JP7252497B2 - hot rolled steel - Google Patents

Description

The present invention relates to hot rolled steel.

Boiler furnaces, incinerators in waste incineration facilities, and the like generate exhaust gases containing water vapor, sulfur oxides, hydrogen chloride, and the like. When this exhaust gas is cooled in an exhaust gas stack or the like, it condenses into sulfuric acid and hydrochloric acid, which causes significant corrosion of the steel material forming the exhaust gas flow path, known as sulfuric acid dew-point corrosion and hydrochloric acid dew-point corrosion.

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

JP-A-2001-164335 JP 2003-213367 A JP 2007-239094 A JP 2012-57221 A JP-A-7-316745

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

These steel materials are used not only for flue gas stacks, but also for incinerator flues such as gasification and melting furnaces, heat exchangers, gas-gas heaters, desulfurizers, and electrostatic precipitators.

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

The present invention has been made to solve the above problems, and the gist thereof is the following hot-rolled steel.

(1) A hot-rolled steel material having oxide scale on at least part of the surface of the base material,
The chemical composition of the base material is, in mass%,
C: 0.01 to 0.10%,
Si: 0.04 to 0.40%,
Mn: 0.30-1.50%,
Cu: 0.02-0.50%,
Sb: 0.01 to 0.30%,
Al: 0.005-0.055%,
P: 0.020% or less,
S: 0.0005 to 0.015%,
N: 0.010% or less,
O: 0.0005 to 0.0035%,
Mo: 0-0.50%,
W: 0 to 0.50%,
Ni: 0 to 0.50%,
Sn: 0-0.50%,
As: 0 to 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%,
Ca: 0-0.010%,
Mg: 0-0.010%,
REM: 0-0.010%,
balance: Fe and impurities,
having a concentrated layer of Si, Cu and Sb at the interface between the base material and the oxide scale;
Hot rolled steel.

(2) the chemical composition, in mass %,
Mn: 0.50-1.50%,
Cu: 0.05-0.50%,
Al: 0.005 to 0.050%,
sum of one or both of Mo and W: 0.01 to 0.30%;
N: 0.005% or less,
Ni: 0 to 0.30%, containing
The mass ratio Si/Al between the Si content and the Al content is 6.0 to 16.0,
AI defined by the following formula (i) is 0.06 to 0.21,
At least one of EI defined by the following formula (ii) is 2.5 to 6.0, or the total content of Cu and Sb is 0.10 to 0.25% by mass. satisfied,
Ceq defined by the following formula (iii) is 0.180 to 0.330,
The hot-rolled steel material according to (1) above.
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 (% by mass) of each element contained in the steel material, and 0 shall be substituted when it is not contained.

(3) the chemical composition, in mass %,
Sn: 0.001 to 0.50%, containing
The hot-rolled steel material according to (2) above.

(4) the chemical composition, in mass %,
Contains Ca: 0.00005 to 0.010%,
XI defined by the following formula (iv) is 5.0 to 16.0,
The hot-rolled steel material according to (2) or (3) above.
XI=(Si/28)/((Al/27)+(Ca/40)) (iv)
However, the element symbol in the above formula represents the content (% by mass) of each element contained in the steel material, and 0 shall be substituted when it is not contained.

(5) the chemical composition, in mass %,
Contains Ca: 0.00005 to 0.010%,
The mass ratio Ca/O between the Ca content and the O content is 1.00 or less,
The hot-rolled steel material according to (2) or (3) above.

(6) the chemical composition, in mass %,
Cu: 0.05-0.50%,
Sb: 0.03 to 0.30%,
Ni: 0.01 to 0.50%,
Cr: 0.02-0.50%,
N: 0.002 to 0.010%,
Sn: 0 to 0.30%, containing
The mass ratio Si/Al between the Si content and the Al content is 7.0 to 15.0,
BI defined by the following formula (v) 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 hot-rolled steel material according to (1) above.
BI=(Cr/52)/(N/14) (v)
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 (% by mass) of each element contained in the steel material, and 0 shall be substituted when it is not contained.

ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the hot-rolled steel material which has favorable corrosion resistance in an acid corrosive environment.

In order to solve the above-described problems, the present inventors have made detailed investigations into the corrosion resistance of steel materials, and as a result, have obtained the following findings.

The present inventors used hot-rolled steel manufactured by hot rolling under various conditions to investigate a method for improving the corrosion resistance of steel in an acid-corrosive environment.

Concentrated layers of Si, Cu and Sb are formed between the oxide scale formed on the surface of the steel base material and the base material by containing Cu and Sb at the same time and by appropriately controlling the hot rolling conditions. found to be It was also found that the formation of such a thickened layer exhibits a barrier effect against sulfuric acid and hydrochloric acid, further improving corrosion resistance in an acid corrosive environment.

The present invention has been made based on the above findings. Each requirement of the present invention will be described in detail below.

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

C: 0.01-0.10%
C is an element that improves the strength of steel. However, when C is contained excessively, carbide increases and corrosion resistance deteriorates. Therefore, the C content should be 0.01 to 0.10%. The C content is preferably 0.03% or more, more preferably 0.05% or more. Also, the C content is preferably 0.09% or less, more preferably 0.08% or less.

Si: 0.04-0.40%
Si is an element that contributes to deoxidation and strength improvement, and controls the morphology of oxides. However, when Si is contained excessively, oxides increase and the corrosion resistance is impaired. Therefore, the Si content should be 0.04 to 0.40%. The Si content is preferably 0.05% or more, more preferably 0.10% or more. Also, 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 contained excessively, coarse MnS is formed, degrading corrosion resistance and mechanical properties. Therefore, the Mn content should be 0.30 to 1.50%. The Mn content is preferably 0.50% or more, more preferably 0.60% or more, even more preferably 0.80% or more. Also, the Mn content is preferably 1.20% or less, more preferably 1.00% or less.

Cu: 0.02-0.50%
Cu is an element that remarkably exhibits corrosion resistance to sulfuric acid and hydrochloric acid when contained together with Sb. However, when Cu is contained excessively, the hot workability is deteriorated and the productivity is impaired. Therefore, the Cu content is set to 0.02 to 0.50%. The Cu content is preferably 0.05% or more, more preferably 0.10% or more, even more preferably 0.20% or more. Also, the Cu content is preferably 0.40% or less, more preferably 0.30% or less.

Sb: 0.01-0.30%
Sb is an element that remarkably exhibits corrosion resistance to sulfuric acid and hydrochloric acid when contained together with Cu. However, when Sb is contained excessively, 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 even more preferably 0.10% or more. Also, the Sb content is preferably 0.20% or less, more preferably 0.15% or less.

In addition, in the present invention, Cu and Sb are contained in a composite manner. % or more. On the other hand, when emphasizing hot workability, the total content of Cu and Sb is 0.50% or less, 0.40% or less, 0.30% or less, 0.25% or less, 0.22% or less Alternatively, it is preferably 0.20% or less.

Al: 0.005-0.055%
Al is added as a deoxidizing agent. However, when Al is contained excessively, corrosion resistance is impaired due to an increase in inclusions. Therefore, the Al content is set to 0.005 to 0.055%. The Al content is preferably 0.010% or more, more preferably 0.020% or more. Also, the Al content is preferably 0.050% or less, more preferably 0.045% or less, and even more preferably 0.040% or less.

P: 0.020% or less P is an impurity and lowers the mechanical properties and productivity of steel materials. 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. In addition, it is preferable to reduce the P content as much as possible. Therefore, the P content may be 0.001% or more.

S: 0.0005-0.015%
S is generally an impurity and reduces the mechanical properties and productivity of steel materials. However, in the present invention, when S is contained together with Cu and Sb, it has the effect of improving corrosion resistance in an acid corrosive environment. Therefore, the S content should be 0.0005 to 0.015%. The S content is preferably 0.0010% or more, 0.0050% or more, or 0.010% or more. Also, the S content is preferably 0.013% or less, more preferably 0.011% or less.

N: 0.010% or less N is an impurity and reduces the mechanical properties and productivity of steel materials. Therefore, the upper limit of the N content is set to 0.010% or less. Preferably, the N content is 0.008% or less, 0.006% or less, 0.005% or less, or 0.004% or less. The N content may be 0%, but an extreme reduction will lead to an increase in steelmaking costs. Therefore, the N content may be 0.001% or more. In addition, N has the effect of contributing to the improvement of mechanical properties and the like by precipitating as fine nitrides. To obtain the effect, the N content may be 0.002% or more.

O: 0.0005 to 0.0035%
O is an element that has the effect of rendering MnS harmless by bonding with MnS and preventing deterioration of corrosion resistance and mechanical properties. However, when O is contained excessively, coarse oxides are formed that serve as starting points for corrosion in an acid corrosive environment. Therefore, the O content should be 0.0005 to 0.0035%. The O content is preferably 0.0010% or more, more preferably 0.0015% or more. Also, the O content is preferably 0.0030% or less, more preferably 0.0025% or less.

In the chemical composition of the steel of the present invention, in addition to the above elements, one or more selected from Mo, W, Ni, Sn, As, and Co are further added in order to improve corrosion resistance in an acid corrosion environment. may be contained within the range shown in . In addition, since these elements are not necessarily essential in the steel material, the lower limit of the content is 0%. The reason for limiting each element will be explained.

Mo: 0-0.50%
Mo is an element that improves the corrosion resistance in an acidic environment, particularly the corrosion resistance to hydrochloric acid, by containing it together with Cu, Sb, and Cr, so it may be contained as needed. However, since Mo is an expensive element, excessive content causes a decrease in economic efficiency. Therefore, Mo content shall be 0.50% or less. The Mo content is preferably 0.30% or less, more preferably 0.10% or less. To obtain the above effects, the Mo content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.

W: 0-0.50%
Like Mo, W is an element that improves corrosion resistance in an acidic environment, particularly corrosion resistance to hydrochloric acid, by containing Cu, Sb, and Cr at the same time, so it may be contained as necessary. However, since W is also an expensive element, excessive content causes a decrease in economic efficiency. Therefore, the W content is made 0.50% or less. The W content is preferably 0.30% or less, more preferably 0.10% or less. To obtain the above effects, the W content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.

Sum of one or both of Mo and W: 0.01 to 0.30%
Furthermore, one of Mo and W may be contained alone, or both may be contained at the same time. In this case, the total content of Mo and W is preferably 0.01-0.30%. The total content of Mo and W is more preferably 0.05% or more, even more preferably 0.10% or more. Further, the total content of Mo and W is more preferably 0.25% or less, even more preferably 0.20% or less.

Ni: 0-0.50%
Ni is an element that improves corrosion resistance in an acid corrosive environment, and additionally has the effect of improving manufacturability in steel containing Cu. Cu has a large effect of improving corrosion resistance, but it tends to segregate, and if it is contained alone, it may promote cracking after casting. In contrast, Ni has the effect of reducing the surface segregation of Cu. By containing Ni, in addition to suppressing the segregation of Cu and slab cracking, the occurrence of localized corrosion caused by segregation is also suppressed, so the effect of improving the corrosion resistance can be obtained.

Therefore, Ni may be contained as necessary. However, Ni is an expensive element, and a large amount of Ni causes an increase in steelmaking costs. Therefore, the Ni content is set to 0.50% or less. The Ni content is preferably 0.30% or less, more preferably 0.25% or less. In order to obtain the above effect, the Ni content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more. .

Sn: 0-0.50%
Sn is an element that improves the corrosion resistance in an acid corrosive environment when contained together with Cu, so it may be contained as necessary. However, when Sn is excessively contained, the hot workability deteriorates. 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, even more preferably 0.20% or less. In order to obtain the above effects, 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: 0-0.30%
As is less effective than Sb and Sn, but it is an element effective in improving corrosion resistance in an acid corrosive environment, so it may be contained as necessary. However, when As is contained excessively, the hot workability deteriorates. 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 effects, the As content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more. .

Co: 0-0.30%
Co does not have a remarkable effect as compared with Sb and Sn, but it is an element that improves corrosion resistance in an acid corrosive environment, so it may be contained as necessary. However, when Co is contained excessively, the economy 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 effects, the Co content is preferably 0.01% or more, more preferably 0.02% or more, and even more 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 are added below in order to improve mechanical properties and the like. It may be contained within the indicated range. In addition, since these elements are not necessarily essential in the steel material, the lower limit of the content is 0%. The reason for limiting each element will be explained.

Cr: 0-0.70%
Cr is an element that enhances hardenability and strength, so it may be contained as 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, even more preferably 0.10% or less. In order to obtain the above effects, the Cr content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more. .

Ti: 0-0.050%
Ti is an element that forms nitrides and contributes to refinement of crystal grains and improvement of strength, and thus 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 set to 0.050% or less. The Ti content is preferably 0.040% or less, more preferably 0.030% or less, even more preferably 0.020% or less. In order to obtain the above effects, the Ti content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more. .

Nb: 0-0.10%
Nb, like Ti, is an element that forms nitrides and contributes to refining crystal grains and improving strength, so it 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 set to 0.10% or less. The Nb content is preferably 0.050% or less, more preferably 0.030% or less, even more preferably 0.020% or less. In order to obtain the above effects, the Nb content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more. .

V: 0-0.10%
V, like Ti and Nb, is an element that forms nitrides and contributes to refining crystal grains and improving strength, so it 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 set to 0.10% or less. The V content is preferably 0.050% or less, more preferably 0.030% or less, even more preferably 0.020% or less. In order to obtain the above effects, the V content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more. .

Zr: 0-0.050%
Zr, like Ti, Nb, and V, is an element that forms nitrides and contributes to refining crystal grains and improving strength, so it may be contained as necessary. However, Zr is an expensive element, and a large amount of Zr leads to an increase in steelmaking costs. In addition, when Zr is contained excessively, the nitride becomes coarse and the mechanical properties deteriorate. Therefore, the Zr content should be 0.050% or less. The Zr content is preferably 0.040% or less, more preferably 0.030% or less, even more preferably 0.020% or less. In order to obtain the above effect, the Zr content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more. .

Ta: 0-0.050%
Ta is an element that contributes to an improvement in strength, and also contributes to an improvement in corrosion resistance, although the mechanism is not necessarily clear, so it may be contained as necessary. However, Ta is an expensive element, and a large amount of Ta leads to an increase in steelmaking costs. Therefore, the Ta content should be 0.050% or less. The Ta content is preferably 0.040% or less, more preferably 0.030% or less, even more preferably 0.020% or less. In order to obtain the above 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, so it may be contained as necessary. However, even if B is contained excessively, the effect may be saturated and the toughness of the base material and HAZ may be 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, even more preferably 0.0020% or less. In order to obtain the above effects, the B content is preferably 0.0003% or more, more preferably 0.0005% or more.

In the chemical composition of the steel of the present invention, in addition to the above elements, one or more selected from Ca, Mg, and REM may be contained within the ranges shown below for the purpose of deoxidizing and controlling inclusions. good. In addition, since these elements are not necessarily essential in the steel material, the lower limit of the content is 0%. The reason for limiting each element will be explained.

Ca: 0-0.010%
Ca is an element mainly used for controlling the morphology of sulfides, and may be contained as necessary in order to form fine oxides. However, when Ca is excessively contained, 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 effects, the Ca content is preferably 0.00005% or more, 0.0001% or more, or 0.0005% or more, and more preferably 0.001% or more. , more preferably 0.002% or more.

Mg: 0-0.010%
Mg may be contained as necessary in order to form fine oxides. However, excessive addition of Mg causes an increase in steelmaking costs. 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 order to obtain the above effects, the Mg content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more. .

REM: 0-0.010%
REM (rare earth element) is an element mainly used for deoxidation, and may be contained as necessary in order to form fine oxides. However, excessive addition of REM causes an increase in steelmaking costs. Therefore, the REM content is set to 0.010% or less. The REM content is preferably 0.005% or less, more preferably 0.003% or less. In order to obtain the above effects, the REM content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more. .

Here, REM is a generic term for a total of 17 elements of Sc, Y and lanthanoids, and the content of REM means the total amount of the above elements. Incidentally, lanthanoids are industrially added in the form of misch metals.

In the chemical composition of the hot-rolled steel material of the present invention, the balance is Fe and impurities. The term "impurities" as used herein refers to components that are mixed from raw materials such as ores, scraps, and other factors during the industrial production of steel materials, and that are allowed within a range that does not adversely affect the steel materials according to the present invention. means.

Moreover, in the hot-rolled steel material according to one embodiment of the present invention,
The chemical composition, in mass %,
C: 0.01 to 0.10%,
Si: 0.04 to 0.40%,
Mn: 0.50-1.50%,
Cu: 0.05-0.50%,
Sb: 0.01 to 0.30%,
Al: 0.005 to 0.050%,
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%,
Ni: 0 to 0.30%,
Sn: 0-0.50%,
As: 0 to 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%,
Ca: 0-0.010%,
Mg: 0-0.010%,
REM: 0-0.010%,
balance: Fe and impurities,
The mass ratio Si/Al between the Si content and the Al content is 6.0 to 16.0,
AI defined by the following formula (i) is 0.06 to 0.21,
At least one of EI defined by the following formula (ii) is 2.5 to 6.0, or the total content of Cu and Sb is 0.10 to 0.25% by mass. satisfied,
Ceq defined by the following formula (iii) is 0.180 to 0.330.

Si/Al: 6.0 to 16.0
The Si/Al ratio (mass ratio) is an important index for suppressing oxides that tend to serve as corrosion starting points on the steel material surface. In order to suppress the formation of oxides, it is effective to use Si, which has a weaker oxidizing power than Al, and the corrosion resistance is remarkably improved by setting the Si/Al ratio to 6.0 or more. On the other hand, even if the Si/Al ratio exceeds 16.0, the effect saturates, and as the amount of Al decreases, deoxidation becomes insufficient, and oxides may lower the corrosion resistance. Therefore, the Si/Al ratio is preferably 6.0 to 16.0. The Si/Al ratio is preferably 6.7 or higher, 8.0 or higher, 8.5 or higher or 9.0 or higher. Also, the Si/Al ratio is preferably 14.0 or less, 13.5 or less, 13.0 or less, or 12.0 or less.

AI: 0.06-0.21
The acid corrosion resistance index AI is an index derived to suppress carbides that tend to serve as corrosion starting points on the steel material surface. Mo and W are effective in improving corrosion resistance, but if their content is excessive, they tend to form carbides that can initiate corrosion. In order to remarkably improve the corrosion resistance in an acid corrosion environment, the acid corrosion resistance index AI is preferably 0.06 to 0.21. The acid corrosion resistance index AI is preferably 0.08 or more, more preferably 0.10 or more, and even more preferably 0.12 or more. Also, 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 corrosion resistance 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 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.
AI=((Mo/96)+(W/184))/(C/12) (i)

EI: 2.5-6.0
The workability index EI is an index that takes into consideration the influence of Sb and Sn that promote the deterioration of hot workability due to Cu. If the contents of Sb and Sn are too large relative to the content of Cu, the hot workability may deteriorate. 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 are insufficient, the effect of improving corrosion resistance in an acid corrosive environment may become insufficient. From the viewpoint of achieving both hot workability and corrosion resistance, the workability index EI is preferably 2.5 to 6.0. The workability index EI is preferably 2.55 or more, more preferably 2.6 or more. Also, the workability index EI is preferably 6.0 or less, more preferably 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 contents of Cu, Sb, and Sn by the mass number of each element, respectively.
EI=(Cu/64)/((Sb/122)+(Sn/119)) (ii)

Cu+Sb: 0.10-0.25%
Combining Cu and Sb improves the acid resistance of the steel. 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 deteriorate. It is preferable to have

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

Furthermore, when Ca: 0.00005 to 0.010% is contained, XI defined by the following formula (iv) is 5.0 to 16.0, or Ca content and O content is preferably 1.00 or less.

XI: 5.0 to 16.0
Ca is an element that forms an oxide like Al. Therefore, in the case of containing 0.00005% or more of Ca, in order to suppress the generation of oxides, in addition to Al and Si, Ca is also considered, and specifically, XI defined by the following formula (iv) is preferably 5.0 to 16.0. XI is more preferably 6.0 or more, and even more preferably 7.0 or more. Also, XI is more preferably 15.0 or less, even more 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 become corrosion starting points on the steel material surface. Ca increases the cleanliness of steel by forming fine oxides that do not affect corrosion resistance. , reducing corrosion resistance. In particular, when the Ca content is 0.00005% or more, the Ca/O ratio is preferably 1.00 or less in order to suppress the formation of excessive coarse oxides. More preferably, the 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 if the Ca/O ratio is too low, oxides other than Ca are formed and the corrosion resistance is reduced. or more or 0.015 or more.

Further, in the hot-rolled steel material according to another embodiment of the present invention,
The chemical composition, in mass %,
C: 0.01 to 0.10%,
Si: 0.04 to 0.40%,
Mn: 0.30-1.50%,
Cu: 0.05-0.50%,
Sb: 0.03 to 0.30%,
Ni: 0.01 to 0.50%,
Cr: 0.02-0.50%,
Al: 0.005-0.055%,
N: 0.002 to 0.010%,
P: 0.020% or less,
S: 0.0005 to 0.015%,
O: 0.0005 to 0.0035%,
Mo: 0-0.50%,
W: 0 to 0.50%,
Sn: 0 to 0.30%,
As: 0 to 0.30%,
Co: 0-0.30%,
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%,
Ca: 0-0.010%,
Mg: 0-0.010%,
REM: 0-0.010%,
balance: Fe and impurities,
The mass ratio Si/Al between the Si content and the Al content is 7.0 to 15.0,
BI defined by the following formula (v) 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.

Si/Al: 7.0 to 15.0
The Si/Al ratio (mass ratio) is an important index for suppressing oxides that tend to serve as corrosion starting points on the steel material surface. In order to suppress the generation of oxides, it is effective to use Si, which has a weaker oxidizing power than Al, and the corrosion resistance is remarkably improved by setting the Si/Al ratio to 7.0 or more. On the other hand, even if the Si/Al ratio exceeds 15.0, the effect is saturated, and as the amount of Al decreases, deoxidation becomes insufficient, and oxides may lower the corrosion resistance. Therefore, the Si/Al ratio is preferably 7.0 to 15.0. The Si/Al ratio is preferably 8.0 or higher or 9.0 or higher. Also, 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, which tend to serve as corrosion starting points on the steel material surface. Cr is effective in improving corrosion resistance, but if its content is excessive, it tends to form nitrides that serve as starting points for corrosion. In order to remarkably improve the corrosion resistance in an acid corrosion environment, the acid corrosion resistance index BI is preferably 0.55 to 30.0. The acid corrosion resistance index BI is preferably 0.60 or more, more preferably 0.70 or more. Also, the acid corrosion resistance index BI is preferably 15.0 or less, more preferably 10.0 or less, and even more preferably 5.00 or less.

The acid corrosion resistance index BI is the ratio between the number of Cr atoms and the number of N atoms, as defined by the following formula (v). That is, Cr/52 and N/14 are terms obtained by dividing the contents of Cr and N by the mass number of each element, respectively.
BI=(Cr/52)/(N/14) (v)

EI: 1.0-6.0
The workability index EI is an index that takes into consideration the influence of Sb and Sn that promote the deterioration of hot workability due to Cu. If the contents of Sb and Sn are too large relative to the content of Cu, the hot workability may deteriorate. 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 are insufficient, the effect of improving corrosion resistance in an acid corrosive environment may become insufficient. From the viewpoint of achieving both hot workability and corrosion resistance, the workability index EI is preferably 1.0 to 6.0. The workability index EI is preferably 2.0 or more, more preferably 3.0 or more. Also, the workability index EI is preferably 5.9 or less, more preferably 5.8 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 contents of Cu, Sb, and Sn by the mass number of each element, respectively.
EI=(Cu/64)/((Sb/122)+(Sn/119)) (ii)

Ceq: 0.150-0.400
Ceq is an index showing deterioration of weldability due to increase in hardness. If Ceq is excessive, weldability cannot be ensured. On the other hand, if the Ceq is too low, the mechanical properties will be insufficient. Therefore, Ceq is set to 0.150 to 0.400. Ceq is preferably 0.180 or more, more preferably 0.200 or more. Also, 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 formulas (i) to (v) represent the content (% by mass) of each element contained in the steel material, and 0 shall be substituted when not contained.

(B) Oxide scale The hot-rolled steel material of the present invention has an oxide scale on at least a part of the surface of the base material, and a concentrated layer of Si, Cu and Sb at the interface between the base material and the oxide scale. . By having a concentrated layer of these elements, a barrier effect against sulfuric acid and hydrochloric acid is exhibited, and corrosion resistance in an acid corrosive environment is further improved.

Here, the concentrated layer of Si, Cu, and Sb means that Si, Cu, and Sb in the steel are diffused during the heat treatment and concentrated at the interface between the base material and the oxide scale. Specifically, a line analysis is performed by an electron probe microanalyzer (EPMA) on a cross section that is perpendicular to the surface of the steel material and includes the interface between the base material and the oxide scale, and the content of Si, Cu and Sb is , is defined as a concentrated layer as a region in which the content is at least two times higher than the content in the base material. In the present invention, measurement is performed under the conditions of acceleration voltage: 15 kV, beam diameter: ~100 nm, irradiation time: 20 ms, and measurement pitch: 80 nm.

In the case where the base material contains Ni, it is desirable to form a Ni-enriched layer on the base material side of the Si, Cu, and Sb-enriched layers. By having the Ni-concentrated layer, it is possible to further improve the corrosion resistance.

(C) Inclusions MnS may become a starting point of corrosion and deteriorate the corrosion resistance in an acid corrosive environment. Therefore, in the steel material according to the present invention, it is preferable that 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 with a maximum length of less than 2.0 μm hardly affects the corrosion resistance of steel materials, inclusions with a maximum length of 2.0 μm or more are targeted in the present invention.

On the other hand, an extreme reduction in the contents of Mn and S is not preferable in terms of improving the strength, toughness and corrosion resistance of the steel material of the present invention. Therefore, it is preferable to combine MnS and oxygen to form an MnS oxide. This is because MnS oxide is rendered harmless and is less likely to serve as a starting point for corrosion.

This makes it easier to limit the number density of MnS having a maximum length of 2.0 μm or more contained in the steel material to less than 50/mm ² . In the following description, MnS with a maximum length of 2.0 μm or more is simply called MnS, and MnS oxide with a maximum length of 2.0 μm or more is simply called MnS oxide. The number density of MnS is preferably 40/mm ² or less, more preferably 30/mm ² or less.

In order to sufficiently detoxify MnS, the ratio of the number density of MnS oxides with a maximum length of 2.0 μm or more to the number density of MnS with a maximum length of 2.0 μm or more is 0.10 or more. It is preferable to The above ratio is preferably 0.12 or greater, more preferably 0.15 or greater.

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

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

(D) Manufacturing Method A method for manufacturing a hot-rolled steel material according to one embodiment of the present invention will be described. Steel materials according to the present embodiment include steel sheets, shaped steels, steel pipes, and the like manufactured by hot rolling. A thick steel plate having a thickness of 3 mm or more, more preferably 6 mm or more is preferable.

The steel material according to the present embodiment is manufactured by melting steel by a conventional method, adjusting the components, and hot-rolling a steel slab obtained by casting. In order to control the number density ratio of MnS and MnS oxides present in the steel material to the range described above, it is important to set the heating temperature before hot rolling to a relatively low temperature. It is preferable to set the temperature to 1130°C.

By lowering the heating temperature before hot rolling, it is possible to suppress the growth of MnS and to refine it during rolling. Since the miniaturized MnS has a relatively large surface area, it easily bonds with oxygen and easily becomes an MnS oxide. In order to make the number density of MnS less than 30/mm ² and to make the ratio of the number density of MnS oxides to MnS 0.12 or more, the heating temperature before hot rolling is more preferably 1080°C or less. .

After hot rolling, the hot-rolled steel sheet is subjected to subsequent processes such as cutting or coiling. At that time, the temperature of the steel sheet drops, but it is desirable that the time from the completion of hot rolling until reaching 400° C. is 4 hours or more. Exposure to this temperature range promotes bonding between MnS and oxygen.

In addition, the concentration of Si, Cu, and Sb progresses at the interface between the base material and the oxide scale during the period from the completion of hot rolling until the temperature reaches 400°C. In order to form the above-mentioned concentrated layer of Si, Cu and Sb, it is preferable to set the time from the completion of hot rolling until reaching 400° C. to 4 hours or more.

When a steel pipe is produced from the obtained steel plate, the steel plate may be formed into a tubular shape and then welded.

EXAMPLES The present invention will be described in more detail below with reference to examples. It should be noted that the conditions in the examples shown below are an example of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to this example of conditions. Moreover, the present invention can adopt various conditions without departing from the gist of the present invention and as long as the objects of the present invention are achieved.

Steels (A1 to 24, B1 to 28, C1 to 13) having chemical compositions shown in Tables 1 to 3 are melted, and the steel ingots are hot rolled under the conditions shown in Tables 4 to 6 to obtain thicknesses. A hot-rolled steel sheet with a thickness of 20 mm was produced.

A test piece for EPMA measurement was cut out from each obtained steel plate so that a cross section perpendicular to the surface of the steel plate and including the interface between the base material and the oxide scale was the measurement surface, and the measurement surface was polished. Then, EPMA line analysis was performed to determine the presence or absence of a thickened layer at the interface between the base material and the oxide scale. The EPMA measurement conditions were acceleration voltage: 15 kV, beam diameter: ~100 nm, irradiation time: 20 ms, and measurement pitch: 80 nm.

Further, a test piece for SEM observation was cut out from each steel plate, and the number density of inclusions was measured by EDS provided in the SEM. The maximum length of MnS and MnS oxides detected in the field of view is measured at a magnification of 1000 times, and the number of inclusions with a maximum length of 2.0 μm or more is counted and divided by the area of the field of view. to find the number density.

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

<Sulfuric Acid Resistance, Hydrochloric Acid Resistance>
A test piece having a 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 by wet #400 polishing to obtain a test piece for corrosion resistance evaluation. 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.

After that, the corrosion rate was calculated from the corrosion weight loss of the test piece in the sulfuric acid immersion test and the hydrochloric acid immersion test. In this example, when the corrosion rate in the sulfuric acid immersion test is 20.0 mg/cm ² /h or less, the sulfuric acid resistance is judged to be excellent, and the corrosion rate in the hydrochloric acid immersion test is 15.0 mg/cm ² /h. h or less was judged to be excellent in hydrochloric acid resistance.

<Hot workability>
The surface of the hot-rolled material rolled under the above conditions was visually observed, and the hot workability was evaluated by rating x for those with cracks and ◯ for those without cracks.

<Weld crack>
A y-type weld cracking test was performed according to JIS Z 3158:2016. A test piece with a thickness of 20 mm was welded from both sides with a current of 170 A, and after 48 hours had passed, the presence or absence of cracks on the surface and cross section was checked.

<Tensile strength>
A tensile test piece having a thickness of 12 mm was prepared according to JIS Z 2241:2011, and a tensile test was performed to determine the tensile strength. A sample with a tensile strength of 400 MPa or more was rated as ◯, and a sample with a tensile strength of less than 400 MPa was rated as x.

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

As shown in Tables 7-9, test no. 1 to 24, 26 to 53 and 55 to 67 gave excellent results in all performance evaluation tests. On the other hand, Test No. which is a comparative example. 25, 54 and 68 resulted in deterioration in sulfuric acid resistance and hydrochloric acid resistance.

The steel material of the present invention is used for boilers that burn fossil fuels such as heavy oil and coal, gas fuels such as liquefied natural gas, general waste such as municipal waste, waste oil, plastics, industrial waste such as waste tires, and sewage sludge. Can be used for smoke extraction equipment. Specifically, flue ducts, casings, heat exchangers, gas-gas heaters composed of two heat exchangers (heat recovery and reheaters), desulfurization equipment, electrostatic precipitators, and induced draft fans of smoke exhaust equipment. , basket materials and heat transfer element plates of rotary regenerative air preheaters.

Claims (6)

A hot-rolled steel material having oxide scale on at least part of the surface of the base material,
The chemical composition of the base material is, in mass%,
C: 0.01 to 0.10%,
Si: 0.04 to 0.40%,
Mn: 0.30-1.50%,
Cu: 0.02-0.50%,
Sb: 0.01 to 0.30%,
Al: 0.005-0.055%,
P: 0.020% or less,
S: 0.0005 to 0.015%,
N: 0.010% or less,
O: 0.0005 to 0.0035%,
Mo: 0-0.50%,
W: 0 to 0.50%,
Ni: 0 to 0.50%,
Sn: 0-0.50%,
As: 0 to 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%,
Ca: 0-0.010%,
Mg: 0-0.010%,
REM: 0-0.010%,
balance: Fe and impurities,
And one or more of the following conditions 1 to 3 are satisfied,
having a concentrated layer of Si, Cu and Sb at the interface between the base material and the oxide scale;
Hot rolled steel.
Condition 1) Contains one or more selected from the group consisting of Mo: 0.01% or more, W: 0.01% or more, Sn: 0.001% or more, and As: 0.01% or more .
Condition 2) Cr: 0.01% or more, Ti: 0.001% or more, Nb: 0.001% or more, V: 0.001% or more, Zr: 0.001% or more, Ta: 0.001% or more , and B: containing one or more selected from the group consisting of 0.0003% or more.
Condition 3) Contains one or more selected from the group consisting of Ca: 0.00005% or more, Mg: 0.0001% or more, and REM: 0.0001% or more. The chemical composition, in mass %,
Mn: 0.50-1.50%,
Cu: 0.05-0.50%,
Al: 0.005 to 0.050%,
The sum of one or both of Mo and W: 0.01 to 0.30%,
The mass ratio Si/Al between the Si content and the Al content is 6.0 to 16.0,
AI defined by the following formula (i) is 0.06 to 0.21,
At least one of EI defined by the following formula (ii) is 2.5 to 6.0, or the total content of Cu and Sb is 0.10 to 0.25% by mass. satisfied,
Ceq defined by the following formula (iii) is 0.180 to 0.330,
The hot rolled steel material according to claim 1.
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 (% by mass) of each element contained in the steel material, and 0 shall be substituted when it is not contained. The chemical composition, in mass %,
Sn: 0.001 to 0.50%, containing
The hot rolled steel material according to claim 2. The chemical composition, in mass %,
Contains Ca: 0.00005 to 0.010%,
XI defined by the following formula (iv) is 5.0 to 16.0,
The hot-rolled steel material according to claim 2 or 3.
XI=(Si/28)/((Al/27)+(Ca/40)) (iv)
However, the element symbol in the above formula represents the content (% by mass) of each element contained in the steel material, and 0 shall be substituted when it is not contained. The chemical composition, in mass %,
Contains Ca: 0.00005 to 0.010%,
The mass ratio Ca/O between the Ca content and the O content is 1.00 or less,
The hot-rolled steel material according to claim 2 or 3. The chemical composition, in mass %,
Cu: 0.05-0.50%,
Sb: 0.03 to 0.30%,
Ni: 0.01 to 0.50%,
Cr: 0.02-0.50%,
N: 0.002 to 0.010%,
The mass ratio Si/Al between the Si content and the Al content is 7.0 to 15.0,
BI defined by the following formula (v) 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 hot rolled steel material according to claim 1.
BI=(Cr/52)/(N/14) (v)
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 (% by mass) of each element contained in the steel material, and 0 shall be substituted when it is not contained.