CN114599807B

CN114599807B - Steel material

Info

Publication number: CN114599807B
Application number: CN201980101642.1A
Authority: CN
Inventors: 今村淳子; 富尾亚希子; 长泽慎
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2019-11-13
Filing date: 2019-11-13
Publication date: 2023-05-09
Anticipated expiration: 2039-11-13
Also published as: JP6813128B1; KR20220059532A; JPWO2021095184A1; CN114599807A; WO2021095184A1

Abstract

A steel material comprises the following chemical components in percentage by mass: 0.01 to 0.10 percent of Si:0.04 to 0.40 percent of Mn:0.30 to 1.50 percent of Cu:0.05 to 0.50 percent of Sb:0.03 to 0.30 percent of Ni:0.01 to 0.50 percent of Cr:0.02 to 0.50 percent of Al:0.0050.055%, N: 0.002-0.010%, P: less than 0.020%, S:0.0005 to 0.015 percent, O:0.0005 to 0.0035 percent of Mo:0 to 0.50 percent, W:0 to 0.50 percent of Sn:0 to 0.30 percent, as:0 to 0.30 percent of Co:0 to 0.30 percent of Ti:0 to 0.050 percent, nb:0 to 0.10 percent, V:0 to 0.10 percent of Zr:0 to 0.050 percent, ta:0 to 0.050 percent, B:0 to 0.010 percent of Ca:0 to 0.010 percent of Mg:0 to 0.010 percent, REM:0 to 0.010 percent, the balance: fe and impurities, si/Al:7.0 to 15.0, BI:0.55 to 30.0, EI:1.0 to 6.0, ceq:0.150 to 0.400, wherein the steel contains MnS and MnS oxide, and the number density of the MnS is less than 50/mm ² The ratio of the number density of MnS oxide to the number density of MnS is 0.10 or more.

Description

Steel material

Technical Field

The present invention relates to steel.

Background

Exhaust gas containing water vapor, sulfur oxides, hydrogen chloride, and the like is generated in a furnace of a boiler, an incinerator of a waste incineration facility, and the like. The exhaust gas is condensed to form sulfuric acid and hydrochloric acid when cooled in an exhaust gas chimney or the like, and causes significant corrosion to steel materials constituting the exhaust gas flow path in a manner known as sulfuric acid dew point corrosion and hydrochloric acid dew point corrosion.

For this problem, sulfuric acid/hydrochloric acid dew point corrosion resistant steel and highly corrosion resistant stainless steel are proposed. For example, patent documents 1 to 4 propose steel materials having excellent dew point corrosion resistance to sulfuric acid to which Cu, sb, co, cr or the like is added. Patent document 5 proposes a highly corrosion-resistant stainless steel to which Cr, ni, or the like is added.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 2001-164335

Patent document 2: japanese patent laid-open No. 2003-213367

Patent document 3: japanese patent laid-open No. 2007-239094

Patent document 4: japanese patent application laid-open No. 2012-57221

Patent document 5: japanese patent laid-open No. 7-316745

Disclosure of Invention

Problems to be solved by the invention

Steels containing Cu, sb, cr and the like 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 plants, further improvement in corrosion resistance is desired.

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

The present invention aims 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 hot workability and weldability.

Solution for solving the problem

The present invention has been made to solve the above problems, and aims to provide the following steel material.

(1) A steel material having a chemical composition in mass percent

C：0.01～0.10％、

Si：0.04～0.40％、

Mn：0.30～1.50％、

Cu：0.05～0.50％、

Sb：0.03～0.30％、

Ni：0.01～0.50％、

Cr：0.02～0.50％、

Al：0.005～0.055％、

N：0.002～0.010％、

P: less than 0.020%,

S：0.0005～0.015％、

O：0.0005～0.0035％、

Mo：0～0.50％、

W：0～0.50％、

Sn：0～0.30％、

As：0～0.30％、

Co：0～0.30％、

Ti：0～0.050％、

Nb：0～0.10％、

V：0～0.10％、

Zr：0～0.050％、

Ta：0～0.050％、

B：0～0.010％、

Ca：0～0.010％、

Mg：0～0.010％、

REM：0～0.010％、

The balance: fe and impurities are mixed in the alloy,

the mass ratio of Si content to Al content is 7.0-15.0,

BI defined by the following formula (i) is 0.55 to 30.0,

EI defined by the following formula (ii) is 1.0 to 6.0,

ceq defined by the following formula (iii) is 0.150 to 0.400,

the steel contains MnS and MnS oxide, and the number density of MnS with the maximum length of more than 2.0 μm is less than 50/mm ² And 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,

BI＝(Cr/52)/(N/14)(i)

EI＝(Cu/64)/((Sb/122)+(Sn/119))(ii)

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

the symbol of the element in the above formula represents the content (mass%) of each element contained in the steel material, and if not, 0 is substituted.

(2) The steel material according to the above (1), wherein the chemical composition contains, in mass%, a metal selected from the group consisting of Mo:0.01 to 0.50 percent,

W：0.01～0.50％、

Sn：0.01～0.30％、

As:0.01 to 0.30%, and

co:0.01 to 0.30% of more than 1 kind.

(3) The steel material according to the above (1) or (2), wherein the chemical composition contains, in mass%, a component selected from the group consisting of

Ti：0.001～0.050％、

Nb：0.001～0.10％、

V：0.005～0.10％、

Zr：0.005～0.050％、

Ta:0.001 to 0.050%, and

b: more than 1 of 0.0003 to 0.010 percent.

(4) The steel material according to any one of the above (1) to (3), wherein the chemical composition contains, in mass%, a component selected from the group consisting of

Ca：0.0005～0.010％、

Mg:0.0001 to 0.010%, and

REM: more than 1 of 0.0001 to 0.010 percent.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a steel product having excellent corrosion resistance, hot workability and weldability in an acid corrosion environment can be provided.

Detailed Description

The inventors of the present invention have studied the corrosion resistance, hot workability and weldability of steel materials in detail to solve the above problems, and as a result, have found the following.

The present inventors have focused on oxides and carbides that become starting points of corrosion, and studied to improve the corrosion resistance of steel materials in an acid corrosion environment. Thus, it was found that when the Si/Al ratio was in an appropriate range, the formation of oxide, which becomes the starting point of corrosion, was suppressed.

In addition, cr was found to be effective for improving corrosion resistance in an acid corrosion environment, but if it is contained in excess, its nitride becomes a corrosion origin. Therefore, a further detailed study is made on the relationship between the contents of Cr and N to derive a relationship of the acid corrosion resistance index BI defined by the following formula (i). And it was found that by the acid corrosion resistance index BI being in a proper range, corrosion resistance exceeding expected is exhibited.

BI＝(Cr/52)/(N/14)(i)

Further, it was found that when Cu and Sb are contained simultaneously, the corrosion resistance in an acid corrosive environment is improved, but the hot workability is lowered. In contrast, when the workability coefficient EI defined by the following formula (ii) and the Ceq defined by the following formula (iii) are in the appropriate ranges, excellent corrosion resistance, hot workability, 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 securing strength and toughness of steel, but on the other hand forms MnS, deteriorating corrosion resistance in acid corrosive environments. In addition, in the present invention, S has the following effects: since the corrosion resistance is improved by the inclusion of Cu and Sb, extreme reduction is not preferable.

As a result of intensive studies to solve the above problems, the present inventors have found that MnS can be made harmless by making MnS finer and combining with oxygen to form MnS oxide.

The present invention has been made based on the above findings. The following describes each condition of the present invention in detail.

(A) Chemical composition

The reasons for limiting the elements are as follows. In the following description, "%" for the content refers to "% by mass".

C：0.01～0.10％

C is an element for improving the strength of the steel material. However, when the C content is excessive, the carbide content increases and the corrosion resistance deteriorates. Therefore, the C content is set to 0.01 to 0.10%. The C content is preferably 0.03% or more, more preferably 0.05% or more. The C content is preferably 0.09% or less, more preferably 0.08% or less.

Si：0.04～0.40％

Si is an element contributing to improvement of deoxidization and strength and controlling the morphology of oxide. However, if 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. The Si content is preferably 0.30% or less.

Mn：0.30～1.50％

Mn is an element that improves strength and toughness. However, when Mn is excessively contained, coarse MnS is generated, and corrosion resistance and mechanical properties are deteriorated. Therefore, the Mn content is set to 0.30 to 1.50%. The Mn content is preferably 0.50% or more, more preferably 0.80% or more. The Mn content is preferably 1.20% or less, more preferably 1.00% or less.

Cu：0.05～0.50％

Cu is an element that significantly exhibits corrosion resistance to sulfuric acid and hydrochloric acid when it is contained together with Sb. However, when Cu is contained in excess, hot workability is lowered, and productivity is impaired. Therefore, the Cu content is set to 0.05 to 0.50%. The Cu content is preferably 0.10% or more, more preferably 0.15% or more, and still more preferably 0.20% or more. The Cu content is preferably 0.40% or less, more preferably 0.30% or less.

Sb：0.03～0.30％

Sb is an element that significantly exhibits corrosion resistance to sulfuric acid and hydrochloric acid when it is contained together with Cu. However, if Sb is contained in excess, hot workability is lowered, and productivity is impaired. Therefore, the Sb content is set to 0.03 to 0.30%. The Sb content is preferably 0.05% or more, more preferably 0.08% or more, and still more preferably 0.10% or more. The Sb content is preferably 0.15% or less.

Ni：0.01～0.50％

Ni is an element that improves corrosion resistance in an acid corrosion environment, and has an effect of improving manufacturability in Cu-containing steel. Cu has a large effect of improving corrosion resistance, but is liable to segregate, and if it is contained alone, cracks after casting may be promoted. In contrast, ni has an effect of reducing surface segregation of Cu. By containing Ni, the occurrence of localized corrosion due to segregation is suppressed in addition to the segregation of Cu and the cracking of the cast slab, and thus the effect of improving corrosion resistance is obtained. However, ni is an expensive element, and is contained in a large amount, which increases the steel-making cost. Therefore, the Ni content is set to 0.01 to 0.50%. The Ni content is preferably 0.30% or less, more preferably 0.25% or less. The Ni content is preferably 0.03% or more, more preferably 0.05% or more, and still more preferably 0.10% or more.

Cr：0.02～0.50％

Cr is an element that improves corrosion resistance, similarly to Cu and Sb. In particular, when Cu and Sb are contained in combination with Cr, excellent corrosion resistance is exhibited in an acidic environment at a high temperature and a high concentration. However, when Cr is contained in an excessive amount, the corrosion resistance is impaired due to an increase in nitride which becomes a starting point of corrosion. Therefore, the Cr content is set to 0.02 to 0.50%. The Cr content is preferably 0.30% or less, more preferably 0.20% or less. The Cr content is preferably 0.05% or more.

Al：0.005～0.055％

Al is added as a deoxidizer. However, when Al is contained in excess, corrosion resistance is impaired due to the increase of inclusions. Therefore, the Al content is set to 0.005 to 0.055%. The Al content is preferably 0.010% or more, more preferably 0.020% or more. The Al content is preferably 0.050% or less, more preferably 0.040% or less.

N：0.002～0.010％

N as a fine nitride contributes to improvement of mechanical properties. However, when N is contained in excess, a nitride is formed with Cr, which deteriorates corrosion resistance. Therefore, the N content is set to 0.002 to 0.010%. The N content is preferably 0.003% or more. The N content is preferably 0.008% or less, more preferably 0.006% or less.

P: less than 0.020%

P is an impurity, and reduces the mechanical properties and productivity of the steel. Thus, the method is applicable to a variety of applications. The upper limit of the P content is set to 0.020% or less. The P content is preferably as low as possible, that is, the content may be 0%, but extremely low results in an increase in steelmaking cost. Therefore, the P content may be 0.001% or more.

S：0.0005～0.015％

S is generally an impurity, and deteriorates the mechanical properties and productivity of the steel material. However, in the present invention, S has the following effects: the corrosion resistance in an acid corrosive environment is improved 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. The S content is preferably 0.013% or less, more preferably 0.011% or less.

O：0.0005～0.0035％

O is an element having the following effects: by combining with MnS, mnS is rendered harmless, and deterioration of corrosion resistance and mechanical properties is prevented. However, when the content of O is excessive, coarse oxides are generated which become starting points of corrosion in an acid corrosion 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.

In addition to the above elements, the steel of the present invention may contain 1 or more kinds selected from Mo, W, sn, as, co in the following ranges in order to improve corrosion resistance in an acid corrosion environment. These elements are not necessarily essential in the steel material, and therefore the lower limit of the content is 0%. The reason for limiting each element will be described.

Mo：0～0.50％

Mo is an element that improves corrosion resistance in an acidic environment, particularly corrosion resistance to hydrochloric acid, by being contained together with Cu, sb, and Cr, and thus may be contained as needed. However, mo is an expensive element, and thus is excessively contained to cause a decrease in economical efficiency. Therefore, the Mo content is set to 0.50% or less. The Mo content is preferably 0.30% or less, more preferably 0.10% or less. In order to obtain the above-described effects, the Mo content is preferably 0.01% or more, more preferably 0.05% or more, and still more preferably 0.10% or more.

W：0～0.50％

W is an element that improves corrosion resistance in an acidic environment, particularly corrosion resistance to hydrochloric acid, by being contained together with Cu, sb, and Cr similarly to Mo, and thus may be contained as needed. However, W is also an expensive element, and thus is contained in an excessive amount, resulting in a decrease in economy. Therefore, the W content is set to 0.50% or less. The W content is preferably 0.30% or less, more preferably 0.10% or less. In order to obtain the above-described effect, the W content is preferably 0.01% or more, more preferably 0.05% or more, and still more preferably 0.10% or more.

Sn：0～0.30％

Sn is an element that improves corrosion resistance in an acid corrosion environment, and thus may be contained as needed. However, when Sn is contained in excess, hot workability is lowered. Therefore, the Sn content is set to 0.30% or less. The Sn content is preferably 0.20% or less. In order to obtain the above-described effects, the Sn content is preferably 0.01% or more, 0.02% or more, or 0.05% or more.

As：0～0.30％

As is an element that is effective for improving corrosion resistance in an acid corrosion environment, although it is not significantly effective As compared with Sb and Sn, and therefore may be contained As needed. However, when the As is contained in excess, 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, more preferably 0.10% or less. In order to obtain the above-described effect, the As content is preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.05% or more.

Co：0～0.30％

Co is an element that improves corrosion resistance in an acid corrosion environment although it is not significantly effective compared with Sb and Sn, and therefore may be contained as needed. However, if Co is contained in excess, the economical efficiency is lowered. 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 order to obtain the above-described 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 addition to the above elements, the steel of the present invention may contain 1 or more selected from Ti, nb, V, zr, ta, B in the following ranges for the purpose of improving mechanical properties and the like. These elements are not necessarily essential in the steel material, and therefore the lower limit of the content is 0%. The reason for limiting each element will be described.

Ti：0～0.050％

Ti is an element that forms a nitride and contributes to grain refinement and strength improvement, and thus may be contained as needed. 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 still more preferably 0.020% or less. In order to obtain the above-described effects, the Ti content is preferably 0.001% or more, more preferably 0.005% or more.

Nb：0～0.10％

Nb is an element that forms nitride similarly to Ti and contributes to grain refinement and strength improvement, and thus may be contained as needed. However, when Nb is contained in excess, 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 still more preferably 0.020% or less. In order to obtain the above-described effects, the Nb content is preferably 0.001% or more, more preferably 0.005% or more.

V：0～0.10％

V is an element that forms a nitride similarly to Ti and Nb and contributes to grain refinement and strength improvement, and thus may be contained as needed. However, when V is contained in excess, 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 still more preferably 0.020% or less. In order to obtain the above-described effects, the V content is preferably 0.005% or more.

Zr：0～0.050％

Zr is an element that forms nitride similarly to Ti, nb, and V and contributes to grain refinement and strength improvement, and thus may be contained as needed. However, zr is an expensive element, and is contained in a large amount, which increases the steel-making cost. If Zr is contained excessively, 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 still more preferably 0.020% or less. In the case where the above-mentioned effects are to be obtained, the Zr content is preferably 0.005% or more.

Ta：0～0.050％

Ta is an element contributing to improvement in strength, and although the mechanism is not necessarily clear, ta contributes to improvement in corrosion resistance, and therefore may be contained as needed. However, ta is an expensive element, and is contained in a large amount, which increases the steel-making 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 still more preferably 0.020% or less. In order to obtain the above-described effects, the Ta content is preferably 0.001% or more, more preferably 0.005% or more.

B：0～0.010％

B is an element that improves hardenability and strength, and thus may be contained as needed. However, even if the B content is excessive, the effect is saturated, and there is a possibility that toughness of the base material and HAZ is lowered. 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 still more preferably 0.0020% or less. In order to obtain the above-described effect, the B content is preferably 0.0003% or more, more preferably 0.0005% or more.

In addition to the above elements, the steel of the present invention may contain 1 or more kinds selected from Ca, mg, REM in the following ranges for deoxidation and inclusion control. These elements are not necessarily essential in the steel material, and therefore the lower limit of the content is 0%. The reason for limiting each element will be described.

Ca：0～0.010％

Ca is an element mainly used for controlling the morphology of sulfides, and may be contained as needed to form fine oxides. However, if Ca is contained in excess, 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 order to obtain the above-described effect, the Ca content is preferably 0.0005% or more, more preferably 0.001% or more, and still more preferably 0.002% or more.

Mg：0～0.010％

Mg may be contained as needed to form a fine oxide. However, excessive Mg addition causes 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. In the case where the above-described effect is to be obtained, the Mg content is preferably 0.0001% or more, more preferably 0.0003% or more, and still more 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 needed to form a fine oxide. However, excessive addition of REM causes 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. In order to obtain the above-described effect, the REM content is preferably 0.0001% or more, more preferably 0.0003% or more, and still more preferably 0.0005% or more.

Here, REM is a total term of 17 elements in total of Sc, Y and lanthanoid, and the content of REM refers to the total amount of the above elements. The lanthanoid element is industrially added as a misch metal alloy.

The balance of Fe and impurities in the chemical composition of the steel. Here, the impurities refer to components which are allowed to be mixed in by other main causes of raw materials such as ores and scraps in the industrial production of steel materials within a range which does not adversely affect the steel materials of the present invention.

Si/Al：7.0～15.0

The Si/Al ratio (mass ratio) is an important index for suppressing oxides that easily become corrosion starting points on the steel surface. In order to suppress the formation of oxide, it is effective to effectively use Si having a weaker oxidizing power than Al, and the corrosion resistance is significantly improved by Si/Al being 7.0 or more. On the other hand, even if the Si/Al ratio exceeds 15.0, the effect is saturated, and the deoxidization is insufficient with a decrease in the Al amount, and the corrosion resistance may be lowered due to the oxide. Therefore, the Si/Al ratio is set to 7.0 to 15.0. The Si/Al ratio is preferably 8.0 or more or 9.0 or more. The Si/Al ratio is preferably 14.0 or less or 13.0 or less.

BI：0.55～30.0

The acid corrosion resistance index BI is an index derived to suppress nitrides that tend to be corrosion starting points on the surface of steel. Cr is effective for improving corrosion resistance, but if it is contained excessively, nitrides which become starting points of corrosion are easily formed. In order to significantly improve the corrosion resistance in an acid corrosion environment, the acid corrosion resistance index BI is set to 0.55 to 30.0. The acid corrosion resistance index BI is preferably 0.60 or more, more preferably 0.70 or more. The acid corrosion resistance index BI is preferably 15.0 or less, more preferably 10.0 or less, and further preferably 5.00 or less.

The acid corrosion resistance index BI is a ratio of the number of Cr atoms to the number of N atoms as defined in the following formula (i). That is, cr/52 and N/14 are each a product obtained by dividing the content of Cr and N by the mass number of each element.

BI＝(Cr/52)/(N/14)(i)

EI：1.0～6.0

The workability index EI is an index in which the influence of Sb and Sn, which contribute to the reduction in hot workability due to Cu, is considered. If the content of Sb and Sn is too large relative to the content of Cu, the hot workability is lowered. On the other hand, it is preferable to increase the workability index EI in order to secure hot workability, but even if the value is excessive, the effect is saturated. In addition, if Sb and Sn are insufficient, the effect of improving corrosion resistance in an acid corrosion environment is insufficient. From the viewpoint of both hot workability and corrosion resistance, the workability index EI is set to 1.0 to 6.0. The workability index EI is preferably 2.0 or more, more preferably 3.0 or more. The workability index EI is preferably 5.9 or less, more preferably 5.8 or less.

The workability index EI is a ratio of the number of Cu atoms to the number of Sb atoms to the number of Sn atoms as defined by the following formula (ii). That is, cu/64, sb/122, and Sn/119 are each obtained by dividing the content of Cu, sb, and Sn by the mass number of each element.

EI＝(Cu/64)/((Sb/122)+(Sn/119))(ii)

Ceq：0.150～0.400

Ceq is an index indicating 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 are insufficient. Therefore, ceq is set to 0.150 to 0.400.Ceq is preferably 0.180 or more, more preferably 0.200 or more. The Ceq is preferably 0.350 or less, more preferably 0.330 or less. Ceq is defined by the following formula (iii).

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

The element symbols in the above formulae (i) to (iii) represent the content (mass%) of each element contained in the steel material, and if not, 0 is substituted.

(B) Inclusions of

The steel material of the present invention contains MnS and MnS oxides. And the number density of MnS with the maximum length of more than 2.0 mu m is less than 50/mm ² . And 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.

Since MnS having a maximum length of less than 2.0 μm hardly affects the corrosion resistance of steel, inclusions having a maximum length of 2.0 μm or more are targeted in the present invention. In the following description, mnS having a maximum length of 2.0 μm or more is referred to as MnS only, and MnS oxide having a maximum length of 2.0 μm or more is referred to as MnS oxide only.

As described above, in the steel material of the present invention, formation of MnS cannot be avoided. However, mnS becomes a starting point of corrosion and deteriorates corrosion resistance in an acid corrosion environment. Therefore, it is necessary to limit the number density of MnS to less than 50/mm ² . The number density of MnS is preferably 40/mm ² Hereinafter, more preferably 30/mm ² The following is given.

On the other hand, the extreme decrease 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. In order to achieve both of these effects, mnS needs to be rendered harmless. MnS combines with oxygen to form MnS oxide, which is harmless and not likely to serve as a starting point for corrosion. Thus, in the present invention, the ratio of the number density of MnS oxide to the number density of MnS is set to 0.10 or more. The above ratio is preferably 0.12 or more, more preferably 0.15 or more.

The number density of MnS and the number density of MnS oxide were measured by energy dispersive X-ray analysis (EDS) provided in a Scanning Electron Microscope (SEM). The measurement magnification was set to 1000 times, and the maximum lengths of MnS and MnS oxides detected in the field of view were measured. The number of inclusions having a maximum length of 2.0 μm or more was counted and divided by the field area to obtain a number density.

The inclusions were identified by EDS, and the inclusions having a total content of Mn and S of 90 mass% or more were determined as MnS, and further peaks of O were detected, and the inclusions having a total content of Mn, S and O of 90 mass% or more were determined as MnS oxides.

(C) Method of manufacture

A method for producing a steel material according to an embodiment of the present invention will be described. The steel material according to the present embodiment includes a steel sheet, a steel section, a steel pipe, and the like manufactured by hot rolling and further cold rolling as necessary. The thickness of the steel sheet is preferably 3mm or more, more preferably 6mm or more.

The steel material according to the present embodiment is produced by melting steel by a conventional method, adjusting the composition, casting to obtain a slab, hot-rolling the slab, and further cold-rolling if necessary. In order to control the ratio of the number density of MnS and MnS oxide present in the steel material within the above range, it is important that the heating temperature before hot rolling is set to a relatively low temperature, and specifically, it is preferably set to 1000 to 1130 ℃.

By reducing the heating temperature before hot rolling, mnS growth is suppressed and miniaturization can be achieved during rolling. The finely divided MnS has a relatively large surface area, and therefore is easily combined with oxygen, and MnS oxide is easily formed. In order to make the number density of MnS less than 30/mm ² The ratio of the number density of MnS oxide to MnS is 0.12 or more, and the heating temperature before hot rolling is more preferably 1080 ℃ or less.

The hot rolled steel sheet after hot rolling is subjected to the next steps such as cutting and coil winding. In this case, the steel sheet temperature is reduced, but the time from the completion of hot rolling to the 400 ℃ is preferably 4 hours or more. By exposure to this temperature range, the binding of MnS and oxygen is promoted. After hot rolling, cold rolling may be performed to form a cold-rolled steel sheet. Further, heat treatment may be performed 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, and for example, a UO steel pipe, an electric welded steel pipe, a forged steel pipe, a spiral steel pipe, or the like may be formed.

The present invention will be described in more detail with reference to examples. The conditions in the examples shown below are examples of conditions to be employed for confirming the possibility and effect of the present invention, and the present invention is not limited to the examples of conditions. In addition, various conditions can be adopted in the present invention as long as the object of the present invention is achieved without departing from the gist of the present invention.

Examples

Steels (A1 to 26 and B1 to 11) having chemical compositions shown in tables 1 and 2 were melted, and steel ingots were hot-rolled under the conditions shown in tables 3 and 4 to produce hot-rolled steel sheets having a thickness of 20 mm. A part of the steel sheet was cooled by simulated coiling after hot rolling, and then cold rolled to form a cold-rolled steel sheet having a thickness of 3.4 mm.

TABLE 1

TABLE 2

TABLE 3

TABLE 3 Table 3

TABLE 4

TABLE 4 Table 4

Test pieces for SEM observation were cut out from the obtained steel sheets, and the number density of inclusions was measured by EDS provided in the SEM. The maximum lengths of MnS and MnS oxides detected in the visual field were measured at a measurement magnification of 1000 times, and the number of inclusions having a maximum length of 2.0 μm or more were counted and divided by the visual field area to obtain the number density.

Further, using the obtained steel sheets, various performance evaluation tests shown below were performed.

< resistance to sulfuric acid and hydrochloric acid >)

Test pieces 3mm thick, 25mm wide and 25mm long were collected from the center of each steel sheet, and polished by wet #400 polishing to form test pieces for corrosion resistance evaluation. The corrosion resistance was evaluated by a sulfuric acid dip test and a hydrochloric acid dip test. In the sulfuric acid impregnation test, the test piece was immersed in a 50% aqueous sulfuric acid solution at 70℃for 6 hours, and in the hydrochloric acid impregnation test, the test piece was immersed in a 10% aqueous hydrochloric acid solution at 80℃for 5 hours.

Then, the corrosion rates were calculated from the corrosion reductions of the test pieces caused by the sulfuric acid immersion test and the hydrochloric acid immersion test, respectively. In this example, the corrosion rate due to the sulfuric acid immersion test was 20.0mg/cm ² When the ratio was not higher than/h, it was found that the acid resistance was excellent, and the corrosion rate by the hydrochloric acid immersion test was 15.0mg/cm ² When the ratio is not higher than/h, the hydrochloric acid resistance is judged to be excellent.

< Hot workability >)

The appearance of the surface of the hot rolled material rolled under the above conditions was observed visually, and the case where cracking occurred was regarded as good and the case where no cracking occurred was regarded as good, and the hot workability was evaluated.

< welding crack >)

According to JIS Z3158: 2016, a y-type weld crack test was performed. After welding from one side with a current 170A using a test piece having a thickness of 3mm, the presence or absence of cracks on the surface and the cross section was confirmed after 48 hours.

< tensile Strength >

According to JIS Z2241: 2011, and performing a tensile test to obtain tensile strength. Test pieces 12mm thick were collected from hot-rolled steel sheets 20mm thick, and test pieces 3.4mm thick were collected from cold-rolled steel sheets 3.4mm thick, and were subjected to tensile test. The tensile strength was set to be equal to or greater than 400MPa and less than 400 MPa.

The results of measuring the number density of inclusions, and the results of evaluating the sulfuric acid resistance dip test, the hydrochloric acid resistance dip test, the hot workability, the weld crack test, and the tensile test are shown in tables 5 and 6.

TABLE 5

TABLE 6

As shown in tables 5 and 6, excellent results were obtained in any performance evaluation test for all of test nos. 1 to 26 satisfying the regulations of the present invention. In contrast, in test nos. 27 to 40 as comparative examples, the results of deterioration in at least any one of sulfur acid resistance, salt acid resistance, hot workability and weldability were obtained.

Industrial applicability

The steel material of the present invention can be used for a smoke exhaust device of a boiler for burning fossil 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 and waste tires, sewage sludge, and the like. Specifically, the present invention can be suitably used for flue pipes, jackets, heat exchangers, gas-gas heaters composed of 2 heat exchangers (heat recoverer and reheater), desulfurization apparatuses, electric dust collectors, induced fans, casings for rotary regenerative air preheaters, heat transfer element plates, and the like of smoke evacuation apparatuses.

Claims

1. A steel material comprises the following chemical components in percentage by mass: 0.01 to 0.10 percent,

Si：0.04～0.40％、

Mn：0.30～1.50％、

Cu：0.05～0.50％、

Sb：0.03～0.30％、

Ni：0.01～0.50％、

Cr：0.02～0.50％、

Al：0.005～0.055％、

N：0.002～0.010％、

P: less than 0.020%,

S：0.0005～0.015％、

O：0.0005～0.0035％、

Mo：0～0.50％、

W：0～0.50％、

Sn：0～0.30％、

As：0～0.30％、

Co：0～0.30％、

Ti：0～0.050％、

Nb：0～0.10％、

V：0～0.10％、

Zr：0～0.050％、

Ta：0～0.050％、

B：0～0.010％、

Ca：0～0.010％、

Mg：0～0.010％、

REM：0～0.010％、

The balance: fe and impurities are mixed in the alloy,

the mass ratio of Si content to Al content is 7.0-15.0,

BI defined by the following formula (i) is 0.55 to 30.0,

EI defined by the following formula (ii) is 1.0 to 6.0,

ceq defined by the following formula (iii) is 0.150 to 0.400,

BI＝(Cr/52)/(N/14) (i)

EI＝(Cu/64)/((Sb/122)+(Sn/119)) (ii)

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

the symbol of the element in the above formula represents the content of each element in mass% contained in the steel material, and if not, 0 is substituted.

2. The steel product as claimed in claim 1, wherein the chemical composition contains, in mass%, a component selected from the group consisting of

Mo：0.01～0.50％、

W：0.01～0.50％、

Sn：0.01～0.30％、

As:0.01 to 0.30%, and

co:0.01 to 0.30% of more than 1 kind.

3. The steel product as claimed in claim 1, wherein the chemical composition contains, in mass%, a component selected from the group consisting of

Ti：0.001～0.050％、

Nb：0.001～0.10％、

V：0.005～0.10％、

Zr：0.005～0.050％、

Ta:0.001 to 0.050%, and

b: more than 1 of 0.0003 to 0.010 percent.

4. The steel product as claimed in claim 2, wherein the chemical composition contains, in mass%, a component selected from the group consisting of

Ti：0.001～0.050％、

Nb：0.001～0.10％、

V：0.005～0.10％、

Zr：0.005～0.050％、

Ta:0.001 to 0.050%, and

b: more than 1 of 0.0003 to 0.010 percent.

5. The steel product as claimed in any one of claims 1 to 4, wherein the chemical composition contains, in mass%, a component selected from the group consisting of

Ca：0.0005～0.010％、

Mg:0.0001 to 0.010%, and

REM: more than 1 of 0.0001 to 0.010 percent.