducing the surface segregation of Cu. By containing Ni, in addition to suppression of Cu segregation and slab cracking, generation of local corrosion due to segregation is also suppressed, so that the effect of improving corrosion resistance can be obtained. Therefore, you may contain Ni as needed. However, Ni is an expensive element, and its content in a large amount causes an increase in the cost of steelmaking. Therefore, the Ni content is made 0.30% or less. It is preferable that Ni content is 0.25 % or less. Moreover, when trying to acquire the said effect, it is preferable that it is 0.01 % or more, and, as for Ni content, it is more preferable that it is 0.05 % or more, and it is still more preferable that it is 0.10 % or more.

Sn: 0 to 0.50%

Since Sn is an element which improves corrosion resistance in an acid corrosion environment when contained simultaneously with Cu, you may contain it as needed. However, when Sn is contained excessively, hot workability will fall. Therefore, Sn content shall be 0.50 % or less. It is preferable that Sn content is 0.40 % or less, It is more preferable that it is 0.30 % or less, It is more preferable that it is 0.20 % or less. In addition, when it is desired to obtain the above effect, 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 does not have a remarkable effect compared with Sb and Sn, since it is an element effective for the improvement of the corrosion resistance in an acid corrosion environment, you may contain it as needed. However, when As is contained excessively, hot workability will fall. Therefore, the As content is made 0.30% or less. As for As content, it is preferable that it is 0.20 % or less, and it is more preferable that it is 0.10 % or less. Moreover, when trying to acquire the said effect, it is preferable that As content is 0.01 % or more, It is more preferable that it is 0.02 % or more, It is still more preferable that it is 0.05 % or more.

Co: 0 to 0.30%

Although Co does not have a remarkable effect compared with Sb and Sn, since it is an element which improves the corrosion resistance in an acid corrosion environment, you may contain it as needed. However, when Co is contained excessively, economical efficiency is lowered. Therefore, the Co content is made 0.30% or less. It is preferable that it is 0.20 % or less, and, as for Co content, it is more preferable that it is 0.10 % or less. In addition, when it is desired to obtain the above effect, the Co content is preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.05% or more.

In the chemical composition of the steel of the present invention, in addition to the above elements, in order to improve mechanical properties and the like, at least one selected from Cr, Ti, Nb, V, Zr, Ta, and B is further selected from the range shown below. You may contain it in In addition, since these elements are not necessarily essential in steel materials, the lower limit of content is 0 %. The reason for limitation of each element is demonstrated.

Cr: 0 to 0.70%

Since Cr is an element which improves hardenability and improves strength, you may contain it as needed. However, although Cr is an element that improves weather resistance, it may reduce corrosion resistance in an acid corrosive environment. Therefore, the Cr content is made 0.70% or less. The Cr content is preferably 0.50% or less, more preferably 0.30% or less, and still more preferably 0.10% or less. Moreover, when trying to acquire the said effect, it is preferable that it is 0.01 % or more, and, as for Cr content, it is more preferable that it is 0.02 % or more, and it is still more preferable that it is 0.05 % or more.

Ti: 0-0.050%

Since Ti is an element that forms a nitride and contributes to refinement of crystal grains and improvement in strength, it may be contained as necessary. However, when Ti is contained excessively, the nitride becomes coarse and the mechanical properties deteriorate. Therefore, the Ti content is made 0.050% or less. The Ti content is preferably 0.040% or less, more preferably 0.030% or less, and still more preferably 0.020% or less. Moreover, when it is desired to acquire the said effect, it is preferable that Ti content is 0.001 % or more, It is more preferable that it is 0.002 % or more, It is still more preferable that it is 0.005 % or more.

Nb: 0 to 0.10%

Like Ti, Nb is an element that forms a nitride and contributes to refinement of crystal grains and improvement in strength, and thus may be contained as necessary. However, when Nb is contained excessively, the nitride becomes coarse and the mechanical properties deteriorate. Therefore, the Nb content is made 0.10% or less. It is preferable that Nb content is 0.050 % or less, It is more preferable that it is 0.030 % or less, It is more preferable that it is 0.020 % or less. Moreover, when trying to acquire the said effect, it is preferable that it is 0.001 % or more, and, as for Nb content, it is more preferable that it is 0.002 % or more, It is still more preferable that it is 0.005 % or more.

V: 0 to 0.10%

Like Ti and Nb, V is an element that forms a nitride and contributes to refinement of crystal grains and improvement in strength, and thus may be contained as necessary. However, when V is contained excessively, the nitride becomes coarse and the mechanical properties deteriorate. Therefore, the V content is made 0.10% or less. It is preferable that V content is 0.050 % or less, It is more preferable that it is 0.030 % or less, It is still more preferable that it is 0.020 % or less. Moreover, when it is desired to acquire the said effect, it is preferable that V content is 0.001 % or more, It is more preferable that it is 0.002 % or more, It is still more preferable that it is 0.005 % or more.

Zr: 0 to 0.050%

Since Zr is an element that forms a nitride and contributes to refinement of crystal grains and improvement of strength, similarly to Ti, Nb, and V, Zr may be contained as necessary. However, Zr is an expensive element, and its content in a large amount causes an increase in the cost of steelmaking. Incidentally, when Zr is contained excessively, the nitride becomes coarse and the mechanical properties deteriorate. Therefore, the Zr content is made 0.050% or less. The Zr content is preferably 0.040% or less, more preferably 0.030% or less, and still more preferably 0.020% or less. Moreover, when it is desired to acquire the said effect, it is preferable that it is 0.001 % or more, and, as for Zr content, it is more preferable that it is 0.002 % or more, It is still more preferable that it is 0.005 % or more.

Ta: 0 to 0.050%

Ta is an element contributing to the improvement of strength, and although the mechanism is not necessarily clear, Ta also contributes to the improvement of corrosion resistance, and thus may be contained as necessary. However, Ta is an expensive element, and its content in a large amount causes an increase in the cost of steelmaking. Therefore, the Ta content is made 0.050% or less. The Ta content is preferably 0.040% or less, more preferably 0.030% or less, and still more preferably 0.020% or less. Moreover, when it is desired to acquire the said effect, it is preferable that it is 0.001 % or more, and, as for Ta content, it is more preferable that it is 0.005 % or more.

B: 0 to 0.010%

Since B is an element which improves hardenability and raises intensity|strength, you may contain it as needed. However, even if B is contained excessively, the effect may be saturated, and the toughness of a base material and HAZ may fall. Therefore, the B content is made 0.010% or less. It is preferable that B content is 0.0050 % or less, It is more preferable that it is 0.0030 % or less, It is more preferable that it is 0.0020 % or less. Moreover, when trying to acquire the said effect, it is preferable that it is 0.0003 % or more, and, as for B content, it is more preferable that it is 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 within the range shown below for the purpose of deoxidation and control of inclusions. In addition, since these elements are not necessarily essential in steel materials, the lower limit of content is 0 %. The reason for limitation of each element is demonstrated.

Mg: 0 to 0.010%

Mg may be contained as needed in order to form a fine oxide. However, excessive addition of Mg causes an increase in the cost of steelmaking. Therefore, the Mg content is made 0.010% or less. It is preferable that it is 0.005 % or less, and, as for Mg content, it is more preferable that it is 0.003 % or less. Moreover, when it is desired to acquire the said effect, it is preferable that Mg content is 0.0001 % or more, It is more preferable that it is 0.0003 % or more, It is still more preferable that it is 0.0005 % or more.

REM: 0 to 0.010%

REM (rare earth element) is an element mainly used for deoxidation, and in order to form a fine oxide, you may contain it as needed. However, excessive addition of REM results in an increase in the cost of steelmaking. Therefore, the REM content is made 0.010% or less. It is preferable that it is 0.005 % or less, and, as for REM content, it is more preferable that it is 0.003 % or less. Moreover, when it is desired to acquire the said effect, it is preferable that REM content is 0.0001 % or more, It is more preferable that it is 0.0003 % or more, It is still more preferable that it is 0.0005 % or more.

Here, REM is a generic term for a total of 17 elements including Sc, Y, and lanthanoids, and the content of REM means the total amount of the elements. In addition, lanthanoids are industrially added in the form of mish metal.

In the chemical composition of the steel of the present invention, the remainder is Fe and impurities. Here, the impurity is a component that is mixed by raw materials such as ore, scrap, and other factors when industrially manufacturing steel materials, and means that it is allowed in a range that does not adversely affect the steel material according to the present invention.

Si/Al: 6.0 to 16.0

Si/Al ratio (mass ratio) is an important parameter|index in order to suppress the oxide which tends to become a corrosion origin on the steel material surface. In order to suppress the formation of oxides, it is effective to utilize Si, which has a weaker oxidizing power compared to Al, and by setting Si/Al to 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, and deoxidation becomes insufficient with a decrease in the amount of Al, and corrosion resistance may be deteriorated by the oxide. Therefore, the Si/Al ratio is set to 6.0 to 16.0. It is preferable that Si/Al ratio is 6.7 or more, 8.0 or more, 8.5 or more, or 9.0 or more. Moreover, it is preferable that Si/Al ratio is 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-resistant corrosion index (AI) is an index derived in order to suppress carbides that tend to become corrosion origins on the surface of steel materials. Mo and W are effective in improving corrosion resistance, but when their content is excessive, it becomes easy to form the carbide used as the origin of corrosion. In order to remarkably improve corrosion resistance in an acid corrosion environment, the acid corrosion resistance index (AI) is set to 0.06 to 0.21. The acid corrosion resistance index (AI) is preferably 0.08 or more, more preferably 0.10 or more, and still more preferably 0.12 or more. Moreover, it is preferable that it is 0.20 or less, It is more preferable that it is 0.19 or less, It is more preferable that acid-resistant corrosion-resistant index (AI) is 0.18 or less.

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

AI=((Mo/96)+(W/184))/(C/12) ...(i)

In this invention, in order to make corrosion resistance and hot workability compatible, it is necessary to satisfy|fill at least either one of EI:2.5-6.0 and Cu+Sb:0.10-0.25%. Each regulation is explained in detail.

EI: 2.5 to 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. When there is too much content of Sb and Sn with respect to content of Cu, hot workability may fall. On the other hand, it is preferable to increase the workability index (EI) in order to ensure hot workability, but even if the value is excessive, the effect is saturated. Moreover, when Sb and Sn run short, the effect of the improvement of corrosion resistance in an acid corrosion environment may become inadequate. From a viewpoint of making hot workability and corrosion resistance compatible, it is preferable that workability index (EI) sets it as 2.5-6.0. It is preferable that it is 2.55 or more, and, as for workability index (EI), it is more preferable that it is 2.6 or more. Moreover, it is preferable that it is 5.9 or less, and, as for the workability index (EI), it is more preferable that it is 5.7 or less.

The workability 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 formula (ii). That is, Cu/64, Sb/122, and Sn/119 are terms obtained by dividing the content 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%

By containing Cu and Sb in combination, the acid resistance of steel is improved. In order to acquire this effect, it is preferable that the total content is 0.10 % or more, 0.12 % or more, 0.14 % or more, or 0.16 % or more. On the other hand, since hot workability may fall when there is too much total amount of Cu and Sb, it is preferable that the total content of Cu and Sb is 0.25 % or less, 0.22 % or less, or 0.20 % or less.

Ceq: 0.180 to 0.330

Ceq is an index indicating deterioration of weldability due to increase in hardness. If Ceq is excessive, weldability cannot be ensured. On the other hand, if Ceq is too low, the mechanical properties become insufficient. Therefore, Ceq shall be 0.180-0.330. It is preferable that it is 0.200 or more, and, as for Ceq, it is more preferable that it is 0.220 or more. Moreover, it is preferable that it is 0.330 or less, and, as for Ceq, it is more preferable that it is 0.300 or less. Ceq is defined by the following formula (iii).

Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/15 ... (iii)

XI: 5.0 to 16.0

Ca, like Al, is an element that forms oxides. Therefore, when Ca is contained in 0.00005% or more, in order to suppress the formation of oxides, in addition to Al and Si, Ca is also taken into consideration. Specifically, XI defined by the following formula (iv) is 5.0 to 16.0. It is preferable to do As for XI, it is more preferable that it is 6.0 or more, and it is still more preferable that it is 7.0 or more. Moreover, it is more preferable that it is 15.0 or less, and, as for XI, it is still more preferable that it is 14.0 or less.

XI = (Si/28)/((Al/27) + (Ca/40)) ... (iv)

Ca/O: 1.00 or less

Ca/O ratio (mass ratio) is a parameter|index for suppressing the oxide which tends to become a corrosion origin on the steel material surface. Ca improves the cleanliness of steel by forming fine oxides that do not affect corrosion resistance. However, when Ca is contained excessively with respect to the amount of O in the steel, coarse oxides are excessively generated, thereby reducing corrosion resistance. In particular, when Ca is contained 0.00005% or more, in order to suppress the formation of an excessive coarse oxide, it is preferable to set the Ca/O ratio to 1.00 or less. It is more preferable that Ca/O ratio is 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 when the Ca/O ratio is too low, oxides other than Ca are formed and corrosion resistance is lowered. Therefore, the Ca/O ratio is preferably 0.005 or more, 0.010 or more, or 0.015 or more. .

In addition, the element symbol in said (i)-(iv) formula shall show content (mass %) of each element contained in steel materials, and shall substitute 0 when it does not contain.

(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 .

In addition, since MnS whose maximum length is less than 2.0 micrometers hardly affects the corrosion resistance of steel materials, in this invention, the maximum length shall target inclusions 2.0 micrometers or more.

MnS becomes a starting point of corrosion and deteriorates corrosion resistance in an acid corrosive environment. On the other hand, since extreme reduction of the content of Mn and S is undesirable from the viewpoint of improving the strength, toughness and corrosion resistance in the steel materials of the present invention, it is necessary to make them compatible.

Therefore, as mentioned above, in the steel materials of this invention, by containing Ca, part or all of S contained in steel materials is fixed as CaS. In addition, about MnS, it is made harmless by combining with oxygen and setting it as MnS oxide. When it becomes MnS oxide, it becomes harmless, and it becomes difficult to become a starting point of corrosion.

Thereby, the maximum length contained in the steel material limits the number density of MnS of 2.0 micrometers or more to less than 50/mm< ² >. In the following description, MnS having a maximum length of 2.0 µm or more is simply called MnS, and an MnS oxide having a maximum length of 2.0 µm or more is simply called MnS oxide. It is preferable that it is 40/mm< ² > or less, and, as for the number density of MnS, it is more preferable that it is 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 preferably 0.10 or more. It is preferable that said ratio is 0.12 or more, and it is more preferable that it is 0.15 or more.

The number density of MnS and the number density of MnS oxide are measured by the energy dispersive X-ray analysis (EDS) with which a scanning electron microscope (SEM) is equipped. The measurement magnification is set to 1000, and the maximum length of MnS and MnS oxide detected within the field of view is measured. And the number density is calculated|required by each counting the number of inclusions whose maximum length is 2.0 micrometers or more, and dividing by the viewing area.

Identification of inclusions is performed by EDS, inclusions having a total content of Mn and S of 90% by mass or more are judged as MnS, and a peak of O is detected, and inclusions having a total content of Mn, S and O of 90% by mass or more is determined to be MnS oxide.

(C) manufacturing method

A method for manufacturing steel according to an embodiment of the present invention will be described. The steel materials according to the present embodiment include a steel sheet, a section steel, a steel pipe, and the like, which are manufactured by performing hot rolling and cold rolling as needed. It is preferably a thick steel plate having a plate thickness of 3 mm or more, more preferably 6 mm or more.

The steel material which concerns on this embodiment melts steel by a normal method, hot-rolls the steel piece obtained by casting after adjusting a component, and also cold-rolls as needed, and is manufactured. In order to control the ratio of the number density of MnS and MnS oxides present in the steel material to the above-mentioned range, it is important to make the heating temperature before hot rolling relatively low, and specifically, it is preferable to set it as 1000-1130°C.

By making the heating temperature before hot rolling low, while suppressing the growth of MnS, it becomes possible to refine|miniaturize at the time of rolling. Since micronized MnS has a relatively large surface area, it becomes easy to bond with oxygen and becomes a MnS oxide easily. In order to make the number density of MnS less than 30/mm ² and the ratio of the number density of MnS oxide to MnS to be 0.12 or more, the heating temperature before hot rolling is more preferably 1080°C or less.

The following steps, such as cutting or coil winding, are applied to the hot rolled steel sheet after hot rolling. In that case, although the temperature of a steel plate falls, it is preferable that the time from completion of hot rolling to 400 degreeC is 4 hours or more. The combination of MnS and oxygen is promoted by exposure to this temperature range. After hot rolling, it is good also as a cold rolled steel sheet by cold rolling. Moreover, you may heat-process after cold rolling.

In the case of manufacturing a steel pipe from the obtained steel sheet, the steel sheet may be formed into a tubular shape and welded, for example, it may be a UO steel pipe, an electric resistance steel pipe, a single welded steel pipe, a spiral steel pipe, or the like.

Hereinafter, the present invention will be described in more detail by way of Examples. In addition, the conditions in the Example shown below are one condition example employ|adopted in order to confirm the practicability and effect of this invention, and this invention is not limited to this one condition example. Moreover, various conditions can be employ|adopted for this invention, as long as the objective of this invention is achieved without deviating from the summary of this invention.

[Example]

Steel (A1 to 29, B1 to 26, C1 to 14) having the chemical composition shown in Tables 1 to 3 is melted, and the steel ingot is hot rolled under the conditions shown in Tables 4 to 6 to obtain a hot rolled steel sheet having a thickness of 20 mm. Manufactured. About a part of steel plate, after performing cooling which simulated winding after hot rolling, it further cold-rolled and set it as the cold-rolled steel plate whose thickness is 3.4 mm.

The test piece for SEM observation was cut out from each obtained steel plate, and the number density of inclusions was measured by EDS with which SEM was equipped. The measurement magnification was set to 1000, the maximum lengths of MnS and MnS oxides detected within the field of view were measured, and the number of inclusions each having a maximum length of 2.0 µm or more was counted and divided by the field area to determine the number density.

Moreover, various performance evaluation tests shown below were done using each obtained steel plate.

<Sulfuric acid resistance, hydrochloric acid resistance>

A test piece having a plate thickness of 3 mm, a width of 25 mm, and a length of 25 mm was taken from each steel plate from the central portion of the plate thickness, and finished with wet #400 grinding to obtain a test piece for evaluation of corrosion resistance. 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 70 degreeC 50% sulfuric acid aqueous solution for 6 hours, and in the hydrochloric acid immersion test, the test piece was immersed in 80 degreeC 10% hydrochloric acid aqueous solution for 5 hours.

Then, the corrosion rate was computed from the corrosion loss of the test piece by the sulfuric acid immersion test and the hydrochloric acid immersion test, respectively. In this example, when the corrosion rate by the sulfuric acid immersion test is 20.0 mg/cm ² /h or less, it is judged that the sulfuric acid resistance is excellent, and when the corrosion rate by the hydrochloric acid immersion test is 15.0 mg/cm ² /h or less Yes, it was judged that it was excellent in hydrochloric acid resistance.

<Hot workability>

The external appearance of the surface of the hot-rolled material rolled under the said conditions was visually observed, the thing which cracked was made into X, the thing which a crack did not generate|occur|produce was made into (circle), and hot workability was evaluated.

＜Weld cracks＞

Based on JIS Z 3158:2016, the y-type weld crack test was done. Using a 3 mm-thick test piece, the presence or absence of cracks on the surface and cross section was checked after 48 hours had elapsed after welding from the single side side at an electric current of 170 A.

<Tensile strength>

Based on JIS Z 2241:2011, a tensile test piece was produced, the tensile test was done, and the tensile strength was calculated|required. In addition, a 12 mm thick test piece was taken from a 20 mm thick hot rolled steel sheet, and a 3.4 mm thick test piece was taken from a 3.4 mm thick cold rolled steel sheet, and it used for the tensile test. A thing with a tensile strength of 400 MPa or more was (circle), and a thing less than 400 MPa was made into x.

In Tables 7-9, the measurement result of the number density of inclusions, and the evaluation result of a sulfuric acid resistance immersion test, a hydrochloric acid resistance immersion test, hot workability, a weld crack test, and a tensile test are collectively shown.

As shown in Tables 7-9, in Test Nos. 1-55 which satisfy|filled all the prescription|regulations of this invention, also in any performance evaluation test, it became the outstanding result. On the other hand, in the test Nos. 56-72 which are comparative examples, at least any one of sulfuric acid resistance, hydrochloric acid resistance, hot workability, and weldability WHEREIN: The result which deteriorated was brought.

[Industrial Applicability]

The steel material of the present invention is a gas fuel such as heavy oil and coal, gas fuel such as liquefied natural gas, general waste such as municipal waste, industrial waste such as waste oil, plastic, waste tire, etc., and a flue gas facility for a boiler that burns sewage mud, etc. can be used for Specifically, the flue duct of a flue gas facility, a casing, a heat exchanger, a gas-gas heater composed of two heat exchangers (a heat recoverer and a reheater), a desulfurization device, an electrostatic precipitator, a manned blower, and a rotary regenerative air preheater. It can be suitably used for basket materials and heat transfer element plates.

Claims (4)

The chemical composition, in mass %,
C: 0.01-0.10%,
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: 0.005-0.050%,
The sum of one or both of Mo and 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: Exceeds 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: 0-0.050%,
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;
The mass ratio of Si content and Al content Si/Al is 6.0 to 16.0,
AI defined by the following (i) formula is 0.06 to 0.21,
EI defined by the following formula (ii) is 2.5 to 6.0, or the total content of Cu and Sb satisfies at least one of 0.10 to 0.25% by mass%,
Ceq defined by the following formula (iii) is 0.180 to 0.330,
A steel material having a number density of less than 50/mm ² of MnS having a maximum length of 2.0 μm or more contained in the 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 said formula shows content (mass %) of each element contained in steel materials, and shall substitute 0 when it does not contain. The method according to claim 1,
The chemical composition is, in mass%,
Sn: Steel materials containing 0.001 to 0.50%. The method according to claim 1 or 2,
The chemical composition is, in mass%,
Ca: containing 0.00005 to 0.010%,
XI defined by the following formula (iv) is 5.0 to 16.0, steel. The method according to claim 1 or 2,
The chemical composition is, in mass%,
Ca: containing 0.00005 to 0.010%,
Steel materials whose mass ratio Ca/O of Ca content and O content is 1.00 or less.